Internal and external validity are two important concepts in research design that help determine the trustworthiness and generalizability of study results.

Internal validity refers to the degree to which a studys results reflect the true cause-and-effect relationships within the study, without bias. It is concerned with control and is largely determined by the studys experimental design and methods. In other words, internal validity examines whether the study design, conduct, and analysis answer the research questions without bias.

External validity, on the other hand, refers to the extent to which the results of a research study can be applied or generalized to other contexts, situations, people, stimuli, and times. It is the validity of applying the conclusions of a scientific study outside the context of that study. In other words, it is the extent to which the results of a study can be generalized to and across other situations, people, stimuli, and times.

It is important to note that a studys external validity is limited by its internal validity. If a causal inference made within a study is invalid, then generalizations of that inference to other contexts will also be invalid. In many studies and research designs, there may be a trade-off between internal validity and external validity: attempts to increase internal validity may also limit the generalizability of the findings, and vice versa.

Threats to internal validity include history, maturation, instrumentation, testing, selection bias, regression to the mean, social interaction, and attrition.

In summary, internal validity examines whether the study design, conduct, and analysis answer the research questions without bias, while external validity examines whether the results of a research study can be applied or generalized to other contexts.