Calculating coinjars revisited

Last year I tinkered with figuring out how much money was in a coin jar by weight. The numbers I came up with were reasonably close – within 10-15%.
Some folks have come up with an online coin calculator that does something very smart. You weigh the total pile, then pick a handful of coins out of it, randomly. Count the number of coins in your hand. That’ll be a statistical sampling of the rest of the coins in the pile, and the page extrapolates based on that sample to the total weight.
Quite accurate, and pretty durned smart.
Thanks to BoingBoing for the link.