A researcher is comparing two antihypertensive drugs A and B with regards to reduction of patient's systolic blood pressure (SBP). Baseline SBP is measured for two groups of patients. One group is given Drug A and another group is given Drug B for six months. At the end of six months SBP was measured again for two groups of patients. The researcher compared the difference in SBP of patients with Drug A to difference in SBP of patients with Drug B. Which factors can potentially impact the power of the analysis?