How to select avg(select max(something) from ...)

[dfgpostgres <dfgpostgres3@xxxxxxxxx>]

Hi:

psql 15.3

I have a table with sets of observations, each set sharing an id.

I want to get the average of the max of each set.

id  |  val

-----------

1     5.0

1     4.3

1     3.8

2     4.8

2     6.0

2     2.9

3     4.1

3     4.4

3     8.0

So I want the avg of the max of the set where id=1 (5.0), where id=2 (6.0), where id=3 (8.0) ~= 6.33...

I tried this...

              select
                 avg(x.maxsz)
               from
                 dvm.dvm_events d,
                 (select cast(max(size_g) as int) as maxsz 

                   from dvm.wa_du_profile_data 

                   where dvm_id=d.dvm_id) x
               where

                  d.project='foo' and  
                 <more conditions on d>

It doesn't like that reference to "d.dvm_id) in that subquery.