I made a reference to "Seven Skills" in the post about Jose Reyes, and a couple of people have asked about this. Here is an excerpt from the 2007 book about the Seven Skills:
EVALUATING POSITION PLAYERS: THE SEVEN SKILLS
Here is how I evaluate position players.
Traditionally, scouts use the "Five Tools" to evaluate position players. These tools are hitting for average, hitting for power, running, throwing, and fielding. It makes sense to look at amateur players in this way, since statistics at the high school and college level can be unreliable due to great variations in the quality and context of competition. But judging players already in professional ball solely by the Five Tool Approach is misleading and can lead to bad results.
I've taken the Five Tool concept and tried to refine it into what I call the Seven Skills. The choice of the word "skill" is deliberate. A hammer is a tool, but knowing what to do with it is a skill. The Seven Skills in baseball are: controlling the strike zone, hitting for power, hitting for average, offensive speed, fielding range, fielding reliability, and throwing utility.
Strike zone judgment is extremely important. Statistically, we should look at the ratio of walks and strikeouts to at-bats and plate appearances, and walks and strikeouts to each other. Generally, a hitter should have a walk total equal to at least 10 percent of his plate appearances. You can eyeball this by just looking at at-bats and increasing the relevant ratio to at least 12 percent if you like.
The relationship between walks and strikeouts is also important. Some successful hitters don't draw many walks, but most of them don't strike out much, either, making their living on contact. Some successful hitters will strike out a lot, but most of them draw walks as well.
There are few examples of consistently successful hitters who strike out frequently AND don't draw walks. There are a few, but not many. Ideally, we want hitters who draw walks AND make contact, which is usually a sign of extremely good plate discipline. Players like that also tend to be the best prospects.
Players with plate discipline problems often fail to reach their ultimate physical ceilings.
Statistically, I like to look at OPS (on-base percentage plus slugging percentage) as a shorthand stat for overall productivity, compared to league average. OPS is not a perfect stat. There IS no perfect stat. But as short-hand and a way to make comparisons, it is useful.
Numbers that are good in one league may not necessarily be good in another; context must always be considered. For example, the league OPS in the Midwest League this year was .725, but the league average in the California League was .809.
Another number I look at is Secondary Average (SEC) devised by Bill James. This measures the contributions not directly measured by batting average: power, patience, and speed. I use SEC as a shorthand for the "broadness" of a player's skill base; a player with a high SEC often is doing more offensively than a player with a low SEC in comparison to their batting average. Look at it this way: a player with a batting average of .300 and a SEC of .400 is a much more "complete" offensive player than a guy with a batting average of .320 but a SEC of .180.
SEC also shows us players whose offensive value is not fairly reflected in their batting average. A .250 hitter can be extremely valuable if he has a high SEC, even if traditional batting average is understating his performance.
The SEC formula I use is
As with OPS, I compare this to league average to get a grasp on how the player is doing in comparison to his peers.
Speed is important in certain tactical situations, but in the big picture it doesn't have a huge impact on how effective a player is offensively. Stealing bases only helps an offensive if done at a success level of 70 percent or better. If a guy steals 30 bases but is caught 21 times, he's hurting the team, not helping.
I use the term "offensive speed" because some players are very fast runners, but lack baserunning instincts and don't help the team much. Other guys may not have terrific running speed, but if they get good jumps, read pitchers well, and take the extra base when offered, they help the team even if they aren't blazing fast in a physical sense. The best runners have both pure speed and good instincts, of course. They are also the rarest.
Defense is difficult to measure using statistics, at least the minor league level. Reliability can be measured fairly well using traditional fielding percentage. But range is more important, developmentally, than reliability, and it is hard to measure range at the minor league level. Range factor is a flawed stat since it doesn't account well for team context. Zone ratings are more useful, but we don't have access to these at the minor league level yet. Generally, I rely on scouts for information about arm strength and range, since the numbers are not consistently reliable at this point.
For all players, age relative to league is a critical factor. A 20-year-old in Double-A is almost always a better prospect than a 23-year-old in Class A, even if the older player has better numbers.