translate grouped data to their centroid
On 01/01/2013 19:50, Michael Friendly wrote:
Given a data set with a group factor, I want to translate the numeric variables to their centroid, by subtracting out the group means (adding back the grand means). The following gives what I want, but there must be an easier way using sweep or apply or some such.
Yes. It is part of the calculation of within-group covariances in LDA,
so (from MASS:::lda.default)
group.means <- tapply(x, list(rep(g, p), col(x)), mean)
x - group.means[g, ]
shows the idiom.
iris2 <- iris[,c(1,2,5)] means <- colMeans(iris2[,1:2]) pooled <- lm(cbind(Sepal.Length, Sepal.Width) ~ Species, data=iris2)$residuals pooled[,1] <- pooled[,1] + means[1] pooled[,2] <- pooled[,2] + means[2] pooled <- as.data.frame(pooled) pooled$Species <- iris2$Species
Brian D. Ripley, ripley at stats.ox.ac.uk 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