[R] Extrapolated regression lines

[Gavin Simpson]

On Thu, 2006-10-12 at 15:41 +0200, Johan A. Stenberg wrote:
> Dear list members,
> 
> When I create a simple scatterplot with a regression line (se below) the 
> line is automatically extrapolated outside the range of data points. Why 
> is this and how can I prevent R from extrapolating the regression line?
> 
> Thank you in advance,
> Johan
> 
> model<-lm(Herb~Para)
> plot(Para,Herb)
> abline(model)

Hi Johan,

Simply predict over the range of your predictor (Para). As I don't have
those data, the example below uses dummy data

X <- rnorm(500) # dummy data
Y <- X + rnorm(500)
mod <- lm(Y ~ X) # fit model
# regular sequence over range of predictor X
newdat <- seq(min(X), max(X), length = 100)
# predict Y for each of these new data points
pred <- predict(mod, newdata = list(X = newdat))
# plot the results
plot(Y ~ X)
lines(pred ~ newdat, col = "red")

HTH

G
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