Relationship between reliability and validity
If data are valid, they must be reliable. If people receive very different scores on a test every time they take it, the test is not likely to predict anything. However, if a test is reliable, that does not mean that it is valid. For example, we can measure strength of grip very reliably, but that does not make it a valid measure of intelligence or even of mechanical ability. Reliability is a necessary, but not sufficient, condition for validity.