15.8 A school psychologist wishes to determine whether a new antismoking film actually reduces the daily consumption of cigarettes by teenage smokers. The mean daily cigarette consumption is calculated for each of eight teenage smokers during the month before and the month after the film presentation, with the following results (Note: When deciding on the form of the alternative hypothesis, H1, remember that a positive difference score (D = X1 − X2) reflects a decline in cigarette consumption.) (a) Using t, test the null hypothesis at the .05 level of significance (b) Specify the p-value for this test result. (c) If appropriate (because the null hypothesis was rejected), construct a 95 percent confidence interval for the true population mean for all difference scores and use Cohen’s d to obtain a standardized estimate of the effect size. Interpret these results. (d) What might be done to improve the design of this experiment?