byMean

<code>numeric</code> vector to be averaged

(list of) <code>factor(s)</code> for which the mean
  will be calculated

<code>logical</code> If <code>TRUE</code> <code>NA</code>
  values will be removed

na.rm

<code>numeric</code> with of same length as
  <code>x</code>

<code>byMean</code> works like a very fast version of tapply
  with (weighted) <code>FUN=mean</code> or
  <code>FUN=weighted.mean</code>.

Basic (statistical) functionality for package ff