This article is part of our The Saber's Edge series.
While I eventually decided to use contact (Contact%) and swing (Swing%) rates, I dug around a bit see what other stats could be used. Bottlecap brought up "deterioration in vision" and "bat speed." The problem with these two data sources is it takes knowing the hitter's vision test or multiple years of scouting reports to get the information.
The key was to find an easy to follow source on the information, and I used the Pitchf/x Plate Discipline values available at FanGraphs (here's Carlos Gomez's, for example). I will look at the three swinging and three contact values to see which ones might indicate a change in a player's performance. The reason I like to use the Pitchf/x plate discipline values instead of the normal "Plate Discipline" values (just above the linked table) is the strike zone size was inconsistent for several years.
Besides being readily available in a simple table at FanGraphs, both of the stats stabilize at a quicker rate