thinking...

say you kept a running count of total minutes and also the average so at any point you can say:

43,343 minutes of statstics with the current average online as 8.4 users.

Now, if you have the next stat of 3 minutes at 20 users then you can do:

((43,343 x 8.4) + (3 x 20) )/ 43,346

and that should give you back the new average and new total, so the process just continues. Make any sense? server intensive? would require two queries me thinks for every loading of forumhome, once to get the stat and second to update it. Wondering if it would perhaps be better to have a cron script run every 5 minutes say that does the updating and then just pull the results (which you might be able to do without another query if you can on your first bit) and that does it?