I read the quoted sentence as "25 of the 52" -- not an additional 25 of the 87 -- because of the word "these". 52 pairs similar schooling 25 of these had very

I read the quoted sentence as "25 of the 52" -- not an additional 25 of the 87 -- because of the word "these". 52 pairs similar schooling 25 of these had very similar schooling Thus the math becomes "87-52=35". 35 data points. Not spectacularly high regardless.