When trying to decide who to play as your flex, there are two basic factors to consider: the candidate player's ability and his matchup. Late last season, I did a series of articles on who to start at RB, WR, and TE that looked at quantifying those factors. You might want to look over those articles first, but I'll try to make this one stand on its own.
I'm going to use PPR scoring and only compare RBs to WRs as flex choices. The data is based on the past ten seasons, 2006-2015 (updated from the articles linked above) and the Top 100 RBs and Top 100 WRs in those years. So that's a total of 1000 players at each position. “RB1s” were the top 100 overall, “RB2s” were ranked RB101-200, etc. On a seasonal basis, the would be the Top 10 RBs, RB11-20, etc. To further differentiate the “Studs” from the rest of the RB1s, I broke the top 10% in half, so “Stud RBs” are the top 5% or top 5 backs in a year, ...
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