These two terms, reliability and validity, are often usedinterchangeably when they are not related to statistics. When critical readersof statistics use these terms, however, they refer to different properties ofthe statistical or experimental method.
Reliability is another term for consistency. If one person takes the samepersonality test several times and always receives the same results, the test isreliable.
A test is valid if it measures what it is supposed to measure. If theresults of the personality test claimed that a very shy person was in factoutgoing, the test would be invalid.
Reliability and validity are independent of each other. A measurement maybe valid but not reliable, or reliable but not valid. Suppose your bathroomscale was reset to read 10 pound lighter. The weight it reads will be reliable(the same every time you step on it) but will not be valid, since it is notreading your actual weight.