On 10/08/2010 06:55 AM, Jonathan DuBois wrote: > Hi, > > I have been using R to do multiple analyses of variance with two > covariates, but recently found that the results in SPSS were very > different. I have check several books and web resources and I think > that both methods are correct, but I am less familiar with R, so I was > hoping someone could offer some suggestions. Oddly simple ANOVA is the > same in SPSS and R. Including covariates improves the main effect > (p-value) in R and diminishes it in SPSS.. > > The formula I have been using is: >> Y = cbind(dV1, dV2, dV3) >> aov(lm(Y~iV1+cV1+cV2))
I wouldn't use aov() and lm() in combination like that. I'm a bit surprised that it actually does something, in fact -- the argument to aov() is documented to be a model formula and aov() is not a generic function. Anyways, what you do get is sequential (type1) ANOVA for each variable, and these depend on the order of terms in the model. What I would do is explicitly to compare the the models with and without the group effect: fit1 <- lm(Y~iV1+cV1+cV2) fit2 <- lm(Y~cV1+cV2) anova(fit1, fit2) which will give you a multivariate test of iV1 specifically. > The main independent variable is disease group and the covariates are > continuous nuisance variables such as age. Both nuisance variables > interact with the dependent variable but not each other. The frequency > distribution of the covariates is similar for each group, but the > groups are not matched 1 to 1. Therefore we would like to control for > these factors statistically. Is this the proper formula for such a > test? If so, what might be cause of major discrepancy with SPSS? -- Peter Dalgaard Center for Statistics, Copenhagen Business School Phone: (+45)38153501 Email: pd....@cbs.dk Priv: pda...@gmail.com ______________________________________________ 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.
