Mining Pools on the other hand typically calculate their hash rate based on the number of shares turned in.  For example: https://www.btcguild.com/index.php?page=pool_stats and http://eligius.st/~gateway/stats/recent-blocks

Interestingly both of those pools have had an increase of their hash rate, and a drop in pool luck over the same time frame.



What if a major pool, or private miner (Ghash, CoinTerra, etc), mined with half of their hardware on a public pool but executed a withholding attack on the pool?  They would collect BTC rewards for the other blocks found by the public pool, but withhold all blocks found and transfer thier BTC rewards to their own pool.  This would make the public pool's luck look bad, and their own pool luck look good.  

They could even delay the block release until a time when their own pool luck was bad, or the public pool's luck was good, to prevent it from looking too obvious.  This delay would cause the entire combined network hash speed to appear much lower than it actually is, at least until they release the found blocks.  They would only need to release them before the difficulty increased when the blocks might become invalid.




Disclaimer: I have done no research beyond the links listed above, and this is pure speculation.