It's an easy enough experiment to do. Just get a few hundred raw cards, randomly assign each of them to SGC or PSA, wait for them to come back, enter each of the individual grades, compare the SGC mean to the PSA mean (indepedent samples t-test), voila, you have your answer.
In practice it may reliably go in one direction for some issues and the other way for others, but I'd be interested in the main effect (i.e., who is more lenient on average).
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