```Date: Fri, 22 Oct 2010 05:33:11 -0700 Reply-To: Bruce Weaver Sender: "SPSSX(r) Discussion" From: Bruce Weaver Subject: Re: same data, same test with different procedures =different results USING 19.0 In-Reply-To: <6CB840B99C41D64197D3778C545C70C8536C55237E@EXLOYCMS.ad.loyola.edu> Content-Type: text/plain; charset=us-ascii msherman wrote: > > Dear List; I just tested a sample distribution using the One Sample > Kolmogorow-Smirnov Test under Non-Parametric Tests-One sample K-S test and > get a K-S Z of .933 with a p value of .349. Indicating that my sample is > consistent with a population that is normally distributed. I then went > and used the Explore procedure and looked at the results of the K-S test > and get some very different results. The statistic reported is .070 and > the p value is .034. I am not sure what is happening here. Does anyone > have any ideas. Thanks, martin > Hi Martin. Marta has given a very thorough answer (as usual) to your question. But I'm still wondering why you want to test for normality. If you are doing it as a precursor to a t-test (or some other parametric procedure), I would advise you to not bother. Sampling from (approximately) normal populations is most important when sample sizes are small. It becomes less and less important as the sample size increases, because the sampling distribution converges on the normal. Now consider the test of normality. When sample sizes are small (and normality is most important for the t-test), the test of normality has very low power, and will fail to detect important departures from normality. But when sample sizes are large (and normality is not so important for the t-test), the test of normality has too much power--i.e., it will throw up the red flag of non-normality for unimportant departures from normality. So testing for normality as a precursor to a t-test or ANOVA is just about the most pointless statistical exercise one can engage in. IMO, at least. ;-) ----- -- Bruce Weaver bweaver@lakeheadu.ca http://sites.google.com/a/lakeheadu.ca/bweaver/ "When all else fails, RTFM." NOTE: My Hotmail account is not monitored regularly. To send me an e-mail, please use the address shown above. -- View this message in context: http://spssx-discussion.1045642.n5.nabble.com/same-data-same-test-with-different-procedures-different-results-USING-19-0-tp3231572p3232172.html Sent from the SPSSX Discussion mailing list archive at Nabble.com. ===================== To manage your subscription to SPSSX-L, send a message to LISTSERV@LISTSERV.UGA.EDU (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD ```

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