OBP refers to how frequently a batter reaches base per plate appearance. Times on base include hits, walks and hit-by-pitches, but do not include errors, times reached on a fielder's choice or a dropped third strike. (Separately, sacrifice bunts are removed from the equation entirely, because it is rarely a hitter's decision to sacrifice himself, but rather a manager's choice as part of an in-game strategy.)
Now that we’ve covered slash lines, plate discipline, and batted ball data, that about does it for the hitting side of advanced baseball stats, but before I wrap up, I need to mention one important thing. All of the statistics that I used were from players who had a qualifying season (3.1 plate appearances per team game which is roughly 500 PA). The pre-mentioned stats are most effectively used when you have a good sample size of data to work with, and you should watch out for stats that are skewed by small sample sizes. Make sure that when you are evaluating a player’s skill set, they have accumulated enough plate appearances (usually you want to aim for a minimum of 100) to make the data you’re working with relevant.
The best way I can explain “Hitting for Average” is that this tool is not just solely focused on a person’s batting average. This tool is more about having the ability to have a consistent swing, the ability to keep the bat on-plane for a long period of time, and the ability to square up baseballs on a regular basis. I wrote another article about having the ability to “Repeat Your Best Swing.”
Henry Chadwick, an English statistician raised on cricket, was an influential figure in the early history of baseball. In the late 19th century he adapted the concept behind the cricket batting average to devise a similar statistic for baseball. Rather than simply copy cricket's formulation of runs scored divided by outs, he realized that hits divided by at bats would provide a better measure of individual batting ability. This is because while in cricket, scoring runs is almost entirely dependent on one's own batting skill, in baseball it is largely dependent on having other good hitters on one's team. Chadwick noted that hits are independent of teammates' skills, so used this as the basis for the baseball batting average. His reason for using at bats rather than outs is less obvious, but it leads to the intuitive idea of the batting average being a percentage reflecting how often a batter gets on base, whereas in contrary, hits divided by outs is not as simple to interpret in real terms.
These are important because they show you how often (on average) a player walks and strikes out. Unlike batting average, K% and BB% are given in a direct percentage format, so there’s no need to translate it. The application of these stats is pretty straightforward; a player with a high BB% and low K% would typically have a good batting eye, and a player with opposite-type numbers would typically have a poor batting eye.
The main application of slugging percentage is to go beyond just being able to tell how good a player is at getting hits, but how good they are at getting quality hits. For example, Robinson Cano and Andrew McCutchen both had a batting average of .314 last year; however Cano slugged just .454 opposed to McCutchen who finished with a .542 mark. While both players got hits just as often, McCutchen got the more valuable kinds of hits more often (he had more doubles, triples, and homers than Cano), so he was the better hitter in 2014.
When it comes to hitting for contact, Chris Carter is pretty definitively the worst in the league right now. If we usual a literal interpretation of the hit tool and say that it indeed relates to specifically the statistic of batting average, Carter could be heading for the basement if his BABIP regresses to something more in line with his profile. Whichever way you slice it he is the very deserving winner of the final "MLB's Worst Tools" award. I'm sure it will adorn the mantlepiece of the of the Carter household for centuries to come.
While batting average is a useful tool for measuring a player's ability at the plate, it isn't all-encompassing. For instance, batting average doesn't take into account the number of times a batter reaches base via walks or hit-by-pitches. And it doesn't take into account hit type (with a double, triple or home run being more valuable than a single).