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\title{Trade Liberalisation and The Formal-Informal Sector Dichotomy in Nigeria}
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             \author[1]{A.  Oluwatoyin}

             \author[2]{Dr.  MATTHEW}

             \affil[1]{  Covenant University, Ota, Nigeria}

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\date{\small \em Received: 6 June 2011 Accepted: 29 June 2011 Published: 13 July 2011}

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\begin{abstract}
        


This study assesses the role of trade liberalization on the formal-informal sector of Nigeria. It looks at whether or not the trade liberalization process have any effect on both the reduction in the wage differential between registered and non-registered (roughly formal and informal) workers and the fall in the proportion of registered workers. The study uses both secondary and primary data via the administration of questionnaires to discuss the channels through which trade liberalization could affect these two variables and put forward an empirical approach to test the existence of any correlation between them. The results suggest that the fall in the wage gap between registered and non-registered workers in the manufacturing sector was affected by trade-related variables, particularly, by the import penetration ratio. However, we do not find robust evidence that trade liberalization had a substantial effect on the fall in the proportion of registered workers.

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\keywords{Trade Liberalization, Registered and Non-registered workers, Manufacturing Sector}

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\let\tabcellsep& 	 	 		 
\section[{INTRODUCTION}]{INTRODUCTION}\par
common feature of several middle-income developing countries in the late 1980's and early 1990's was the undertaking of several structural reforms, particularly trade liberalization measures. Many recent studies have tried to assess the impact of these reforms on the labour market of these countries. Basically, researchers have looked for evidences of any of the Hecksher-Ohlin/Stolper-Samuelson (HOS) framework's implications in developing country labour markets. More specifically, they have looked for evidence that trade liberalization has triggered the following sequence of events : (1) increases in the relative price of unskilled intensive products/firms/industries; (2) a positive effect of these price increases on the demand for unskilled workers; (3) a reduction in the wage premium of skilled workers, leading to lower wage inequality in these countries and finally, \hyperref[b3]{(4)} an increase in the share of skilled workers in all sectors due to the increase in the relative price of unskilled workers. Besides this interest on the distributional consequences of trade liberalization based on the HOS framework, researchers have also tried to measure the impact of trade liberalization on employment and on the wage structure as a way to assess the importance of rent-sharing in the protected sectors. Despite these numerous studies on the impact of trade liberalization on developing country labour markets, several questions remain to be explored. In particular, as pointed out by \hyperref[b4]{Behrman (1999)}, the impact of trade liberalization on the "informal" manufacturing sector and the existence of possible spillover effects on the rest of the economy have been overlooked. This is important because if dual labour markets are important in developing countries, then to overlook the implications of trade liberalization for the wage differential between "formal" and "informal" workers and on their mobility pattern may yield an incomplete description of its impact on the entire labour market.\par
The objective of this paper is to fill the gap on the empirical literature using the Nigerian trade liberalization experience as a quasi-natural experiment. We assess whether trade liberalization can be considered a serious candidate to explain both the fall in the wage differential between registered (formal) and non-registered (informal) workers, and the fall in the proportion of registered workers in the economy. The episode of trade liberalization in Nigeria is particularly interesting for this task because it implied a huge fall in tariff and non-tariff barriers and it occurred during a short period of time, basically within three years. Besides, the schedule of tariff reduction announced in 1990 was brought forward several times, making a strong case for the exogeneity of the trade reforms.\par
The structure of the paper is as follows: First, we will describe the evolution of the proportion of registered workers and of the wage gap between registered and nonregistered workers in the manufacturing sector during the 1980's and 1990's. Second, we will discuss the literature on the impact of the trade liberalization on the labour market of developing countries, and the channels through which trade liberalization could affect both the wage differential between registered and non-registered workers and the proportion of registered workers. Third, we will put forward a procedure to identify whether or not trade liberalization had any impact on the fall in the wage differential between registered and non-registered workers and on the increase of the proportion of nonregistered workers. This procedure is based on; 1) exploiting industry variation of trade-related measures orientation ratios for a panel of 10 tradable manufacturing industries.\par
Our results suggest that trade liberalization had a statistically significant impact on the reduction of the wage differential between registered and non-registered workers in the manufacturing sector. However, we do not find evidence of spillover to entire economy. As for the impact on the proportion of registered workers, the results are not very robust, and in our opinion, it is not possible to make a strong case for the link between trade liberalization and this phenomenon. 
\section[{II.}]{II.} 
\section[{LITERATURE REVIEW a)}]{LITERATURE REVIEW a)}\par
Registered and Non -registered Workers in the Manufacturing Sector in Nigeria\par
The proportion of non-registered workers increased -at least, for non-farming activities -and the wage differential between the two groups diminished during the 1990's. The proportion of non-registered workers increased from 30 percent in 1981 to about 60 percent in 2008, and the bulk of this increase was concentrated after 1990, just after the country started the market-oriented reforms, such as the programmes of privatization and the process of trade liberalization. While the proportion of non-registered workers increased, the raw wage gap between registered and non-registered workers fell between 1981 and 2008. In 1981, the raw ratio of log real hourly-wage between registered and non-registered workers was 1.08, but in 2008 it was down to 0.71. As non-registered workers are less likely to be found in the manufacturing sector, one could associate the fall in the proportion of registered workers with the reduction in the number of workers in the manufacturing sector. A lower proportion of "manufacturing jobs" would lead to a higher proportion of non-registered jobs in the whole economy. 
