Hard to find anyone more tested last year.
Simpson 2
Solomon 3
Jager 4
Centrowitz 6
Lomong 7
Rowbury 7
Manzano 8
Symmonds 9
Hall 9
Cain 9
Lagat 10
Ritzenhein 11
Keflezighi 12
Rupp 15
Massive outlier, any way you slice it.
Don't say he races more than all of these guys, because (1) he doesn't and (2) highly-tested Ritzhein barely raced in 2014.
USADA has a history of "turning up the heat" where there is suspicion. Is there suspicion around Rupp? Overall the NOP group seems to be highly tested compared with other US elites.
Other quick takes: Marathoners are very highly tested. Being out for a season doesn't mean less testing.