One of the most important factors impacting external validit

One of the most important factors impacting external validity is:

Manipulation of the IV

The sampling strategy used by the researcher

Random assignment

The use of inferential statistics

Solution

The sampling strategy used by the researcher is an extremely important factor impacting external validity, since that determines whether the sample obtained is truly random or not. A random sample is necessary for external validity, as it ensures that the results are generalisable. If the sample is hand picked, or such that it favours an outcome, the research cannot be generalised.