Question

what should researchers do to the research setting to maximum external validity?

Answer 1

External validity is the extent to which a test results can be generalized to a larger population. This will become very difficult and impossible if the researcher doesn't take care of certain elements of the research.
Firstly, the population size should be large enough before jumping into conclusions.
Secondly, the research should take care of the cultural issues of various cultures.
Thirdly, the research should be conducted in the natural setting an not in a laboratory.
Finally, the researcher should maintain objectivity.