In order to study the relations between variables for the time period under study, the optimum lag is determined. For this purpose, the model selection criteria are used: Akaike information criterion, Schwartz criterion, Hannan- Quinn criterion and final prediction error. The results show that the optimum lags, according to SC criterion, is 2 lags, whereas the FPE, HQ & AIC criterion indicate the optimal lag number is p=4.