\section[{b)}]{b)}\par
The Impact of Trade Liberalisation on Country Labour Market\par
The Hecksher-Ohlin theorem states that a country will tend to export goods that are relatively intensive in the abundant factor. The Stolper-Samuelson theorem shows that changes in the output price have a more than proportional effect on the return of the relatively abundant factor in the industry where the shock occurred. The combination of these two theorems yields the prediction that trade policy changes that lead to a higher relative price of unskilled-intensive goods should bring about an increase in the relative wage of unskilled workers. Assuming the special case where the functional form of the production functions for all sectors and for the aggregate utility function is Cobb-Douglas, the proportional change in the relative wage rate an open economy can be expressed as:1 1 1 2 2 2 1 s u W P A W P A ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?\textbf{(1)}\par
For the Mexican experience in the mid-1980s,  {\ref Hanson and Harrison (1999)} show that the reduction in tariff protection disproportionately affected low-skilled industries, contrary to what one would expect for a developing country. This is so, because the Mexican import substitution strategy extended trade protection preferentially to industries that made relatively intensive use of unskilled labour. The relative higher protection of industries in which the countries would, in principle, have comparative advantage was also noticed by \hyperref[b24]{Currie and Harrison (1997)} for the Moroccan manufacturing sector. \hyperref[b14]{Goldberg and Pavcnik (2003)} also found that the structure of tariff protection benefited more the industries with a higher share of unskilled workers in Colombia and in Brazil. These studies highlight the necessity to understand the previous structure of protection before assuming that any trade liberalisation reform would trigger a reduction in wage inequality, as measured by the relative wage between skilled and = unskilled workers. If the protected sectors were the ones in which the country already had comparative advantage, then the openness measures could lead to a fall in their product prices and then trigger an increase in where ? 1 and ? 2 are the proportion of skilled workers in the skilled-intensive sector and in the unskilled-intensive sector, respectively; A 1 and A 2 are technology parameters in these same sectors, and P 1 and P 2 the respective product prices. Since ? 1 > ? 2 changes in prices and/or technology have a more than proportional effect on changes in the relative wage, an increase in P 2 , the price of the product in the unskilledintensive sector, should lead to a more than proportional fall in the relative wage of the skilled workers. However, the above result only holds if we assume that: 1) the economy is small so that it cannot affect the international price of the product, which is assumed to be exogenous, 2) the economy is inside the cone of diversification, meaning that tradable goods intensive in both factors, skilled and unskilled labour, are produced in that economy; 3) there is no product differentiation, i.e., foreign and domestic goods are perfect substitutes; and 4) there are no mobility barriers for workers to respond to wage changes. A corollary of this theory is that changes in the supply of different factors do not alter their relative prices (as changes in the relative price do). Changes in the factor endowment of a country would increase the production in the industries intensive in the factor, without altering its relative price \hyperref[b17]{(Johnson and Stafford, 1999)}. So far the empirical literature on the developing countries has found at best mixed results regarding HOS predictions.\par
the relative demand for the scarce factor (skilled workers). \hyperref[b5]{Behrman et al. (2001)} do not find evidence that trade liberalization has any overall widening effect on wage differentials for a panel of 18 Latin American countries -including Brazil -for the period 1977 to 1998. Robbins (1996a) also fails to find any relationship between trade liberalisation and wage inequality for Colombia. Gindling and Robbins (2001) find evidences consistent with a positive correlation between trade liberalization and higher returns to education in Chile. \hyperref[b13]{Galiani and Sanguinetti (2001)} find that manufacturing sectors where the import penetration increased the most, wage inequality also widened relatively more in favour of the most skilled workers in Argentina.\par
According to \hyperref[b31]{Pavcnik et al (2002)}, they showed that the increase in the return of the college-educated workers coincides with the trade liberalization in Brazil. They do not find any relationship between trade related measures and the increase in wage premium in sectors more affected by the reform, but they do find that the sector specific skill premium did rise for skilled workers. \hyperref[b16]{Green et al. (2001)} also stress the coincidence between trade liberalisation in Brazil and the increase in the relative wage of college-educated workers, but fail to find any causal relationship. Unlike \hyperref[b31]{Pavcnik et al (2002)}, however, they do find that the wage premium increased in sectors more affected by the trade reform. Dickerson et al. (  {\ref 2001}) using a pseudo panel approach find that the returns to education for college-educated workers fell after the trade liberalization in Brazil, but do not find any correlation between trade measures and the return to education for college workers. Note that this result is at odds with \hyperref[b16]{Green et al. (2001)}. This is so due to the fact that the pseudo-cohort approach adopted by \hyperref[b9]{Dickerson et al. (2001)} points to an overestimation of the returns to education yielded by the Ordinary Least Square (OLS) method employed by \hyperref[b16]{Green et al. (2001)}. \hyperref[b2]{Arbache et al. (2004)} reinforce the results in  {\ref