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Unformatted text preview: PSTAT 120C: Midterm Solutions May 20, 2009 1. (a) For W = 99 , 235 the appropriate U is U = 99 , 235 298(299) 2 = 54 , 684 The expected value of this statistic is n 1 n 2 / 2 = 49 , 021 so we really should use U = 298(329) 54 , 684 = 43 , 358 (b) The standard deviation of the U statistic here is p n 1 n 2 ( n 1 + n 2 + 1) / 12 = p (298)(329)(628) / 12 = 2 , 265 . 141 Thus the test statistic Z = 43 , 358 49 , 021 2 , 265 . 141 = 2 . 50 which gives a P value of P = 2 P { Z < 2 . 50 } = 2( . 0062) = 0 . 0124. (c) The advantage of the MannWhitney test is that we do not have to assume that the observations are normally distributed. (d) The disadvantage is that the MannWhitney test has less power than the ttest when the data is normally distributed. 2. A survey asked people to rate their opinion on the economy on a scale from 1 to 10 where 10 meant they were extremely optimistic, 1 meant they were extremely pessimistic, and 5 indicated that they were neutral. Here are the results in the order they were collected:indicated that they were neutral....
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 Spring '08
 EduardoMontoya

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