I haven't got any studies, but I am a monetary economist. Recently what's happened is that the prime has fallen because of the fall in economic activity, but the risk in the credit card business has also gone up. You would not expect to have a one-to-one correlation between the fall in prime and the fall in the interest rate charged on credit cards. It's more expensive now, given the high risk among those cards. That has to be taken into account.
When you have no change in the risk, you would expect closer correlation, but when they move in opposite directions, you wouldn't expect them to move together.