The best research designs are those that can assure high levels of internal and external
validity. Such designs would guard against spurious correlations, inspire greater faith in the
hypotheses testing, and ensure that the results drawn from a small sample are generalizable to
the population at large. Controls are required to assure internal validity (causality) of research
designs, and can be accomplished in four ways: (1) manipulation, (2) elimination, (3) inclusion,
and (4) statistical control, and (5) randomization.