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Unformatted text preview: Alternatives to the tTools Stat 511 Chapter 4 Case Study 1: ORing Failures Observational study Highly unbalanced Far from normal Apparently unequal standard deviations and larger sample is associated with smaller standard deviation Case Study 2: Cognitive Load Randomized experiment Students not randomly selected Highly skewed distributions Censored data Case Study 1: Assumptions What assumptions are violated? normality equal standard deviation Will the log transformation help? Case Study 2: Assumptions What assumptions are violated? normality data are censored Will the log transformation help? e RankSum Test e ranksum test has several names Wilcoxon test MannWhitney Developed in the 1940s by Frank Wilcoxon (a emist, not a statistician) A nonparametric or distribution free tool No speci f c distributional assumptions required Tests the null hypothesis of identical population distributions e RankSum Test Implied assumption: populations have the same shape and spread Performs nearly as well as the twosample ttest when the data are normal and considerably better when there are extreme outliers Resistant (because it is based on ranks) Can deal with some kinds of censored data Disadvantages Con f dence intervals di cult to compute by hand (and many statistics pa ages dont do them either) its a trialanderror calculation (see pp. 9394) Does not extend easily to more complex situations. e RankSum Test: Calculate T 1. List all observations from both samples in increasing order (Display 4.5 page 91)....
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 Winter '08
 Eggett,D
 Standard Deviation

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