"Any research can be affected by different kinds of factors which, while extraneous to the concerns of the research, can invalidate the findings" (Seliger & Shohamy 1989, 95).

Let's take a look on the the most frequent uses of validity in the scientific method:

External Validity

External validity [3] is about generalization: To what extent can an effect in research, be generalized [4] to populations, settings, treatment variables, and measurement variables?

External validity is usually split into two distinct types, population validity [5] and ecological validity [6] and they are both essential elements in judging the strength of an experimental design [7].

Internal Validity

Internal validity [8] is a measure which ensures that a researcher's experiment design closely follows the principle of cause and effect [9].

“Could there be an alternative cause, or causes, that explain my observations and results?”

Test Validity

Test validity [10] is an indicator of how much meaning can be placed upon a set of test results.

Criterion Validity

Criterion Validity [11] assesses whether a test reflects a certain set of abilities.

• Concurrent validity [12] measures the test against a benchmark test and high correlation indicates that the test has strong criterion validity.

• Predictive validity [13] is a measure of how well a test predicts abilities. It involves testing a group of subjects for a certain construct and then comparing them with results obtained at some point in the future.

Content Validity

Content validity [14] is the estimate of how much a measure represents every single element of a construct.

Construct Validity

Construct validity [15] defines how well a test or experiment measures up to its claims. A test designed to measure depression must only measure that particular construct, not closely related ideals such as anxiety or stress.

• Convergent validity [16] tests that constructs that are expected to be related are, in fact, related.

• Discriminant validity [16] tests that constructs that should have no relationship do, in fact, not have any relationship. (also referred to as divergent validity)

Face Validity

Face validity [17] is a measure of how representative a research project is ‘at face value,' and whether it appears to be a good project.