DSest = data structure and estimation methods	The existing studies have based their work on various types of data, such as cross-section, time-series and panel data. To each type of data corresponds an econometric methodology. Among different methodology of empirical of empirical studies, panel data method (fixed effects … etc.), ordinary least-squares and others estimates often vary greatly as a result. Our dummy variable distinguishes by coding 1 if panel data method and 0 for others.
Npoints = number of observations	It seems important to argue that the size of the sample can lead to different conclusions about factors leading to R&D activities.
USNUS = US or not US country variable	Most studies have documented the effectiveness of tax credit in the United States. Over time there was an increasing tendency to examine firms from elsewhere, such as Europe, Asia, Africa and South America. To distinguish the studies with and without United States data, we include a dummy variable.
Idum = includes industry dummy	Some omitted factors may influence both R&D expenditure and tax credit variables, such as the nature of competitive rivalry. To address concerns with the endogeneity of tax credit variable, most studies have included dummy variables to capture those effects, such as industry dummy. R&D investment might be determined by a variety of firm or industry specific characteristic in the policy-making processes, such as the supply of researchers, the degree of concentration, technology opportunities, industrial pressure … etc. We try to capture these characteristics by this dummy variable.
TDY: includes time dummy	Some studies include time variables to represent the effect of omitted variables and control the effect that is common across time. Since various common shocks, like business cycle or technology shocks, would affect the ability of firms to perform R&D, time dummy variable is included in the model to get rid of annual macroeconomic shocks. Time dummy capture the changes in public policy, technological progress or other time-specific effects which authors were not able to incorporate into the measure of tax credit.
BERD: business enterprises expenditure in R&D	R&D personnel in the total workforce is one of the major determinants in R&D intensity. Scientists undertake R&D research and also expand absorptive capacity. This variable measures the R&D activity of the firm.
GDERD: gross domestic exp on R&D total	The theory of industrial cluster argues that R&D investment depends on interactions between input supply and local demand conditions and the competition of industries, but those factors are complicated and immeasurable. To account for these omitted factors, the gross domestic expenses is included in the model to interpret fluctuations in economic health over the sample period. Its growth may be relevant to determine the market size for future inventions, which in turn is decisive to forecast the profitability of R&D project.
HE: high Edu expenditure by gov.	One of the key elements in the production of ideas is human capital which is dedicated to the production of R&D activity. The higher education influences the size of R&D employment and event stimulates R&D activity. The development of human capital generates a highly educated labour and increase the productivity of labour over time, so the efficient labour approaches the success in the innovation-based growth model. As a result, human capital consists of skills and knowledge which workers acquire through education and training.