3.3 Net Income Differential over Time
In order to determine any significant change in net income differential over the period studied and reduce the impact of one-year anomalies,average net income differential was calculated for the 2-year period at the beginning of the period
studied (2003 and 2004) and at the end of the period studied (2010 and 2011). A paired t-test was conducted to test whether the 2-year average net income differential at the beginning of the period studied is significantly different from that
at the end of the period studied when examined in aggregate and by industry. Table 9 shows the correlations and paired-sample t-test results of 2-year average net income differential of the total sample and each of the three industries at
the beginning and that at the end of the period studied.