A study about the effects of marijuana use concludes that persons who became persistent users before they reached the age of 18, and who remained persistent users until age 38, will lose several IQ points over that time span.5 The summary data, for the change in IQ from before marijuana use to age 38, are

(a) Do the data provide strong evidence that the mean difference is not equal to 0?
(b) Construct a 95% confidence interval for the mean difference.

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n 238.0 14.5 SD145