The most drastic change in baseball since testing for performance-enhance drugs was first initiated over a decade isn't ago isn't testing for performance-enhancing drugs.
It's the change in the size of the strike zone that began with the advent of the PITCHf/x era -- where the location of every pitch is tracked -- and the evidence is leading us down this irrefutable acceptance: It's this increasing size of the strike zone that, more than anything, has led to the drastic decline in offense from the pumped-up days of Barry Bonds and Mark McGwire.
Jon Roegele did important work in this area last year and he's back with more data on the 2014 season. The bad news for major league hitters: The average size of the strike zone continued to increase, going from 459 square inches in 2013 to 475 square inches in 2014. Jon writes:
We can see that the strike zone has had a full three inches – the diameter of a baseball – tacked on to the bottom within half a decade. The called zone was wider than it was tall in 2009, but with the trimming of the sides and the massive expansion at the bottom, its width and height have become virtually equal in 2014.
The strike zone is now 40 square inches larger than in 2009, coming on the bottom of the zone -- a benefit to pitchers, since low pitches are better for pitchers than hitters. Batters hit .244/.317/.358 on low pitches (lower third of the strike zone or below) in 2014 compared to .241/.372/.387 on pitches in the upper third of the strike zone or above.
No wonder offense decreased from 4.17 runs per game to 4.07, continuing the downward trajectory:
2000: 5.14 runs per game
2001: 4.78
2002: 4.62
2003: 4.73 (first year of anonymous testing with no punishment)
2004: 4.81 (mandatory testing with punishment begins)
2005: 4.59 (lengths of suspensions for positive tests increased)
2006: 4.86 (random testing for amphetamines begins)
2007: 4.80
2008: 4.65 (first season of complete PITCHf/x data)
2009: 4.61
2010: 4.38
2011: 4.28
2012: 4.32
2013: 4.17
2014: 4.07
As you can see, the initiation of PED testing and later amphetamine testing had little immediate effect on run scoring (although offense had already started declining a bit in the early 2000s after topping 5.0 runs per game three seasons between 1996 and 2000).
Once MLB started collecting the PITCHf/x data and started training its umpires to call a more uniform strike zone, run scoring went down. The low strike zone probably isn't the only reason for the decline in offense -- pitchers continue to throw harder than ever, defensive shifting has had a small impact, more pitchers seem to be throwing cutters and splitters and wicked changeups, and, yes, drug testing has probably made an impact -- but it's no doubt a huge factor. (Jon estimates that 31 percent of the lower run enviroment between 2008 and 2013 was due to the bigger strike zone, while Brian Mills, another researcher, estimated the impact at 24 to 41 percent.)
As Jon reports, hitters have had to adjust to the new lower strike zone by swinging more often at low pitches. Before, they could wait and force the pitcher to come closer to their sweet spot. Now pitchers are forcing them to swing more often. In 2008, batters swung at 45.6 percent of pitches 18 to 24 inches off the ground; in 2014, that number was 49.2 percent.
It's become a pitcher's era. The pitchers are better but they're also receiving a lot of help.