Green et al. (2002)} and argue that within the traded sector, increasing openness was associated with lower wages but the downward impact of openness on wages was insignificant at the highest two education groups. \hyperref[b15]{Gonzaga et al. (2002)} argue that the wage differential between skilled and non-skilled workers fell after trade liberalisation in Brazil and that the mechanism of transmission of this fall through tariff to prices and prices to wages is in line with HOS predictions. The lack of strong evidence for HOS implications in developing countries has been rationalized via three hypotheses. The first is related to the perception that developing countries with higher proportion of semi-skilled workers may have been suffering strong competition from countries with a higher proportion of unskilled workers  {\ref (Wood, 1997 and} {\ref Hanson and} {\ref Harrison, 1999)}. This halfway position of some industrialized developing countries, mainly in Latin America, would prevent HOS framework from working.\par
The second hypothesis assumes that trade may have caused a higher contact with leading-edge technology. In order to install this newly available technology, firms might have demanded more skilled workers to operate them and to adapt the production process to this more efficient technology. Such reasoning is advocated by the skill-enhancing trade hypothesis due to capital-skill complementarity \hyperref[b40]{(Robbins, 1996b)} and by the learningby-trade hypothesis \hyperref[b33]{(Pissarides, 1997)}.\par
The third hypothesis argues that empirical studies fail to find any HOS trade related impact on income distribution because of global/pervasive skill biased technological change \hyperref[b21]{(Berman and Machin, 2000)}. In this case, no correlation between trade measures and the increase in the premium of skilled workers would be observed, since this would be an economy-wide phenomenon. A second point that has been emphasized in the literature is the role of trade liberalization in changing institutional features of industrial relations and then indirectly affecting the wage and employment structure (mainly in the manufacturing sector) of countries that have undertaken trade reforms. The main hypothesis has to do with the loss of union power triggered by trade reforms. Both the fall of trade barriers and tariff reductions increase the price elasticity of product demand, hence reducing rents that sustained the union wage premium. We will briefly report some results of empirical studies on the impact of trade reforms on relative wages. Driffil et al.  {\ref (1998)} show that when non-tariff barriers were reduced in Britain, the wages in the relevant establishments fell significantly Revenga (1992) observes a similar effect associated with falls in import prices in the United States of America. \hyperref[b20]{Lang (1998)} argues that the small effect of trade liberalization in New Zealand on the composition of employment suggests that the effect of tariffs on wages and firms' monopoly power, reducing both of them, eliminated any effect on the distribution of employment. \hyperref[b23]{Borjas and Ramey (1995)} show that the impact of international trade on relative wages (skilled/unskilled wages) depends on the market structure of the industry affected. They argue that many of durable goods industries in the United States of America in the 1980s that employed a disproportionate share of less educated workers were highly concentrated, earned significant rents, and shared those rents with their workers by paying them higher-than-average wages. Their empirical evidence shows that employment changes in a small group of trade-impacted concentrated industries can explain not only part of the aggregate rise in wage inequality in the United States of America, but also some of the differences in trends in wage inequality across metropolitan areas. Somewhat against those findings, Johnson and Stafford (1999) in their review of the impact of trade on labour market institutions argued that despite the theoretical negative relationship between increased international competition and "monopoly rents" enjoyed by the firms protected in the past, there is no strong evidence of a negative effect of increased trade on unionism either in the United States of America or in the United Kingdom.\par
The findings on the impact of trade-related variables on wages and on employment for developing countries also tend to place this sort of rent-sharing argument as a possible explanation for decreases in the average wage, at least, in the manufacturing sector. \hyperref[b0]{Arbache (1999)} argues that the marked-oriented reforms in Brazil, and particularly trade liberalisation, led to a higher demand for skilled workers that ended up increasing union power. This happened because unlike developed countries, the unionized workers are relatively more educated in Brazil than non-unionized workers. \hyperref[b35]{Revenga (1997)} finds evidences that the (negative) impact of trade liberalisation on wages in Mexico was higher than the (negative) effect on employment. She argued that this fact may be explained by the prevalence of rent-sharing schemes in the period previous to trade liberalization. Such schemes would have allowed unions and firms to agree in cutting "excessive wages" rather than adjusting the employment margin after the reforms. Similarly, \hyperref[b24]{Currie and Harrison (1997)} analysing the Moroccan trade liberalization argue that in an imperfect competitive framework where some rents were captured by workers in the form of higher wages, firms could also respond to their rent loss by cutting wages and substituting temporary workers for permanent ones. Menezes-Filho and \hyperref[b28]{Arbache (2002)} show evidences of rent-sharing for unionized workers in the manufacturing sector in Brazil. However, they also find that the increase in quasi-rents brought about by trade liberalization was not shared with unionized workers. 
