[R] Help with predict.lm

[TEMPL Matthias]

> 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

lm.calibration

predict(lm.calibration, new.abs)

Is this the reason for not working?

Best,
Matthias

> 
> 
> Thanks
> Mike White
> 
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