Re: [R] Regression lines for differently-sized groups on the same plot

From: Laura M Marx <marxlau1_at_msu.edu>
Date: Thu 21 Jul 2005 - 03:58:37 EST

Sundar Dorai-Raj writes:

> Hi, Laura,
>
> Would ?predict.glm be better?
>
> plot(logarea, hempresence,
> xlab = "Surface area of log (m2)",
> ylab="Probability of hemlock seedling presence",
> type="n", font.lab=2, cex.lab=1.5, axes=TRUE)
> lines(logarea, predict(hemhem, logreg, "response"), lty=1, lwd=2)
> lines(logarea, predict(hemyb, logreg, "response"), lty="dashed", lwd=2)
> lines(logarea, predict(hemsm, logreg, "response"), lty="dotted", lwd=2)
>
> Without seeing more description of your data, this is still a guess.
>
> --sundar
>

YES! Thank you. That solves all of the problems I was having. Thanks also to Tom Mulholland and Bill Venables for their replies.

     Laura
The final code is now:

logreg <- read.table("C:/Documents and Settings/Laura/Desktop/logreg.txt", header=TRUE, sep=",", na.strings="NA", dec=".", strip.white=TRUE)

#This is the full dataset.  A hemlock row followed by a birch row might look 
#like:
#Hemybsm     logarea    hempresence
#1           0.054      0
#2           1.370      1 

hemhem=glm(hempresence~logarea, family=binomial(logit), subset=Hemybsm<2) #This pulls out only the hemlocks from the full dataset and calculates the #regression.
hemyb=glm(hempresence~logarea, family=binomial(logit), subset=Hemybsm==2) #Same code, with only the birches from the full dataset. hemsm=glm(hempresence~logarea, family=binomial(logit), subset=Hemybsm>2)

plot(logarea, hempresence, xlab = "Surface area of log (m2)",

    ylab="Probability of hemlock seedling presence",     type="n", font.lab=2, cex.lab=1.5, axes=TRUE)

lines(logarea, predict(hemhem, logreg, "response"), lty=1, lwd=2)
lines(logarea, predict(hemyb, logreg, "response"), lty="dashed", lwd=2)
lines(logarea, predict(hemsm, logreg, "response"), lty="dotted", lwd=2)

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