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>.

Extends the out of memory vectors of 'ff' with
statistical functions and other utilities to ease their usage.