\section[{c) Trade Liberalisation and the Segmented Labour Market}]{c) Trade Liberalisation and the Segmented Labour Market}\par
The impact of trade liberalisation on the informal sector is less understood and less documented. \hyperref[b4]{Behrman (1999)} points out that most studies have focused on the impact of trade liberalization on the formal manufacturing sector, but less is known about its effect on the informal sector, both in the manufacturing sector17 and in the entire labour market. In this section, we will highlight possible effects of trade liberalization on segmentation -in terms of the registered versus non-registered classification -in Brazil. The first thing to notice about the relationship between trade liberalization workers is that such a classification is an institutional feature and not a skill-based classification. Unlike the classifications by educational attainment; occupational categories; production and non-production workers as traditionally used in the literature and that are based on productivity related features; the classification between registered and non -registered workers parallels the classifications between union/non-union workers and/or temporary/permanent workers, which are much more related to institutional features of the labour market. The second thing to notice is that the informal sector has been traditionally linked to the non-tradable sector, leading to the view that the informal sector would not be affected directly by trade liberalisation. The presence of a non-tradable sector where non-registered status is prevalent, however, would not affect the predictions according to the model we presented in the last section, i.e. the world prices of goods 1 and 2 would still determine the relative wage as in equation \hyperref[b0]{(1)}. Nevertheless, the presence of a non-tradable sector with such characteristic would make the cone of diversification of that country thinner, and more likely that a fall in the relative price of the "non-registered" prevalent good would lead that industry to close down so that non-registered workers would be employed only in the non-tradable sector \hyperref[b17]{(Johnson and Stafford, 1999)}. In this case, equation (1) would no longer represent the skilled/unskilled (or registered/nonregistered) wage rate. The relative wage would be determined as in the case of a closed economy:4 4 4 4 1 s u W U W S ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? (2)\par
Midway situations between the result of the closed model represented by equation (  {\ref 2}) and the open model in equation ( \hyperref[formula_0]{1}) arise if one assumes that a) domestic and foreign goods are not perfectly substitutes and b) labour types cannot move in response to wage changes \hyperref[b17]{(Johnson and Stafford, 1999)}. In fact, some commentators have used the tradable versus non-tradable approach to explain the dichotomy between formal versus informal sector in Nigeria. The idea behind this correspondence is that the earnings of workers in the informal (non-tradable sector) and the relative wage of registered and non-registered is determined by supply and demand in that sector, = whereas the earnings of workers in the formal sector is determined by the external demand for the export goods. The demand for the national manufactured product would be lower after the trade reform due to the access to cheaper products, whereas the non-tradable sector would be protected from that competition. The difficulty in such argument is how to justify the lack of mobility between workers from the formal to the informal sector. It could be argued that the informal sector would act as a cushion for workers displaced from the tradable where the subscript 4 stands for the nontradable sector 4, which is "non-registered" prevalent and unskilled intensive. It is clear from equation (2) that regardless of the changes in prices in the tradable sector, the relative wage would be unaffected. In this context, one should not expect to find any effect of trade-related variables on the relative wage of skilled/unskilled workers or registered/non-registered workers.\par
sector, leading to a downward pressure on wages in that sector. Therefore, it is not clear how trade liberalization would affect the wage of the employees in the tradable sector, but would not affect the wage of the employees in the non-tradable sector.\par
This sort of argument would be more justifiable in a context of imperfect competition where one would focus on the effect of trade reforms on institutional features of developing country labour markets. In this framework, trade reform would squeeze rents that would have been captured by protected firms and shared with their employees. As seen above, if the protected firms were the ones abundant in the scarce factor (skilled labour) then one should expect under HOS assumptions that the reduction or elimination of trade barriers would reallocate resources to the now more competitive firms based on the abundant factor and hence increase the demand for unskilled labour. However, as many papers have shown this is not necessarily the case, and the protected sector may have been in fact the one that had a higher proportion of the abundant factor. Alternatively, if the protected sector cannot be characterised as perfectly competitive, then there is space for some sort of rent sharing. Therefore, workers in most affected industries would experience a reduction in their bargaining power -and so in their wages -since the ground for rent sharing would be reduced. Assuming that the most protected sectors are the ones with a higher proportion of registered workers, or where registered workers profit more from rents thanks to the market power of their firms, one should observe a reduction in the wage premium for registered workers after the reforms. In Nigeria, registered workers are more likely to be unionized and to work in large firms, which are more likely to have market power and therefore to have some loss due to trade liberalization.\par
Therefore, it would be reasonable to assume that the degree of segmentation, as measured by the wage premium for registered workers, is positively correlated with trade protection measures. The process of trade liberalization should hence curb the wage differential between registered and non-registered workers. Besides this direct effect, skilled workers displaced from registered jobs may have joined the pool of non-registered workers, increasing their average skill differential due to changes in the composition of the two groups.\par
