To assess whether net income differential has
increased signifi cantly over time, a paired t-test
will be conducted to test whether average net
income differential as a percentage of sales at the beginning of the period studied is signifi cantly
different from that at the end of the period
studied at the fi ve percent level of confi dence
when examined in aggregate for the total sample
and separately for each of the three industries. It
was hypothesized that: