Based on some recent r-help discussions, I have been trying out plotting confidence intervals using predict and matplot. Matplot appeared to not be plotting the linear regression when using the default column names generated by read.table (V1, V2, etc). On further inspection, the error seemed to be with predict and vector names (V1, V2) rather than with matplot. I was using some textbook data sets that were read with read.table, so my data.frame columns were named by default. The problem seems to be related to the name of the vector only (though I may be wrong).
The example below, based on that in ?predict.lm, better describes the problem. Using predict with V2~V1 & y~x produces identical output (when x<-V1 and y<-V2). However, using predict with interval="confidence" results in different output from the same data. That with y~x is correct, and V2~V1 is "incorrect". This may be related to a previous post on r-help: http://finzi.psych.upenn.edu/R/Rhelp02a/archive/56982.html I can't figure out why there would be a difference in predict when only the vector name seemingly changes. Granted, I could just read the data.frame is as "x" and "y." Thanks Kevin ###################### set.seed(10) # For example purposes, plot side by side par(mfrow=c(1,2)) V1 <- rnorm(15) V2 <- V1 + rnorm(15) new <- data.frame(x = seq(min(V1), max(V1), length.out = length(V1))) pred.w.clim <- predict(lm(V2 ~ V1), new, interval="confidence") matplot(new$x, pred.w.clim, lty=c(1,2,2), type="l", col=c("black", "red", "red"), ylab="predicted y") points(V1,V2) # Create x & y equal to V1 & V2 x <- V1 y <- V2 pred.w.clim2 <- predict(lm(y ~ x), new, interval="confidence") matplot(new$x, pred.w.clim2, lty=c(1,2,2), type="l", col=c("black", "red", "red"), ylab="predicted y") points(x,y) # Test if V1=x, V2=y all.equal(x,V1) all.equal(y,V2) # Same output with predict predict(lm(V2 ~ V1)) predict(lm(y ~ x)) all.equal(predict(lm(V2 ~ V1)), predict(lm(y ~ x))) # Different output with interval="confidence" pred.w.clim pred.w.clim2 all.equal(pred.w.clim, pred.w.clim2) ______________________________________________ R-help@stat.math.ethz.ch 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.
