Board statistics as of right now:

Board Visible Pruned    Rate
----- ------- ------  ------
a        1674  27951  6.8180
m        1330   2351  1.3931
cats     1295    148  0.7135
w        1172    594  0.9881
big       947    173  0.7419
o         881     48  0.7589
ma        765   1993  0.5615
j         664    199  0.5889
cs        541    349  0.3954
z         481   4616  0.5860
prog      282      2  0.3726
dis       195     22  0.2237
p         154      9  0.2026
f         122   2474  1.4138

• "Visible" is the number of posts currently existing on the board.
• "Pruned" is the total of posts not on the board, calculated as the largest post ID minus the number of visible posts. (This might be one off, come to think of it.)
• "Rate" is the average posts per day of existing posts, i.e. total visible posts divided by the timespan between oldest and newest.

You need to change the way you calculate rates, it doesn't seem quite right, maybe break it into past week/month/(half?)year.

Wow, that was some complicated SQL. I think this is only the boards that have had activity over the past 30 days, it didn't give any data for some fields.

Board  7 days 30 days
----- ------- -------
a     36.5975 37.9541
big    5.6850  1.6333
cats           0.5308
dis    0.3159  0.3712
f      1.6373  0.5962
m      1.6769  2.2300
ma     1.0307  0.1758
p              0.2736
w      2.2696  1.9325
z      2.0427  1.9255

For the record, that was:

select t7.boardname as "Board", t7.rate as "7 days", t30.rate as "30 days"       --*-sql-*-
from (
    select boardname, count(postid) / ((max(posttime) - min(posttime)) / 60 / 60 / 24) rate
    from posts
    where posttime > unix_timestamp() - (60 * 60 * 24 * 7)
    group by boardname
) as t7, (
    select boardname, count(postid) / ((max(posttime) - min(posttime)) / 60 / 60 / 24) rate
    from posts
    where posttime > unix_timestamp() - (60 * 60 * 24 * 30)
    group by boardname
) as t30
where t7.boardname = t30.boardname
order by t7.boardname;