How come a reliable test can be invalid, but an unreliable test cannot be valid?
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A test is reliable if it gets the same value over and over. A test is valid if it is truly measuring what the researcher thinks it is measuring.
A test can be reliable but not valid. Let's say I wanted to measure how smart people are by measuring their heads. I'd get the same value (in inches or centimeters or whatever) every time I measured their head so the test would be reliable. But I wouldn't be actually measuring their intelligence so the test wouldn't be valid.
A test cannot be valid if it's not reliable. If the test is not reliable, that means it gives different results every time I do it. If it keeps giving different results, it cannot possibly be measuring what I think it is. Let's say I give a person multiple tests to measure intelligence and they get wildly different results the tests. Clearly, the test is not really measuring intelligence because if it truly measured intelligence it would have to yield results that were nearly the same every time (because we assume a person's intelligence doesn't change from moment to moment).
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