As mentioned above, the variables concerning the characteristics of a student’s family and a student’s SES were not entered in any of the regression models, due to the multicolinearity of variables. The indirect effect of SES on students’ achievement in the post-test, especially the reduction of this effect by the intervention of differentiated instruction, was further investigated in order to determine its existence and its effect on students’ achievement. From the regression analysis, which was performed to determine the predicting value of students’ prior achievement on the posttest results, the values of the non-weighted residuals that were extracted, showed the difference in students’ achievement between the pre- and post-test. Following this, two multiple regression analyses were conducted in order to determine the degree of impact of SES on students’ progress in the experimental and control groups. Both analyses generated similar results, with the multiple regression model of the experimental group showing a regression coefficient (R2 = 0.064) lower by 1.1% compared to the control group regression model (R2 = 0.075). The difference between the two models was too small to answer our research question and thus it was necessary to further investigate this matter.