<P> External validity is the validity of generalized (causal) inferences in scientific research, usually based on experiments as experimental validity . In other words, it is the extent to which the results of a study can be generalized to other situations and to other people . Mathematical analysis of external validity concerns a determination of whether generalization across heterogeneous populations is feasible, and devising statistical and computational methods that produce valid generalizations . </P> <P> "A threat to external validity is an explanation of how you might be wrong in making a generalization ." Generally, generalizability is limited when the cause (i.e. the independent variable) depends on other factors; therefore, all threats to external validity interact with the independent variable - a so - called background factor x treatment interaction . </P>

Whether laboratory results are applicable in a real world setting is an issue of