How to subset a range of values in lm()

Question

The help file for lm() doesn't go into the syntax for the subset argument. I am not sure how to get it to find the line of best fit for only a portion of my data set. This question is similar, but I wasn't able to solve my particular problem using it. How does the subset argument work in the lm() function?

Here is my code:

    with(dat[dat$SIZE <7 & dat$SIZE > 0.8 ,], plot(SP.RICH~SIZE, log="x",
      xlim=c(1,9), ylim=c(60,180), ylab="plant species richness", 
      xlab="log area (ha)", type="n"))
   with(dat[dat$SIZE <7 & dat$SIZE > 0.8 ,], points(SP.RICH~SIZE, pch=20, cex=1))
   fit=lm(SP.RICH~SIZE, subset=c(1:7))

I would like to make sure that the regression line is drawn only for the values that I subset above in the plot() and points() commands.

Answer 1

The subset parameter in lm() and other model fitting functions takes as its argument a logical vector the length of the dataframe, evaluated in the environment of the dataframe. So, if I understand you correctly, I would use the following:

fit <- lm(SP.RICH~SIZE, data=dat, subset=(SIZE>0.8 & SIZE<7))

Answer 2

But the above solution does not help if you want to run one lm for each group in your data - lets say that you have different countries as a column and you want to understand the relationship between richness and size within each country.

For that I recommend following the help for the function by in R http://astrostatistics.psu.edu/su07/R/html/base/html/by.html:

require(stats)
attach(warpbreaks)
by(warpbreaks[, 1:2], tension, summary)
by(warpbreaks[, 1], list(wool = wool, tension = tension), summary)
by(warpbreaks, tension, function(x) lm(breaks ~ wool, data = x))

## now suppose we want to extract the coefficients by group
tmp <- by(warpbreaks, tension, function(x) lm(breaks ~ wool, data = x))
sapply(tmp, coef)

From the list tmp you can extract any lm parameters you like.