In a study, validity is the degree to which the conclusions that the researchers make can be considered to be ‘true’, based on how well the study was designed and how well the study matched ‘real life’ situations.
External validity is the extent to which the cause‐and‐effect relationships in a study are true for a wider population beyond the study. For example, the external validity of the study may be questioned if the population is people in Australia and the study was in Spain, or for old people if the study was in young people.
Internal validity is the extent to which the cause‐and‐effect relationships in a study are true for the people and conditions of the study. HTAI