Correlated independence

Several opinions can agree because they share a source, a method, or a paycheck, so they feel like proof when they are really one opinion repeated.

There is a crack running up the basement wall, and you did the responsible thing. You got three quotes. All three foundation companies came out, all three said piers, all three landed near $27,000. Three strangers agreeing feels like the end of the question. You sign. Later you learn all three came off the same contractor-matching site, all three use the same inspection script, and all three earn nothing if the answer is a $900 drainage fix.

Correlated independence is when checks that look separate share a source, a method, an incentive, or a blind spot, so they fail together. The tell is agreement that arrives too easily, in the same words and the same reasoning, from people you found the same way. When you notice it, ask three questions of each check: where did the information come from, what method was used, and how is this person paid. Then buy one check that differs on all three. Here that is a structural engineer on a flat fee who sells no repairs. What changes is that you learn something new instead of hearing one claim louder. Unnoticed, the cost is high confidence aimed at the wrong repair. It is a false alarm when the checks agree because the answer is genuinely obvious, and hunting for a truly separate opinion there only burns time and money.