average/stddev on all values returned by a select distinct

[ann hedley <ann.hedley@xxxxxxxx>]

I have this query which returns the length of the longest consensus for 
each id.

lumbribase=# select distinct on (id) length(consensus) from cluster 
order by id,length(consensus) desc;
length
--------
   647
   680
   273
   384
  1285
<snip>

What I want is the average and stddev of the set of lengths returned by 
this query.  Something like...

select average(select distinct on (id) length(consensus) from cluster 
order by id,length(consensus) desc);

Could someone tell me how it's done?

Thanks

--
Ann

"In a world without walls and fences - who needs Windows and Gates ?"
                                                          (unknown)