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The change is usually to a different address, and it could be more or less than the amount sent to the recipient. It seems like a crude estimate would be to just randomly remove one output (assuming num outputs >= 2), and hope it worked out on average but I was curious if anyone had better ideas. Thanks!
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The satoshi client always sends change to an address that has never been used before.
Often the amount being sent has less significant decimal digits than the change. If you see 2 outputs, one for 1.235 and one for 1.19403995 then the change is probably the 2nd one.
The satoshi client will try to minimise the change. Usually the change will be relatively small, and is often smaller than any of the inputs.
Interesting, thanks Chris. The comment about significant digits is a great idea. Do you think (assuming significant digits the same) selecting the smaller output as the change would cause your estimate to be too high on average? – Brian Armstrong – 2012-07-20T05:41:06.493
Probably, yes. Sometimes you see a chain of transactions where a single very large input is used to make a series of much smaller payments, with the change being very big each time. – Chris Moore – 2012-07-20T05:59:26.247