[R] best way to calculate per-parameter differences in across-subject means

[René J.V. Bertin]

Hello,

I have some data in a data.frame where for each of a number of
subjects, I have scores for all of a number of symptoms.

Subjects are subdivided in a number of groups, which have unequal sizes.

I'd like to plot between-group differences in the scores on the
various symptoms. Ideally, that would be in a form as would be
produced by

> bwplot( Score~Symptom )

but I'm not sure one can say anything about the distribution of
differences when the sample sizes differ as much as they do.

So I will start plotting the per-group differences in the per-symptom
mean scores.

Is there a better way (rather than using loops) to get a table of
those per-symptom means, something like

> with( subset(dat, group==1, drop=FALSE), Score~Symptom )

Thanks in advance,
René Bertin