What’s the difference between validity and reliability?

Research in psychology needs to produce results that are both reliable and valid. In order to do so, also the methods (measures) used in the studies need to reliable and valid.

Reliability refers to how consistent (i.e. reproducible/repeatable) the data is. If a method is reliable, results will be consistent so the measure will produce the same results on different occasions.

Validity refers to the ability to measure what was set out to be measured (i.e. a measure on self-esteem should measure self-esteem and not be measuring happiness). A test is valid when it measures what it intends to measure.