"avg" function for arrays?

I know that there's an "average" function (avg) for some datatypes.
Is there something comparable for float or int arrays?

e.g.
select avg(time_instants[1:5]) from ellipse_proc where rep = 1;
time_instants
-----------------------------------------
{"148","167.8","187.6","207.4","227.2"}
(1 row)

select avg(time_instants[1:5]) from ellipse_proc where rep = 1;
ERROR: Unable to select an aggregate function avg(_float4)

Thanks.
-Tony