How do reliability and validity differ? How do they complement each other? Can a measure be reliable ...

•   Reliability means dependability or consistency. It suggests that the same thing is repeated or recurs under the identical or very similar conditions. The opposite of reliability is a measurement that yields erratic, unstable, or inconsistent results.
•   Validity suggests truthfulness and refers to the match between a construct, or the way a researcher conceptualizes the idea in a conceptual definition, and a measure. It refers to how well an idea about reality “fits” with actual reality.
•   Reliability is necessary for validity and is easier to achieve than validity. Although reliability is necessary in order to have a valid measure of a concept, it does not guarantee that a measure will be valid. It is not a sufficient condition for validity. A measure can produce the same result over and over (i.e., it has reliability), but what it measures may not match the definition of the construct (i.e., validity).
•   A measure can be reliable but invalid. For example, you can get on a scale and get weighed. The weight registered by the scale is the same each time you get on and off. But then you go to another scale—an “official” one that measures true weight—and it says that your weight is twice as great. The first scale yielded reliable (i.e., dependable and consistent) results, but it did not give a valid measure of your weight.