So here's my idea, let me know what you think. I want to devise a system that takes yearly batter-vs-pitcher statistics and splits them into two categories: Good and Bad opponent.
Example: If a player hit .300/.400/.500 during a season, how many of those hits came against pitchers considered "bad" and how many came against pitchers considered "good"? Maybe he was 0-8 with 6 K's against Justin Verlander but 3-3 with 2 HRs against Jonathan Sanchez. (I know what you must be thinking, how did his third hit not leave the park? It was a double that just missed the top of the fence. There were probably 3 runners on base.)
Point being, there have got to be players that live or die by the level of competition they're playing against. Sure a pitcher can strike out 12 Royals and throw a shut out, but what's he going to do against the Yankees? So I want to split season stats into Good and Bad Opposition. How will I go about doing that?
My first thought was simple. I'll take the batter vs pitcher data provided by sites like baseball-reference and split the pitchers into Above 100 ERA+ and Below 99 ERA+ to get the "below average" and "above average" pitchers, eliminating anyone under an arbitrary number of IP like 20. Then it's a simple recalculation of the batter's stats against those pitchers, and you get something like this, for Josh Hamilton's 2010 MVP year:
(Any data that looks like it's missing is because it came from the pitchers who had below 20 IP on the year. Their sample size is too small to include in either good or bad.)
The first thing I want to say about this data is that, yes, it's not a surprise at all that Hamilton, or anyone else for that matter, hit 80 points better and slugged 130 points better against worse pitching. That's what they're supposed to do. I'll be interested to see what the league average is in the difference between good and bad pitching, and who some of the outliers are on each side.
My question, faithful Royals fans (are there any other kinds?) is if you think ERA+ and innings pitched are what I should filter "good" and "bad" pitching by. I have some issues with the idea that ERA is determining who is good and bad, and also because relief pitchers usually have a lower ERA and so a higher ERA+, but I'm not sure how else to easily filter it.