Not going to claim they aren't skimming, but is it not possible to calculate expected number of blocks from declared GH/s, expected earnings from user provided MH/s and tell if the pool is excessively "unlucky"?
Yeah I thought of that too after posting. I think it comes down to how transparent the pool is with their data.
The obvious thing to do would be to tell everyone that my 500 GH/s pool is 400 GH/s, and reward everyone an 80% share on every hash. If you're sophisticated enough you might notice that my pool is mining blocks about 25% above what you'd expect, but how many data point do you need for that, and it's statistical, so I'll have runs of good or bad luck.
Another option is to dilute the pool of contributors, but again you might be able to detect that either I'm misreporting your hash rate, or the sum of all contributions doesn't line up.
Assuming I give you enough tools to figure any of this out.
If you report the pool's hash rate as lower, the users should demand a higher fair share, as they know their own hash rate.
Either way, if a user knows his hash rate, he can calculate expected earnings and their presumed share. You could fudge a bit, but go too much, especially on a large pool, and it will be apparent. There are probably lots of users doing calculations, willing to call you out.
err, right. If you want to convince people they're getting a fair share, you have to downplay their contributions, and since they know what they did you have to make the pool bigger, not smaller. How long would it take to notice someone was fluffing the pool by 10%?