Posted By:
Eric BrehmJust in case anyone is interested in the precise details, the Set Rating is not quite equal to the Grade Point Average (GPA) multiplied by the Percent Complete, as stated above, although it is approximately equal to that in some cases.
The calculations that PSA actually does are as follows:
-- GPA = Total Grade Points / Total Weight (your set)
-- Set Rating = Total Grade Points / Total Weight (entire set)
where.. Total Grade Points = sum of the Grade Points for all cards you have in the set
and..... Total Weight (your set) = sum of the Card Weights for all cards you have in the set
and..... Total Weight (entire set) = sum of the Card Weights for all cards in the entire set
The Grade Points you get for each card in your set is the numerical Grade of the card multiplied by the Card Weight assigned to that card by PSA. According to PSA, the Card Weights that are assigned within each set are based on the relative values of the cards in NM-MT (PSA 8) condition. So, GPA and Set Rating are weighted averages of card quality within your set, with the weights being in proportion to the relative difficulty of obtaining each card in high grade. Set Rating also takes your degree of set completion into account, since it divides by the Total Weight for the entire set, which by the way is referred to as "the divisor for this set for weighting" by PSA.
If you want to calculate Set Rating as a function of GPA, the following formula would apply:
-- Set Rating = GPA * (Total Weight (your set) / Total Weight (entire set))
Thus, if you have every card in the set, Set Rating = GPA.
This is a reasonable, though certainly not perfect, system for ranking the quality of sets, and one could imagine numerous modifications that might improve it. When I was at the National Convention last year, I attended the PSA Set Registry luncheon, and during the Q&A session several people were complaining to Joe Orlando, President of PSA, about the set rating calculations. They wanted the weighting scheme to be modified in various ways, to better reflect relative values and so on. Too late now, I would think, better to leave it as is.