Bootstrap or Wilcoxons' test?
Charlotta, I'm not sure what you mean when you say simple linear regression. From your description you have two groups of people, for which you recorded contaminant concentration. Thus, I would think you would do something like a t-test to compare the mean concentration level. Where does the regression part come in? What are you regressing? As for the Wilcoxnin test, it is often thought of as a nonparametric t-test equivalent. This is only true if the observations were drawn, from a population with the same probability distribution. The null hypothesis of the Wilcoxin test is actually "the observations were drawn, from the same probability distribution". Thus if your two samples had say different variances, there means could be the same, but since the variances are different, the Wilcoxin could give you a significant result. Don't know if this all makes sense, but if you have more questions, please e-mail your data and a more detailed description of what analysis you used and I'd be happy to try and help out. Murray M Cooper, Ph.D. Richland Statistics 9800 N 24th St Richland, MI, USA 49083 Mail: richstat at earthlink.net ----- Original Message ----- From: "Charlotta Rylander" <zcr at nilu.no> To: <r-help at r-project.org> Sent: Friday, February 13, 2009 3:24 AM Subject: [R] Bootstrap or Wilcoxons' test?
Hi! I'm comparing the differences in contaminant concentration between 2 different groups of people ( N=36, N=37). When using a simple linear regression model I found no differences between groups, but when evaluating the diagnostic plots of the residuals I found my independent variable to have deviations from normality (even after log transformation). Therefore I have used bootstrap on the regression parameters ( R= 1000 & R=10000) and this confirms my results , i.e., no differences between groups ( and the distribution is log-normal). However, when using wilcoxons' rank sum test on the same data set I find differences between groups. Should I trust the results from bootstrapping or from wilcoxons' test? Thanks! Regards Lotta Rylander [[alternative HTML version deleted]]
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