A psychologist wonders whether midsemester warning notices affect performance. She decides to select a sample of delinquent students and, at random, to send such notices to half of them and no notices to the other half. Experience suggests that among such delinquent students, GPA .30. She decides to adopt .05 and power .95. If the difference between the warned and unwarned students is as great as .075 grade points, she would want to know it.
(a) State the null hypothesis and the alternative hypothesis for a two-tailed test.
(b) What should the size of the sample be to test this null hypothesis? Use Cohen's power curves.