where S1 and S3 denote the residual sum of squares from the variable coefficient model and the pooled model, respectively; N denotes the number of cross-sectional data observations; T denotes the total number of years; and k denotes the number of arguments in the model. If the F2 value is lower than the corresponding critical value at a given confidence level, H1 cannot be rejected. Namely, the pooled model is selected. Otherwise, one new Ftest should be further conducted between the variable intercept model and variable coefficient model. The new F-test statistic (denoted by F1) can be expressed by