a 39 game can be divided by as many ways as your imagination allows...
i personally divide it two ways to get an angle...
1) i divide by 13, that gives three equal areas.
2) i divide by units...1 to 9
then 10 19
then 20 to 29 and 30 to 39...
granted one group is smaller than the others...but that doesnt really throw off the data too much.
the point is to see if the current average is different than the all time average...If the 15 day moving average all time is 15 hits...and the last 15 days have only 10 hits...then you expect this group to "make up some hits"
if you expect more hits than average from this group you would thus filter your wheel groups accordingly.
A+ software shows this automatically using the bar charts...when ever a "valley appears" that has gone below the 15 day average...that is when you expect the trend to start changing upward for that group.
I hope this helps.