LungCap Age Height  Smoke   Gender  Caesarean
6.475   6   62.1    no      male    no
10.125  18  74.7    yes     female  no
9.55    16  69.7    no      female  yes
11.125  14  71      no      male    no
4.8      5  56.9    no      male    no
6.225   11  58.7    no      female  no

> attach(LungCap6)
> tapply(Age, Smoke, mean, T)
Error in mean.default(X[[i]], ...) : 'trim' must be numeric of length one
> length(Age)
[1] 6
> tapply(X=Age, INDEX=Smoke, FUN=mean, na.rm=T)
  no  yes
10.4 18.0
> tapply(Age, Smoke, mean, T)
Error in mean.default(X[[i]], ...) : 'trim' must be numeric of length one
>