Cecile: On Wed, Apr 18, 2012 at 8:21 AM, Cecile De Cat <c.de...@leeds.ac.uk> wrote: > Hello, > > I'm analysing reaction time data from a linguistic experiment (a variant of > a lexical decision task). To ascertain that the data was normally > distributed, I used *shapiro.test *for each participant (see commands > below), but only one out of 21 returns a p value above p.0 05. > >> f = function(dfr) return(shapiro.test(dfr$Target.RTinv)$p.value) >> p = as.vector(by(newdat, newdat$Subject, f)) >> names(p) = levels(newdat$Subject) >> names(p[p < 0.05]) > > Removing a few outliers
!! Yikes!! I won't say "Don't do this." But I will say that this can be a very dangerous and unscientific thing to do, leading to biased, misleading results. per subject doesn't make a difference, and > "aggressive" removal of outliers (done by subject, for each of the 6 > conditions ) still results in non-normally distributed data by subject. > > Does this invalidate any attempt at multi-level modelling? How can we possibly know without knowing in detail the objectives of the investigation, the nature of the data, and the details of the analysis you did??! On general principles, normality is rarely of any real importance; lack of independence (or, in general, non-adherence to the covariance structures specified) usually is. So "any attempt" seems too general a claim to support. Indeed, a good graphical analysis -- often the most scientifically informative thing to do anyway -- is almost always a good thing to do. As this has little to do with R, you should follow up on a statistical list, like stats.stackexchange.com . -- Bert > > Many thanks in advance for your help. > > Cecile > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. -- Bert Gunter Genentech Nonclinical Biostatistics Internal Contact Info: Phone: 467-7374 Website: http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