So far we have focused on wage differential, but as mentioned above, trade liberalization can also affect the allocation of different type of workers between and within industries. In particular, Nigerian firms may have reacted to the trade liberalization shock not only by substituting non-registered workers for registered workers, but also sub-contracting part of the tasks that they could have performed earlier in an attempt to reduce costs. The reallocation of part of the production to smaller firms may have led to a higher participation of the non-registered workers in the pool of manufacturing employee. Similarly, the reduction in absolute terms of the number of jobs in the manufacturing sector due to adjustments in the size and composition of its workforce may have led to an increase in the proportion of nonregistered workers in the entire economy. Most of these channels are hard to assess since there is no available and compatible data on "informal firms" in Brazil that would allow one to compare its performance over the recent period with the performance of medium to large firms.\par
Nevertheless, data from the Brazilian annual household survey (PNAD) shows that the proportion of non-registered workers increased in the manufacturing sector from 15 percent to 24 percent from 1981 to 2008, which is a clear indication of a possible lower degree of compliance within the manufacturing sector. Furthermore the proportion of workers in small firms (up to ten employees) increased from 40 percent to 50 percent in the entire economy and from 15 percent to 23 percent in the manufacturing sector during the same period. These changes may have two causes: a) more firms decided to contract workers illegally; b) the balance on the birth and death of firms favoured smaller firms that are more likely to employ non-registered workers. Somewhat supporting this latter hypothesis, \hyperref[b30]{Muendler (2001)}, based on an unbalanced panel of medium to large size manufacturing firms, finds that in the period 1992-1998, the probability of transition from active status to extinct (shut down), and from suspended to extinct had increased considerably in comparison to the period 1986-1990. This evidence lends some support to the argument that there was a cleanup effect among medium to large firms after trade liberalisation that may have led to a reduction in the proportion of registered workers.\par
IV. 
\section[{METHODOLOGY a) Model Specification}]{METHODOLOGY a) Model Specification}\par
In order to test whether or not trade liberalisation have an effect on the degree of segmentation and on the expansion of the informal level and contributing to the reduction in the wage sector in either the manufacturing sector or in the entire labour market, we use the strategy which relies on the variation of the degree of protection enjoyed by different industries, and on the different speed of the reform for different industries, since tariffs are uniform within the country in a given period of time .\par
This strategy allows us to check whether the industries most affected by the trade reform were also those that experienced the strongest reduction in both the degree of segmentation as measured by the wage differential and in the proportion of registered workers. Thus, in order to avoid bias due to the correlation between unobserved industry specific characteristics and trade related variables, we estimate this relationship using industry fixed-effect models and time dummies.\par
Controlling for time invariant unobserved industry characteristics is important because industry features that affect the relative wage of registered workers (and its proportion) may also affect their ability to lobby the government and/or the government priority in tariff reduction. Likewise, time dummies would control for common macroeconomic shocks that would affect both the relative wage and the proportion of registered workers and the behaviour of trade related variables. For instance, during a recession it is likely that the import penetration ratio would fall as well as the relative wage of informal sector workers, whereas the proportion of workers in that sector increases. If the recessive period coincides with the trade liberalization measures and we do not control for this common macroeconomic shock we would find a spurious relationship between the wage gap and the size of the informal sector and the trade measures. 
\section[{i. The Impact of Trade Liberalisation on the Sector}]{i. The Impact of Trade Liberalisation on the Sector}\par
To test whether or not trade liberalisation had any impact on the fall in the wage differential between the registered and non-registered workers and on the increase in the proportion of the non-registered workers in the pool of manufacturing employees between 1981 and 2008, we run a fixed-effect model for both the coefficient of the "registered worker" dummy variable obtained from a standard semi-log Mincerian wage equation and the proportion of registered workers on a set of variables related to trade: effective tariff, nominal tariff, import penetration ratio and export orientation for a panel of 10 industries. The industry classification was developed in order to make the data from the household survey compatible with the trade-related data used in this paper. The sample is restricted to employed individuals between 14 and 65 years old with positive earnings and who worked more than 20 hours per week. In the case of the effect of trade openness measures on the wage differential, we first estimate the following log wage equation for each pair of industry j and year t separately between 1981 and 2008.\par
Re ijt jt jt ijt jt ijt ijt w X g ? ? ? = + Î?" + +\textbf{(3)}jt jt j t jt TM ? ? ? ? ? ? = + + + +\textbf{(4)}\par
Saxonhouse  {\ref (1976)} shows that estimations where the dependent variable is estimated in a first step and then regressed against a set of variables intended to explain it, suffer from heteroscedasticity because the stochastic term in the first stage is individual specific (in our case, industry specific). Similarly, equation ( 5) is estimated using the share of the industry in the total manufacturing employment as weight. Besides this weighting scheme, the standard errors in both equations are Huber-White corrected for any other source of general heteroscedasticity. We run separate regressions for each trade measure and a joint one with effective tariff, import penetration and export orientation; we also include other variables related to the structure of the industry in order to test the robustness of the results. These variables are value-added and the proportion of workers who earn less than the minimum wage. Additionally, we run one specification adding the industry specific nominal exchange rate to the joint specification. The set of regressions also contains year dummies aimed at capturing aggregate shocks that may have had some impact on all manufacturing industries. where w ijt is the log of the real hourly-wage for the individual i in industry j and year t and Î?" is a vector of coefficients of the following independent variables X: region, gender, education (6 groups), experience, experience squared and metropolitan area, and ? jt is the coefficient for the dummy variable Reg that indicates whether the individual is a registered worker. In a second step, we regress the estimate coefficient ? jt on the trade-related variables :\par
