N reachable alters 3 steps: TFN
A step refers to the length of a path from i to j. If i nominates j and j nominates k and k nominates l, then i and l are three steps apart. The value of the variable REACH3 is the total number of alters ego can reach in three steps.
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Taken from: National Longitudinal Study of Adolescent Health (Add Health), 1994-2008: Constructed Variables [Restricted Use] - School Network Variables Data.