Javier Acuña <javier.acuna.o <at> gmail.com> writes: > > Hi, I'm a new user of R. My background is Electrical Engineering, so > please bear with me if this is a silly question. > > I'm trying to assess whether the results of an experiment satisfy the > hypothesis of homoscedasticity (my ultimate goal is to use ANOVA). > > The result of the experiment is mean delay (dT), which depends on > three factors, topology, drift, and lambda. The first two factors are > categorical (with 4 levels each) and the last one is numerical, with > two levels. > > A sample of my data is as follows: > > dT Topology Drift lambda > 258.789 Tree b1 .43 > 244.195 Tree b1 .43 > 115.961 Tree b2 .3 > 115.183 Tree b2 .3 > > I would like to separate dT in the 32 samples (4x4x2), and test if the > variance of each sample is equal to the other 31 samples. > I tried using fligner.test and bartlett.test, but either test seems to > only work for one factor: > > > fligner.test( dT ~ Topology + Drift + lambda) > > Fligner-Killeen test of homogeneity of variances > > data: dT by Topology by Drift by lambda > Fligner-Killeen:med chi-squared = 15.4343, df = 2, p-value = 0.0004451 > > > fligner.test( dT ~ Topology ) > > Fligner-Killeen test of homogeneity of variances > > data: dT by Topology > Fligner-Killeen:med chi-squared = 15.4343, df = 2, p-value = 0.0004451 > > As I see from the previous two outputs, fligner.test only takes into > account the first factor. Similar results are obtained for > bartlett.test.
I would try fligner.test(dT ~ Topology:Drift:lambda) there's also lots of advice floating around in the archives about not taking these homogeneity of variance tests *too* seriously: for small data sets they are underpowered, for large data sets they are overpowered (i.e., they will detect departures from normality that are not actually a problem for ANOVA results). good luck, Ben Bolker ______________________________________________ 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.
