who is being measured
Next, you need to ask who is being measured? Selection bias is a real problem when it comes to measuring dicks. If you’re selecting men at random to submit to having their dicks measured you can bet that the only ones likely to step-up are men who are comfortable with their size. So the results from that group are likely to be skewed towards a larger average. Conversely, if your study group is a bunch of guys who are signing up for dick enlargement surgery, you can expect the average to be skewed in the other direction.
Then you have to ask where the study is being conducted. Different populations may have different average sizes. You’ve probably heard rumors about African men being proportionately large and East Asian men being proportionately small. These rumors seem to have some basis in fact, although the extent of the difference appears to be exaggerated.