It's meaningful, but not under the name 'interaction'. The model is the same as
y ~ x1 + x2 + I(x1*x2) that is, 'including a product term'. However, needing product terms often indicates a missed transformation of y (e.g. it might be better to use log(y)). On Wed, 28 Nov 2007, Irene Mantzouni wrote: > Hi all! > > this is a rather statistical question: > is it meaningful to consider an interaction effect between 2 continuous > covariates? > for example: lm(y~x1+x2+x1:x2) > Should one of continuous x1, x2 be "transformed" to a categorical variable, > i.e. be classified into groups? > Is it easier to interpret the effect if 1 or both are centered to the mean or > z-transformed? Depends on the context, but usually not. -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ 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.
