average/stddev on all values returned by a select distinct

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)