'NF' in MLB stands for 'Nasty Factor'. The Nasty Factor evaluates several properties of each pitch, and rates the "nastiness" of the pitch on a scale from 0-100, based in part on the success or failure of opposing hitters against previous similar pitches. The Nasty Factor incorporates several different factors for each pitch, including: Velocity, Sequence, Location, and Movement. The Nasty Factor also adjusts for how often the pitcher has faced the current batter during the game, as well as how often he has used the same pitch type against the same batter in the current at-bat and previously in the game.
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