One of the features of my Seattle Mariners weekly recaps are the two-week performance summaries for both the hitters and the pitchers. Here I will breakdown each of the statistics I use, how I calculate them, and why. We’ll start with the batting stats.

PA; BF – Plate Appearances and Batters Faced.

1B/2B/3B/HR; K/BB – These are straight counting stats.

AVG/OBP/SLG – This is your standard slash line, Average /On Base Percentage / Slugging Percentage.

ERA/FIP – ERA is a standard pitching statistic. FIP is a ERA estimator that uses strikeouts, walks, and home runs to measure how a pitcher performs based on the events he directly controls. This removes the noise balls in play and play sequencing that can affect ERA.

The next three rate stats have a companion statistic marked with an ‘x.’ Their companions are expected rates based on their career averages. Research has shown that certain statistics become more reliable after a number of plate appearances. What I’ve done is regressed each player’s actual rates to that point using their career average. The reason I’m doing this is to try and see which players might be over performing or under performing our expectations. In the example above, Jesus Montero actual strike out rate is 13.2% where as his expected strike out rate over the same amount of plate appearances is 17.1%. The more plate appearances a player accumulates, the more accurate the expected rate should be.

K%/xK% – Strikeout rate measures how often a player strikes out per plate appearance. A high strike out rate usually means the batter is unable to maintain a high enough contact rate to maintain a high batting average. Strike out rate becomes reliable after 150 plate appearances. The league average strike out rate is 19.6%.

BB%/xBB% – Walk rate measures how often a player walks per plate appearance. A high walk rate usually indicates strong plate discipline and a high on base percentage. Walk rate becomes reliable after 200 plate appearances. The league average walk rate is 8.0%.

HR%/xHR% – Home run rate measures how often a player hits a home run out of their total number of fly balls. A high home run rate  usually indicates strong power numbers and a high slugging percentage. Home run rate for batters becomes reliable after 300 plate appearances. Oddly enough, home run rate for pitchers has no reliability no matter how many batters they face. Therefore, I have regressed pitcher expected home run rate to the league average home run rate of 11.6%.

The last stat is of my own creation, inspired by Carson Cistulli’s work on FanGraphs. Its an attempt to measure the quality of a player’s performance based on their expected rate stats. It is certainly not perfect and will probably change and morph over the season, but its at least a first attempt to give a single number to the performance of a player over a two week period.