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You seem to have thought about this.On some balls, the difference is more than 0.5 psi. Still, a small amount. Such a small amount, in my opinion, that it is unlikely that they were tampered with. But far from proof that they were not tampered with.
What's the Standard Deviation around the expected pressure loss under comparable atmospheric conditions?
For example, if a football is inflated to 12.0 PSI in a room where the temperature is 73 degrees Fahrenheit what would be the Mean expected PSI if the ball were outside in a rainy atmosphere with an average temperature of 42 degrees Fahrenheit for 90 minutes?
Let's say, for the sake of discussion, that the expected PSI for that ball is 10.5.
Now let's assume that the SD is 0.1 psi (I am completely making that up). Then, if a ball weighed in at 10.0 PSI instead of 10.5 (0.5 PSI below the expected Mean), it would be Five Standard Deviations below the mean. That would be material. You'd be at five sigma, where fewer than 1 in a thousand balls would be expected to test that low.
However, if the SD were 0.25, then the ball would fall within two standard deviations of the mean, or within the expected range for 67% of balls tested (one in three balls would be expected to test outside the range).
While all of that information would be useful, it would, unfortunately, not be helpful in this case since the pressure was not recorded before the game.