# [R] CIs in predict?

From: Guy Forrester <ForresterG_at_landcareresearch.co.nz>
Date: Tue 12 Jul 2005 - 08:40:44 EST

I am trying to put some Confidence intervals on some regressions from a linear model with no luck. I can extract the fitted values using 'predict', but am having difficulty in getting at the confidence intervals, or the standard errors.

Cheers

Guy

Using Version 2.1.0 (2005-04-18) on a PC

vol.mod3 <- lm(log.volume~log.area*lake,data=vol) summary(vol.mod3)

plot(c(1.3,2.5),c(-0.7,0.45),type="n",xlab="Log area",ylab="Log volume")

areapred.a <- seq(min(vol\$log.area[vol\$lake=="a"]), max(vol\$log.area[vol\$lake=="a"]), length=100) areapred.b <- seq(min(vol\$log.area[vol\$lake=="b"]), max(vol\$log.area[vol\$lake=="b"]), length=100)

preda <- predict(vol.mod3, data.frame(log.area=areapred.a,interval="confidence" ,lake=rep("a",100)))

1 2 3 4 5 6 7 8 9 -0.562577529 -0.553263576 -0.543949624 -0.534635671 -0.525321718 -0.516007765 -0.506693813 -0.497379860 -0.488065907

10 11 12 13 14 15 16 17 18 -0.478751955 -0.469438002 -0.460124049 -0.450810097 -0.441496144 -0.432182191 -0.422868239 -0.413554286 -0.404240333

19 20 21 22 23 24 25 26 27 -0.394926380 -0.385612428 -0.376298475 ETC ETC #As does this, but with no SEs
> preda <- predict(vol.mod3, data.frame(log.area=areapred.a,se.fit=T ,lake=rep("a",100)))
> preda

1 2 3 4 5 6 7 8 9 10 -0.562577529 -0.553263576 -0.543949624 -0.534635671 -0.525321718 -0.516007765 -0.506693813 -0.497379860 -0.488065907 -0.478751955

11 12 13 14 15 16 17 18 19 20 -0.469438002 -0.460124049 -0.450810097 ETC ETC

Guy J Forrester
Biometrician
Manaaki Whenua - Landcare Research
PO Box 69, Lincoln, New Zealand.
Tel. +64 3 325 6701 x3738
Fax +64 3 325 2418
E-mail ForresterG@LandcareResearch.co.nz www.LandcareResearch.co.nz
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