When deciding how to measure things, such as “cognitive ability,” researchers can either use assessments that have already been developed and tested by others, or they can create their own measures. If they use an assessment that has already been developed and tested, they simply cite the fact that it is widely-used and has been shown to measure what it really claims to be measuring (this is called validity). If researchers create their own measure, they need to prove that their instrument actually measures what they say it does. Frequently they do this by comparing their measure to others that are already proven to be valid to show that the two measures are capturing similar phenomena.