3.4 Net Income Differential as a Percentage of Sales over Time
In order to determine any significant change in net income differentials as a percentage of sales over the period studied and reduce the impact of one-year anomalies, average net income differential as a percentage of sales 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 as a percentage
of sales 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 10 shows the correlations and paired-sample t-test results of 2-year average net income differential as a percentage of sales of the total sample and each of the three industries at
the beginning and that at the end of the period studied.