where TM jt stands for trade measure variables in period j and time t and j are industry dummies and ? t are time period dummies and ? jt is assumed to be a white noise. The second reduced-form estimates refer to the impact of trade-related measures on the proportion of registered workers in the industries:\par
where Pregjt is the proportion of registered worker in industry j and year t and the remaining variables are as stated in equation ( \hyperref[formula_3]{4}). The fixed-effect regression versions of equation ( \hyperref[formula_3]{4}) are weighted by the inverse of the sampling variance of the dependent variable. The results in Table1 above suggest that trade liberalisation as measured by the import penetration ratio had a diminishing effect on the wage differential between registered and nonregistered in the manufacturing sector. Overall something between 10 percent and 14 percent of the 42 percent decrease in the wage gap in the manufacturing sector can be attributed to the 10 percent increase in the import penetration ratio observed in the period. As for the other trade measures directly affected by the reform, the coefficients for nominal and effective tariff is correctly signed as we would expect based on our discussion in the last section, i.e., they have a positive impact on the registered workers wage premium, but they are very small and not statistically significant. The regressions for the proportion of registered workers in the pool of employees were weighted by the share of workers in the industry for each industry/year pair. The standard errors are Huber-White corrected. Table  {\ref 2} shows the results for the fixed effect specifications with time dummies. Both effective and nominal tariffs have a negative effect on the proportion of registered workers for all specifications, and their coefficients are significant for most of them. The coefficients for the import penetration ratio and for the export orientation are not significant, but while the import penetration ratio shows a negative impact in most specifications, the export orientation has a positive impact in all specifications. The proportion of workers earning below the minimum wage is, as expected, negatively correlated with the proportion of registered workers. Including the industry-specific exchange rate does not change the coefficient (column \hyperref[b5]{[6]}) of the other variables. Note: As we do not have data on value-added for 1998, column \hyperref[b21]{[7]} specification has only 153 observations. The results in Tables \hyperref[tab_0]{1 and 2} are somewhat puzzling. Whereas the import penetration ratio seems to have had a negative impact on the wage differential, it has not affected (significantly) the proportion of registered workers, despite its negative sign in most specifications. The effective tariff has had no effect on the wage differential (despite its positive sign), but it has had a negative impact on the proportion of registered workers. Thus, industries most affected by the reduction in effective tariff were the same that witnessed an increase in the proportion of registered workers. Therefore, at least for the manufacturing sector, trade liberalisation may have had an impact on cutting the wage premia of registered workers, but if anything, it had the effect of increasing the proportion of registered workers in the most affected industries. In order to check the robustness of these results, allowing for some delay in the adjustments to the new tariffs and to the more competitive environment, and also to avoid problems of simultaneity between import penetration ratio and export orientation and the wage differential, we re-run equations ( \hyperref[formula_3]{4}) and (5) using lagged regressors rather than contemporaneous. As for the results for the wage premium, Table \hyperref[tab_1]{3} reveals that the main difference is that the coefficient of effective tariff is negative in most specifications, but again it is never statistically significant. The coefficients of export orientation turn out to be positive for most the fixed-effect specifications with time dummies, but they are not significant either. The coefficients for import penetration are negative and show a point estimate somewhat higher than the one with contemporaneous effect, ranging from ?0.0048 to ?0.0064. The inclusion of the lagged industry-specific exchange rate does not change this result (see column \hyperref[b5]{[6]}). 
\section[{c) Discussion of Findings}]{c) Discussion of Findings}\par
The study found out the following :\par
Firstly, we found evidence that the trade liberalisation process is behind the fall in the wage gap in the manufacturing sector. It seems that rents that went to registered workers were cut due to the more competitive environment in the economy. This result was found in strategy used in this study. The strategy used only within-manufacturing industry variation and revealed a negative effect of import penetration on the wage gap.\par
Secondly, the fall in the proportion of registered workers, however, does not seem to be correlated with trade liberalisation. The strategy did not yield robust evidence that the trade measures were correlated with the fall in the proportion of registered workers. The evidence found in the strategy that effective tariff led to an increase in the proportion of registered workers in the manufacturing sector was not robust to the use of lagged regressors in the specification with additional controls. Similarly, the evidence yielded by the strategy that the increase in the import penetration ratio led to a fall in the proportion of registered workers in the entire labour market is not robust to the use of lagged regressors.\par
Lastly, the study found out that the weak evidence for the effect of trade liberalization on the proportion of registered workers suggests that the fall in the proportion of registered workers was due to macroeconomic factors or institutional changes that had affected in a homogenous way regions and industries within the country. 
