# [R] CIs in predict?

Guy Forrester ForresterG at landcareresearch.co.nz
Tue Jul 12 00:40:44 CEST 2005

```Dear All,

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.

Any suggestions would be welcome

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)))

#This gives the fitted values as predicted, but no CIs
> preda
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 at LandcareResearch.co.nz
www.LandcareResearch.co.nz

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