You withdrew some money from an ATM last night, right? Thinking it was from your salary? Actually that was drug money. The bank isn't useless, because it keeps

You withdrew some money from an ATM last night, right? Thinking it was from your salary? Actually that was drug money. The bank isn't useless, because it keeps track of how much money it receives and hands out, and from whom. So even if you got a note that has been in some drug smuggler's hands, the bank isn't useless. The researchers do much the same, except with less precision. The bank has full insight into who deposits, transfers and withdraws, the researchers only partial insight into the income and spending of the research subjects. But they can estimate how good their insight is and take steps to improve it. Their results are as precise as their insight is complete. Their results won't be as precise as your bank statement, but they can be precise enough to be useful. After all, the researches can set the terms their experiment so as to get enough data.