There were five objectives in the development of XERA:
1) Find the relative importance (in terms of earned run production) of the various elements (walks, hits, etc.) of a player's statistics.
2) Evaluate hitters and pitchers by the same criteria (relative standards).
Baseball's numbers are beautifully balanced. Whenever a hitter gets a hit, a pitcher is charged with giving up a hit. If one team scores a run, the other team gives up a run. When a pitcher allows an earned run, the hitters on the opposing team must have produced an earned run. Therefore, an earned run average, theoretically, exists for hitters as well as pitchers. That's what XERA is all about - it turns theory into reality.
3) Rate the players by a single, easily recognized, number.
A pitcher, of course, seeks the lowest possible XERA, while a hitter would pursue the highest possible XERA. This single number is much the same as Bill James' Runs Created/27 or Pete Palmer's Linear Weights formula, except that XERA relates to earned runs only (so a hitter's XERA can be equated, directly to the relative Earned Run Averages of his timeframe and ballpark). The same XERA formulas are used for each season and park.
4) Make it possible to decompose the XERA figure into distinct elements to determine a player's specific strengths and weaknesses.
Does this hitter have a good eye at the plate? Does he strikeout too much? This is what turns XERA into a "statistical scouting report." Good baseball fans know that a pitcher who typically walks 5 hitters, strikes out 3, gives up 11 hits and a couple of home runs per game (9 innings), has poor control (location, command) and has little "stuff." By the same token, a hitter who compiles similar stats, would be considered a star.
Just as ERA, IP, and walks, hits, homers, and strikeouts per game are important for evaluating a pitcher, wouldn't it be great to have the same information about a hitter? XERA does that - and it closely approximates batting average, on base percentage, slugging average, and OPS against pitchers, as well! XERA establishes the same objective standards to judge hitters and pitchers.
5) Keep it simple. Calculating XERA should require no more than readily available statistics. It isn't necessary to have box scores or a play-by-play account of every game, to calculate XERA.
Virtually all of the background player statistics used for this compilation came from Lee Sinins' Complete Baseball Encyclopedia. His CD offers the most comprehensive, sortable baseball statistics we've ever seen (this is an unsolicited testimonial). We highly recommend it to every serious baseball fan. If it offered XERA data, too, it would be perfect (a somewhat biased opinion)! His web address is http://www.baseball-encyclopedia.com/.
Would you like to see some examples of a hitter's XERA? We've linked (at http://www.xera-baseball.com/) to a list of more than 300 of baseball's finest hitters based on their XERA's (current players include stats through the 2009 season), along with the calculated number of innings that were pitched against each of them (rounded to the nearest third of an inning), and the number of hits, doubles, triples, home runs, etc. each had per "game" (27 outs). The black figures are traditional baseball stats, the blue figures represent calculations with the XERA formulas. Note that Babe Ruth and Ted Williams produced better than 1 earned run for each inning that was pitched against them (every three outs they produced)! Joe Dimaggio and Stan Musial were extremely tough to strike out, and Ruth, Williams, and Barry Bonds had great "eyes" at the plate (in addition to the fact that there may have been a little fear in facing these guys). Albert Pujols (8.59) is the highest ranked active player (he's fourth on the all-time list!). Todd Helton (7.47), Manny Ramirez (7.45), Alex Rodriguez (7.12), and Lance Berkman (7.08) are the only other active players with XERA's over 7.00 and at least 5,000 plate appearances. Click the Various XERA Stat Sheets Quick Click Link at http://www.xera-baseball.com/ and the Greats tab to see the full list.
We also have detailed stats, including XERA, on more than a thousand hitters and pitchers from the 2009 season - just click the Hitters09 tab or Pitchers09 tab in the Various XERA Stat Sheets Quick Click Link at http://www.xera-baseball.com/.
Check out which hitters have a good eye at the plate, who strikes out too much, who has the most power etc. Check out the pitchers to see who has the best control, the best strikeout ability, the best skills at preventing hits and homeruns etc. See why a pitcher's ERA is good or bad. When a pitcher's XERA is significantly different than his actual ERA, XERA is generally more indicative of future performance than ERA! That's because XERA represents a normal distribution of the numbers he's assimilated.
