An essential concept in experimental design, validity directly relates to the soundness of research. Validity refers to the degree to which a research design measures what it intends to. A good study will always attempt to maximize validity, both internal to the study and external, according to the National Center for Technology Innovation.

Internal Validity

Internal validity refers to the validity of the study itself. In a study that has high internal validity, the outcomes can confidently be said to directly result from the study's manipulation. Problems related to selection, bias, maturation and attrition, among others, can negatively affect a design's internal validity, according to clinical psychologist Steven Taylor and clinical researcher Gordon J. G. Asmundson in their chapter in the “Handbook of Research Methods in Abnormal and Clinical Psychology.”

External Validity

External validity refers to how a study's results can be generalized to a larger population. In this case, validity is determined in part by whether a study's outcomes can be replicated in and across other samples, times and settings. A study with high external validity can therefore be repeated in multiple contexts with similar outcomes.