Concussions and brain size. What is the effect of concussions on the brain? Researchers measured the brain sizes (hippocampal volume in microliters) of 25 collegiate football players with a history of clinician-diagnosed concussion and 25 collegiate football players without a history of concussion. Here are the summary statistics:

(a) Is there evidence of a difference in mean brain size between football players with a history of concussion and those without concussions?

(b) The researchers in this study stated that participants were consecutive cases of healthy National Collegiate Athletic Association Football Bowl Subdivision Division I football athletes with (n = 25) or without (n = 25) a history of clinician-diagnosed concussion ... between June 2011 and August 2013 at a U.S. psychiatric research institute specializing in neuroimaging among collegiate football players. What effect does this information have on your conclusions in part (a)?