The methodology for the study was discussed under the following sub-headings: research design, nature and sources of data, specification of the empirical model and estimation procedure.
3.1 Research Design
The purpose of a research design is to certify that the information gathered allows the researcher to effectively address the research problem as logically and unambiguously as possible. Acquiring information relevant to the research problem in social science research, typically includes determining the type of facts required to test a theory, to evaluate a programme, or accurately characterize and evaluate context related to an observable phenomenon. This study adopted the quantitative method and descriptive research design using already existing data to provide empirical answers to the research problems. Descriptive research designs help provide answers to the questions about who, what, when, where and how connected with a research problem. A descriptive research design cannot conclusively establish answers to the why problems associated with a research. It is used to generate information on the current state of the phenomenon and to explain what exists with respect to variables (Ogunjimi, 2019).
3.2 Nature and Sources of Data
The data for this study which are purely secondary were extracted from the Federal Inland Revenue Service (FIRS), Central Bank of Nigeria (CBN) Statistical Bulletin, National Bureau of Statistics (NBS) and the World Development Indicators statistical database using the desk survey approach. The macroeconomic variables on which data were collected included the Gross Fixed Capital Formation as a percentage of GDP (GFCF), Direct taxes disaggregated into Petroleum Profit Tax (PPT), Corporate Income Tax (CIT) and Personal Income Tax (PIT), Indirect Taxes represented by Customs and Excise Duties (CED), Government Expenditure disaggregated into Government Capital Expenditure (GCE) and Government Recurrent Expenditure (GRE) and Public Debt disaggregated into Public External Debt (PED) and Government Domestic Debt (GDD). All variables were taken on annual basis in millions of Naira and in percentage running from 1980-2017 making a total of 342 observations. Data on GFCF were sourced from the World Development indicators, PPT, CIT, PIT and CED were sourced from the CBN and FIRS while GCE, GRE, PED and GDD were sourced from the CBN and NBS statistical database. Secondary data were selected as these data had already been checked by experts and other regulatory bodies prior to their publication. However, there is no doubt envisaged about the reliability of the secondary data used, but the possibility of random errors has not been overlooked.
3.3 Specification of the Empirical Model
The model designed to capture the effects of fiscal policy variables on private investment in Nigeria leans very closely on the Keynesian-classical crowding-in and crowding-out acceleration theory of investment to justify the introduction of fiscal policy variables in the model. The model was formulated to take into account the individual effects of disaggregated fiscal policy variables on private investment following the lead of Gitahi et al (2013) and Omojolaibi et al. (2016) with few modifications to suit the requirements of the current study. The study modelled private investment represented by GFCF as a function of disaggregated fiscal policy variables. The complete structure of the three standard fiscal policy variables were disaggregated into their various components of taxation, government spending and borrowing. This disaggregation was informed by the need to evaluate the individual effects and determine whether there is a crowding- in or crowding-out effect of these variables on private investment. Such a rich environment can overcome variable omission bias thus allowing for a better assessment of the individual effect of each component of fiscal policy variable on private investment. From the foregoing, the empirical model was formulated and specified as follows:
Where, GFCF is the proxy for private investment and the dependent variable of the model. All variables remain as earlier defined.
Δ = Denotes the first difference operator, t = time trend consisting of years from 1980-2017.
β0 = Intercept µ = stochastic disturbance or error term.
β1, β2, β3, β4, β5, β6, β7 and β8 are the long-run multipliers or coefficients of the explanatory variables.
Ø10 - Ø17 are the short-run dynamics or coefficients of the explanatory variables.
3.4: A Priori Expectation from the Model
The a priori expectations about the signs of the coefficients of the empirical model follow naturally from the analysis of the taxation and investment theories discussed in the theoretical framework. From theoretical literature, the study expects the signs of the coefficients of the various tax revenues to be positively or negatively related to GFCF. That is, β1, β2, β3, and β4 = > or < 0. β5 and β6 are the disaggregated coefficients of government expenditure while β7 and β8 are the disaggregated parameter coefficients of public debts. The study expects the sign of the coefficients of β5 to be positive while β6 is expected to be negatively related to GFCF. From theoretical literature, the coefficients of public debt are expected to be positively or negatively related to GFCF.
3.5 Estimation Procedure
The study uses the Autoregressive Distributed Lag (ARDL) approach to co-integration proposed by Pesaran & Shin (1999) and Pesaran, Shin & Smith (2001) to empirically analyse the long and short-run impact of fiscal policy variables on economic growth in Nigeria. This approach presents three significant advantages over the two alternatives commonly used in the empirical literature: the single-equation technique suggested by Engle & Granger (1987) and the maximum likelihood approach proposed by Johansen (1991, 1995) which are based on a system of equations. First, the ARDL bounds testing approach allows the analysis of long-term relationships between variables in a model to be achieved without the threat of producing false regressions, irrespective of whether they are stationary at levels, I(0), or stationary at first difference, I(1), or mutually co-integrated. Second, the ARDL method allows for the simultaneous estimation of the short-run and long-run components, eliminating the problems associated with omitted variables and the presence of autocorrelation. Finally, the short and long-run parameters estimated using this approach are consistent in small samples. In addition, different optimal lags can be used for different variables as they enter the model, which is not applicable in the standard co-integration test. To use this approach, the study first ensure that none of the variables in the model are I(2), as such data will invalidate the methodology. Then perform a bounds test to see if there is evidence of a long-run relationship between the variables and if the outcome is positive, then the study estimates a long-run levels model, as well as a separate unrestricted ECM. Following these, estimate the equation and ensure the errors of each model are serially independent and stable.