FIP and xFIP and the rejection of 'in-between results'   The art
of pitching has a clear goal: prevent the opponent from scoring
runs. While it may not be possible to stop the opponent from
scoring runs entirely, the pitcher has the clear objective of
keeping the damage done to a minimum. Traditionally, the runs
that a pitcher has given up have been measured by ERA, or Earned
Run Average, which counts the number of runs scored from
baserunners that the pitcher has allowed, excluding those that
have gotten on base because of an error made in the field. The
idea behind this stat, then, is that a pitcher takes full
responsibility for the runs that are scored while he is on the
mound (or later, in the case of inherited runners scoring).
Sabermetricians and their followers reject this "philosophy" of
pitcher responsibility, because they argue that the defense
behind pitchers plays an important role as well. To filter out
the influence of the defense, they assume that all "in-between
results" are not predictive o