Validity in psychology refers to the extent to which a test or measurement tool accurately measures what it is intended to measure. It ensures that the research findings are genuine and not due to extraneous factors. Validity can be categorized into different types based on internal and external validity. The concept of validity was formulated by Kelly (1927), who stated that a test is valid if it measures what it claims to measure.

There are different types of validity in psychology, including:

• Content Validity: This refers to the extent to which a test or measurement represents all aspects of the intended content domain. It assesses whether the test items adequately cover the topic or concept. For example, a 4th-grade math test would have high content validity if it covered all the skills taught in that grade.

• Criterion Validity: This assesses the performance of a test based on its correlation with a known external criterion or outcome. It can be further divided into concurrent (measured at the same time as the criterion) and predictive (measured before the criterion) validity. For example, a test of intelligence should predict academic performance.

• Construct Validity: This asks whether a measure successfully measures the concept it is supposed to. It is concerned with the extent to which a test measures an abstract concept or construct. For example, a questionnaire should measure depression and not anxiety.

• Face Validity: This is a measure of whether it looks subjectively promising that a tool measures what its supposed to. It refers to the extent to which a test appears to measure what it is supposed to measure. For example, a test of intelligence should look like it measures intelligence.

• Internal Validity: This is a measure of whether results obtained are solely affected by changes in the variable being manipulated (i.e., by the independent variable) in a cause-and-effect relationship. It can be assessed based on whether extraneous (i.e., unwanted) variables that could also affect results are successfully controlled or eliminated.

• External Validity: This refers to the extent to which the results of a study can be generalized to other settings (ecological validity), other people (population validity), and over time (historical validity) . It can be improved by setting experiments more naturally and using random sampling to select participants.

Evaluating validity is crucial because it helps establish which tests to use and which to avoid. If researchers use the wrong instruments, their results can be meaningless.