ABSTRACT
Section 40 showed how to conduct a two-tailed z test. The decision rule at the
.05 level was to reject the null hypothesis when the value of z was as extreme as
1.96 or -1.96. The following figure illustrates that the rule is based on the two tails
(Together, they contain 5%.)
In the example in Section 40, the population mean on the SAT-Verbal test was
compared with the mean of a random sample of students from the State of Disrepair
(the 51st state admitted to the union). By using a two-tailed test, we were expressing
interest in one of two types of significant differences:
1. The sample mean is significantly lower than the population mean (as indi-
cated by a z as extreme as -1.96).
