25 Significant does not mean important Never forget that even small effects can be statistically significant if the samples are large To illustrate this fact consider a sample of 148 small businesses During a three year period 15 of the 106 headed by men and 7 of the 42 headed by women failed 22 a Find the proportions of failures for businesses headed by women and businesses headed by men These sample proportions are quite close to each other Give the P value for the z test of the hypothesis that the same proportion of women s and men s busi nesses fail Use the two sided alternative The test is very far from being significant b Now suppose that the same sample proportions came from a sample 30 times as large That is 210 out of 1260 businesses headed by women and 450 out of 3180 businesses headed by men fail Verify that the proportions of failures are exactly the same as in part a Repeat the z test for the new data and show that it is now significant at the a 0 05 level c It is wise to use a confidence interval to estimate the size of an effect rather than just giving a P value Give the large sample 95 confidence intervals for the difference between the proportions of women s of