\section[{d) Recommendations}]{d) Recommendations}\par
Based on the findings of this study, the following recommendations are made:\par
1. The Government should encourage the informal sector by providing the needed financial and infrastructural facilities to facilitate their smooth operations. If this is done these micro enterprises will be able to employ more people and pay the required wages to their employees and compete with their counterparts in the advanced countries.\par
2. The formal manufacturing sector should also be provided with the necessary logistics-in terms of the access to raw materials and infrastructural/social amenities to make their operations effective, this way they will be able to employ more and pay commensurate salaries to their workers. 3. The gap between the wages earned by registered and non-registered workers should be narrowed as much as possible. One of the ways to do this is via trade liberalization which opens up the economy to international trade, where informal sector enterprises will be able to export their goods and earn income with which they can pay their employees.\par
V. 
\section[{CONCLUSION}]{CONCLUSION}\par
Therefore, based on the findings of this study, we conclude that since the fall in the wage differential between registered and non-registered workers as well as the fall in the proportion of registered workers are two stylized facts of the Nigerian labour market in the 1990s and 2000s, the Government should encourage the informal sector by lending out money to its operators in order to be able to set up enterprises that will compete favourably with their counterparts abroad. Furthermore, since the wage differential is one of the main determinants of the queue for formal jobs, it is reasonable to infer that the size of the queue can be reduced after trade liberalization by narrowing of the wage gap between formal and informal workers.\begin{figure}[htbp]
\noindent\textbf{}\includegraphics[]{image-2.png}
\caption{\label{fig_0}}\end{figure}
   \begin{figure}[htbp]
\noindent\textbf{1} \par 
\begin{longtable}{P{0.3073869900771775\textwidth}P{0.16681367144432194\textwidth}P{0.032800441014332965\textwidth}P{0.05810363836824697\textwidth}P{0.03373759647188534\textwidth}P{0.08059536934950386\textwidth}P{0.11152149944873208\textwidth}P{0.05904079382579933\textwidth}}
\tabcellsep {}[1]\tabcellsep {}[2]\tabcellsep {}[3]\tabcellsep {}[4]\tabcellsep {}[5]\tabcellsep {}[6]\tabcellsep {}[7]\\
Effective Tariff\tabcellsep 0.0008 [0.0007]\tabcellsep \tabcellsep \tabcellsep \tabcellsep 0.0007 [0.0007]\tabcellsep 0.0008 [0.0007]\tabcellsep -0.0003 [0.0009]\\
Nominal Tariff\tabcellsep \tabcellsep 0.0009 [0.0013]\tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep \\
Import Penetration\tabcellsep \tabcellsep \tabcellsep -0.0042 [0.0013]**\tabcellsep \tabcellsep -0.0039 [0.0013]**\tabcellsep \multicolumn{2}{l}{-0.0033 [0.0015]* [0.0015]** -0.0064}\\
Export Orientation\tabcellsep \tabcellsep \tabcellsep \tabcellsep -0.0011 [0.0016]\tabcellsep -0.0009 [0.0016]\tabcellsep \multicolumn{2}{l}{-0.0004 [0.0017] [0.0021] -0.0004}\\
\% workers w<mw\tabcellsep {}[0.3437]\tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep 0.6548\\
Value\tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep 0.0131\\
constant\tabcellsep \multicolumn{4}{l}{0.4076 [0.0604]** [0.0844]** [0.0327]** [0.0323]** 0.4152 0.4669 0.4711}\tabcellsep 0.4132 [0.0600]**\tabcellsep 0.2252 [0.1876]\tabcellsep 0.0640 [0.3437]\\
N\tabcellsep 170\tabcellsep 170\tabcellsep 170\tabcellsep 170\tabcellsep 170\tabcellsep 170\tabcellsep 153\\
Adj. R2\tabcellsep 0.71\tabcellsep 0.71\tabcellsep 0.72\tabcellsep 0.71\tabcellsep 0.72\tabcellsep 0.72\tabcellsep 0.73\\
\multicolumn{2}{l}{F test : industry 19.18}\tabcellsep 18.48\tabcellsep 21.24\tabcellsep 16.20\tabcellsep 19.62\tabcellsep 19.86\tabcellsep 15.33\\
Prob> F\tabcellsep 0.000\tabcellsep 0.000\tabcellsep 0.000\tabcellsep 0.000\tabcellsep 0.000\tabcellsep 0.000\tabcellsep 0.000\\
