From: Mike White
Hi
I have measured the UV absorbance (abs) of 10 solutions of a
substance at
known concentrations (conc) and have used a linear model to plot a
calibration graph with confidence limits. I now want to predict the
concentration of solutions with UV absorbance results given
in the new.abs
data.frame, however predict.lm only appears to work for new
"conc" variables
not new "abs" variables.
I have search the help files and did find a similar problem
in June 2000,
but unfortunately no solution was offered.
Any help and how to use predict.lm with the new "abs" data to
predict "conc"
with confidence limits would be appreciated.
conc<-seq(100, 280, 20) # mg/l
abs<-c(1.064, 1.177, 1.303, 1.414, 1.534, 1.642, 1.744,
1.852, 1.936,
2.046) # absorbance units
lm.calibration<-lm(abs ~ conc)
pred.w.plim <- predict(lm.calibration, interval="prediction")
pred.w.clim <- predict(lm.calibration, interval="confidence")
matplot(conc, cbind(pred.w.clim, pred.w.plim[,-1]),
lty=c(1,2,2,3,3), type="l", ylab="abs", xlab=
"conc mg/l")
points(conc, abs, pch=21, col="blue")
new.abs<-data.frame(abs=c(1.251, 1.324, 1.452))
predict(calibration.lm, new.abs) # does not work
Thanks
Mike White