Help with predict.lm

Will this help?
http://finzi.psych.upenn.edu/R/Rhelp02a/archive/1202.html

[Found by RSiteSearch("calibration") in R-2.1.0.]

Andy
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

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Andy
Thanks, the link was very helpful.  I havn't checked the code for the calib
function but it appears to work with my data.
[NB the return function generates a warning message with later verions of R
and needs to be amended to return a list]

In the last line of my code, calibration.lm should have been lm.calibration,
but it still does not work with predict.lm
I presume the solution of reversing the variables as in the linear model to
force predict.lm to work is an invalid statistical method as the regression
line will be slightly different, i.e.

    predict(lm(conc ~ abs), new.abs) # gives slightly different results than
the calib function

Thanks to all who replied,
Mike White

----- Original Message -----
From: "Liaw, Andy" <andy_liaw at merck.com>
To: "'Mike White'" <mikewhite.diu at tiscali.co.uk>; <R-help at stat.math.ethz.ch>
Sent: Tuesday, April 19, 2005 1:53 PM
Subject: RE: [R] Help with predict.lm
Will this help?
http://finzi.psych.upenn.edu/R/Rhelp02a/archive/1202.html

[Found by RSiteSearch("calibration") in R-2.1.0.]

Andy

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

______________________________________________
R-help at 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

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