Regression on subset of data set

Question

I\'d like to do the following and need some help:

Calculate slope and intercept for \"Height\" over \"Age\" [lm(Height~Age)] separately for

(A) each individ

Answer 1

One way to do regression analysis separately for each level and then combine slopes and intercepts in data frame, is to use function ddply() from library plyr.

library(plyr)

ddply(example,"Individual",function(x) coefficients(lm(Height~Age,x)))
  Individual (Intercept)      Age
1       Jack    26.29188 11.11421
2        Jen    22.10660 11.56345
3       Jill    18.33249 12.04315
4       Tony    33.02030 11.96447

ddply(example,"Gender",function(x) coefficients(lm(Height~Age,x)))
  Gender (Intercept)      Age
1      F    20.21954 11.80330
2      M    29.65609 11.53934