In reference to Problem 13, what should the sample size be if the psychologist feels that: (a) She should choose power .80? (b) Is it important to know whether the difference between the means is .15 grade point (or more) and power .95? Problem 13 A psychologist wonders whether mid semester 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?? power curves.