EXAMPLE 7.14

Using software. Figure 7.13 shows JMP and Minitab outputs for the comparison of DRP scores. Both outputs include the 95% confidence interval and the significance test that the means are equal. JMP reports the difference as the mean of treatment minus the mean of control, while Minitab reports the difference in the opposite order.

Recall the confidence interval (treatment minus control) is

= 9.96 ± 4.31t*

From the JMP output, we see that the degrees of freedom under the first approximation are 37.9. Using these degrees of freedom, the interval is (1.2, 18.7). This interval, as expected, is more narrow than the confidence interval in Example 7.12 (page 439), which uses the conservative approach. The difference, however, is pretty small.

For the significance test, the P-value for the one-sided significance test is P(T ≥ 2.31). JMP gives the approximate P-value as 0.0132, again using 37.9 as the degrees of freedom.

Minitab also uses the first degrees of freedom approximation but rounds the degrees of freedom down to the nearest integer (). As a result, the margin of error is slightly wider than that of JMP and the P-value of the significance test is slightly larger.