Validity is one of those concepts that can really tie you up in knots.  The more you think about it the more difficult it can become.  If you know that it simply refers to whether a study measures what it claims to measure you are just about there.

If you read on you will find definitions of some of the other types of validity that are often discussed, but don’t worry if you find it a bit confusing.  Most psychologists and text books don’t always agree about this concept and they often use different terms and definitions.

We can evaluate a study for its validity in a number of ways.  However, it is possible to divide validity into two types.  

1. We can evaluate the validity of the measuring tool (e.g. psychometric test, questionnaire etc)
    

2.  We can also evaluate the validity of the procedure of a study.

1. There are three main ways of assessing the validity of a measuring tool.

(a) Face validity refers to the extent to which a measure appears on the surface to measure what it is suppose to measure.   Face validity (sometimes called surface validity) is probably the most commonly discussed type of validity.

(b) Criterion validity is a way of assessing validity by comparing the results with another measure.  For example, we could compare the results of an IQ test with school results.   If the other measure is roughly compared at the same time we call this concurrent validity.  If the other measure is compared at a much later time we call this predictive validity.

(c) Construct validity is a way of assessing validity by investigating if the measure really is measuring the theoretical construct it is suppose to be.  For example, many theories of intelligence see intelligence as comprising a number of different skills and therefore to have construct validity an IQ test would have to test these different skills.
 

2.  There are two main ways of assessing the validity of a procedure.

(a) Internal validity is related to what actually happens in a study.  In terms of an experiment it refers to whether the independent variable really has had an effect on the dependent variable or whether the dependent variable was caused by some other confounding variable. 

(b) External validity refers to whether the findings of a study really can be generalised beyond the present study.  We can break external validity down into two types.

Population validity - which refers to the extent to which the findings can be generalised to other populations of people.

Ecological validity - which refers to the extent to which the findings can be generalised beyond the present situation.