Without a doubt my most valuable resource when I write about fantasy baseball is the information on the Fangraphs website. I will not make any drastic moves with my teams before consulting the player pages of all involved. Recently I was talking to a reader who admitted that while he would like to use Fangraphs more, he did not have a grasp of which stats were most important and what the numbers really meant. In an effort to rectify that, this is a rundown of the statistics I most commonly use and how I apply them to player evaluation. Just for fun, feel free to check out where Albert Pujols ranks in the more skill based BB/K and ISO.

BABIP: The acronym for Batting Average on Balls In Play, it is receiving more widespread exposure than ever this year. Strikeouts and home runs are not factored into BABIP. A normal range would be anywhere from .290 to .310, but hitters who have established higher levels can maintain them. Joe Mauer has a .348 career BABIP and has never been below .310. Matt Kemp may be the most extreme example; he is at .377 in nearly 1500 major league at bats. It is reasonable to project him at al least a .340 BABIP in future years. Above average speed will help a player increase his BABIP, as will ground balls and, to a much greater extent, line drives. Excessive fly balls, especially popups will hold BABIP down, as will continually hitting into a shift. Carlos Pena and David Ortiz have two of the four lowest BABIPs this season, Adrian Gonzalez is in the bottom 20 also.

BB/K: Walks to strikeout ratio, a more complete measure of strike zone command than isolating BB% or K%. Even when strikeouts are high, walks can offset them because the batter reaches base and has an opportunity to score. A hitter who walks will also have seen more pitches and could use that to his advantage in future at bats. Anything under 0.50 is a poor mark, and 0.33 or less is in the bottom 10%. The top 20% of hitters are above 0.75, the top 15 are at 1.00 or higher.

ISO: Slugging Percentage – Batting Average. Commonly referred to as a measure of a hitter’s raw power, it is only driven up by extra base hits. It is most useful to follow the trends of individual players. In the past three years Russell Martin’s ISO has fallen from .176 to .116 to .058. An ISO under .120 is low, over .220 is above average

GB/FB: Ground ball to fly ball ratio. If you are evaluating a slap hitting speedster a high GB/FB, around 1.50 or more, should help him get on base more often. Hitters counted on to deliver home runs and RBI (which can come more often via extra base hits and sacrifice flies) want a ratio of 1.00 or lower, preferably under 0.80. A couple of months into the 2008 season it was obvious that Delmon Young would not be the power hitter so many scouting reports predicted because he was putting too many balls on the ground. He finished with a 1.98 GB/FB.

HR/FB: A hitter will usually establish a career average in home run to fly ball ratio and stay near it all other factors being equal. In the past seven years Mark Teixeira has played in multiple home ballparks and in both leagues, but his HR/FB has remained between 15.9% and 22.4%. He actually has a lower HR/FB this year than last, but will hit more HR because his GB/FB is lower. Even over a full season HR/FB can appear out of normal ranges because of luck. Many low power hitters are below 6.0%, the top 25 most prolific are between 18% and 30% this year.

Oswing%: One of the more recently introduced stats on Fangraphs, it is how often a hitter swings at pitches that the cameras trained on the strike zone perceive as balls. The higher it is the less selectivity a hitter is displaying. He could be naturally aggressive, or just have poor pitch recognition. Another explanation is the hitter is compensating for a loss of bat speed by starting to swing earlier. David Ortiz’s rising Oswing% over the past two years illustrates that theory. Batters are more likely to miss or connect weakly with pitches outside the zone. Vladimir Guerrero and a handful of others aside, it is difficult for a hitter to sustain success by having a Oswing% at 33% or higher. The league average for Oswing% was near 25% in 2007 and 2008.