To test the effectiveness of a new growth hormone, a researcher selects a sample of n = 20 newborn rats. ten of the rats receive regular injections of the hormone and the other 10 serve as a no-treatment control group. except for the hormone injections, the two groups receivve exactly the same treatment. after 6 months, the researcher weighs each rat and computes the average weight for each group. if the average weight for the hormone group is greater than the average for the control group, can the researcher automatically conclude that the hormone has an effect? (hint: what other factor might explain why the two groups have different averages? )