How are ESPN Player Ratings calculated? : r/fantasybaseball – Reddit

Z-scores! Essentially ESPN will calculate the average and standard deviation for each stat. Then, for each player, you take that player’s stat and subtract it from the league average, then divide that result by the standard deviation. The sum of each players’ z-scores is their total player rating.  

https://www.reddit.com/r/fantasybaseball/comments/4o91bw/how_are_espn_player_ratings_calculated/