\tabcellsep \multicolumn{6}{l}{Source : Researcher's Computations Robust standard errors in brackets}\tabcellsep \\
\multicolumn{8}{l}{* Significant at 5\%; ** Significant at 1\% Industry and time dummy variables not shown.}\\
\multicolumn{8}{l}{Note : As we do not have data on value-added for 1998, specifications that include it have their sample size}\\
\tabcellsep \tabcellsep \tabcellsep \multicolumn{2}{l}{reduced form 170 to 153.}\tabcellsep \tabcellsep \tabcellsep \end{longtable} \par
 
\caption{\label{tab_0}Table 1 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{3} \par 
\begin{longtable}{P{0.18347386172006744\textwidth}P{0.07596964586846543\textwidth}P{0.07310286677908938\textwidth}P{0.09173693086003372\textwidth}P{0.0587689713322091\textwidth}P{0.11897133220910623\textwidth}P{0.1318718381112985\textwidth}P{0.11610455311973017\textwidth}}
\tabcellsep {}[1]\tabcellsep {}[2]\tabcellsep {}[3]\tabcellsep {}[4]\tabcellsep {}[5]\tabcellsep {}[6]\tabcellsep {}[7]\\
Effective\tabcellsep 0.0002\tabcellsep \tabcellsep \tabcellsep \tabcellsep -0.0001\tabcellsep -0.0000\tabcellsep -0.0003\\
Tariff\tabcellsep {}[0.0008]\tabcellsep \tabcellsep \tabcellsep \tabcellsep {}[0.0008]\tabcellsep {}[0.0008]\tabcellsep {}[0.0009]\\
Nominal\tabcellsep \tabcellsep -0.0005\tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep \\
Tariff\tabcellsep \tabcellsep {}[0.0014]\tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep \\
Import\tabcellsep \tabcellsep \tabcellsep -0.0048\tabcellsep \tabcellsep -0.0061\tabcellsep -0.0055\tabcellsep -0.0064\\
Penetration\tabcellsep \tabcellsep \tabcellsep {}[0.0016]**\tabcellsep \tabcellsep {}[0.0016]**\tabcellsep \multicolumn{2}{l}{[0.0019]** [0.0022]**}\\
Export\tabcellsep \tabcellsep \tabcellsep \tabcellsep -0.0001\tabcellsep 0.0013\tabcellsep 0.0018\tabcellsep 0.0018\\
Orientation\tabcellsep \tabcellsep \tabcellsep \tabcellsep {}[0.0019]\tabcellsep {}[0.0022]\tabcellsep {}[0.0023]\tabcellsep {}[0.0026]\\
\% workers w<mw\tabcellsep {}[0.6606]\tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep -0.1119\\
Value\tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep \tabcellsep 0.0418\\
Constant\tabcellsep 0.3861\tabcellsep 0.5788\tabcellsep 0.4703\tabcellsep 0.4693\tabcellsep 0.3928\tabcellsep 0.3931\tabcellsep 1.2582\\
\tabcellsep {}[0.0349]**\tabcellsep {}[0.0979]**\tabcellsep \multicolumn{2}{l}{[0.0328]** [0.0321]**}\tabcellsep {}[0.0367]**\tabcellsep {}[0.1704]*\tabcellsep {}[0.0235]\\
N\tabcellsep 153\tabcellsep 153\tabcellsep 170\tabcellsep 170\tabcellsep 153\tabcellsep 153\tabcellsep 119\\
Adj. R2\tabcellsep 0.71\tabcellsep 0.71\tabcellsep 0.72\tabcellsep 0.71\tabcellsep 0.72\tabcellsep 0.72\tabcellsep 0.70\\
\multicolumn{2}{l}{F test : industry 16.32}\tabcellsep 15.26\tabcellsep 20.32\tabcellsep 15.00\tabcellsep 17. 75\tabcellsep 18.05\tabcellsep 9.0\\
Prob> F\tabcellsep 0.000\tabcellsep 0.000\tabcellsep 0.000\tabcellsep 0.000\tabcellsep 0.000\tabcellsep 0.000\tabcellsep 0.000\end{longtable} \par
 
\caption{\label{tab_1}Table 3 :}\end{figure}
 			\footnote{July 2011 Trade Liberalisation and The Formal-Informal Sector Dichotomy in Nigeria © 2011 Global Journals Inc. (US)} 			\footnote{© 2011 Global Journals Inc. (US)} 			\footnote{July 2011 Trade Liberalisation and The Formal-Informal Sector Dichotomy in Nigeria © 2011 Global Journals Inc. (US)} 			\footnote{July 2011 Trade Liberalisation and The Formal-Informal Sector Dichotomy in Nigeria © 2011 Global Journals Inc. (US)} 			\footnote{July 2011 Trade Liberalisation and The Formal-Informal Sector Dichotomy in Nigeria © 2011 Global Journals Inc. (US) http://www.anpec.org.br/encontro2004/} 			\footnote{© 2011 Global Journals Inc. (US) © 2011 Global Journals Inc. (US) Trade Liberalisation and The Formal-Informal Sector Dichotomy in Nigeria} 		 		\backmatter  			 			 			  				\begin{bibitemlist}{1}
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