-- W. Edwards Deming, Statistical Adjustment of Data, (Wiley, 1943), p. 30.
Only a series of experiments under a wide range of conditions will establish whether a relationship is robust and holds in a variety of circumstances. Alas, no one wants to publish papers that say, "Yeah, I found the same relationship, too." Or to re-run the same experiment under different conditions. Feynman was once critical of an experiment using heavy hydrogen because the comparison was made to another, published experiment using light hydrogen on another apparatus. The comparison should have been made using light hydrogen on the same apparatus as the heavy hydrogen; but time was not made available because "we already know the answer for light hydrogen."
Feynman knew quite well that the same experiment might give different results when run on different apparatus. And if this is an issue for physics, how much more so for social "science"?