To gain a high level of internal validity, a researcher must eliminate or
minimize confounding variables. To accomplish this, a study must be tightly
controlled so that no extraneous variables can infl uence the results. However,
controlling a study may create a research environment that is so artifi cial and
unnatural that results obtained within the study may not occur in the outside
world. Thus, attempts to increase internal validity can reduce external validity.
In general, the results from a tightly controlled research study should be
interpreted as demonstrating what can happen but not necessarily what will
happen in an outside environment where other variables are free to operate.
On the other hand, research that attempts to gain a high level of external
validity often creates a research environment that closely resembles the
outside world. The risk in this type of research comes from the fact that the
real world is often a chaotic jumble of uncontrolled variables, especially in
comparison with the highly regulated environment of a controlled study.
Thus, striving for increased external validity can allow extraneous variables
(potentially confounding variables) into a study and thereby threaten internal
validity.
In very general terms, there tends to be a trade-off between internal and
external validity. Research that is very strong with respect to one kind of
validity often tends to be relatively weak with respect to the second type.
This basic relationship must be considered in planning a research study or
evaluating someone else’s work. Usually the purpose or goals of a study help
you decide which type of validity is more important and which threats must
be addressed.