## Pairing Off

One sample average is the loneliest number that you’ll ever do. Luckily, we’re usually concerned with two. We’re especially keen to compare averages for subjects in experimental treatment and control groups. We reference these averages with a compact notation, writing Y1 for Avgn[YilDi = 1] and Y0 for Avgn[YilDi = 0]. The treatment group mean, Y1, is the average for the n1 observations belonging to the treatment group, with Y° defined similarly. The total sample size is n = n0 + n1.

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