Revisiting the Defensive Spectrum

The idea of the defensive spectrum, first articulated by Bill James in the early 1980s has provided a useful frame for understanding baseball. The defensive spectrum is the relatively simple idea that there are different offensive expectations for players who play defensive positions. It formalizes the intuitive notion a team will sacrifice some offense for a good middle infielder or catcher, corner outfielders, first baseman and DHs are expected to be strong hitters.

The defensive spectrum, however, particularly the notion that middle infielders and catchers are not expected to hit as much as other players, has not been entirely constant over time. The basic framework has applied for over a century, but the extent to which it has been accurate has varied. At first glance it would seem like currently there are more middle infielders who contribute offensively, and fewer who are in the lineups exclusively for their gloves, than a generation ago, but this alone does not represent strong evidence.

A decade by decade analysis provides better evidence. For each decade from 1900-1909 to 2001-2000 I looked at the proportion middle infielders and catchers, and then just middle infielders, of all the seasons for any player who had an OPS+ above 120, which indicated a good offensive season, and then all the seasons of any player who had an adjusted OPS+ of less than 80, which indicates a bad offensive season. If good and bad seasons were randomly distributed over all positions we would expect 37.5% of the seasons in each category to be produced from middle infielders and catchers, for each decade until the 1970s when we would then expect that number to be slightly lower, around 35%, due to the designated hitter rule in the AL.

As expected, the proportion of OPS+ seasons of 120 or better by catchers and middle infielders was always well below 37.5%, while the proportion of seasons of OPS+ of less than 80 by catchers and middle infielders was always well above this figure. However, within this basic framework, there was a fair amount of range over the decades. The general pattern is that the defensive spectrum became more dominant over the 20th century, peaking in the period from 1960-1979 before receding again in recent decades.

In the 1960s, 73% of OPS+ seasons of 80 or lower were by players on the left end of the political spectrum, catchers and middle infielders. In the 1970s, this number rose to 81%. This number, by contrast ranged from 49%-60% for each of the decades before 1960. During the 1960s and 1970s almost all of the offensive seasons that could loosely be described as bad belonged to players on the left end of the defensive spectrum. Interestingly, during these two decades the poor offensive seasons were mostly contributed by middle infielders as 72% and 76%, overall, of these seasons were by middle infielders. So, in the 1960s and 1970s while the defensive spectrum was relatively strong for middle infielders, this was less true for catchers.

In the 1960s and 1970s, it was very unusual for catchers or middle infielders to have OPS+ of over 120, or what might loosely be defined as strong offensive seasons. In the 1960s, only 10% of OPS 120+ seasons were by catchers or middle infielders. In the 1970s, the numbers were a little better, as 13% of these good seasons were by catchers or middle infielders, but only 5% overall were middle infielders, meaning in both these decades it was extremely rare for a middle infielder to have an OPS+ of 120 or better. This occurred 44 times during this 20 year period or less than three times a season.

Beginning in the 1980s, this trend began to reverse itself, so that in the current decade, only 60% of OPS+ seasons of 80 or worse belong to catchers or middle infielders while of OPS+ seasons of 120 18% were done by catchers and middle infielders. The former was the lowest since the 1940s, the latter the highest since the 1930s.

This is a very brief look at the data using measurements that, while reasonable, are certainly not the only ones that could have been used. Changing some of the measurements would change the findings somewhat, but the basic trends would not change much. These findings have import for comparing players across periods, evaluating contemporary players, decisions around player movement and award and Hall of Fame voting. So, for example, Omar Vizquel and Luis Aparacio were very comparable offensive, and for that matter defensive, players, but Aparacio played in a period when a career OPS+ of 82 was better, for a shortstop, than Vizquel’s career OPS+ of 83 during a period when shortstops were expected to hit. Although their numbers suggest they were more or less equally valuable, Aparicio was more valuable because, during the time he played, he hit better for his position than Vizquel did. Similarly, it would be a mistake to trade as much talent for a league average hitting shortstop today as one would have half a century ago.

At this moment, the defensive spectrum is less extreme than it has been since the first half of the twentieth century. Perhaps this is cyclical and we will soon see the return of poor hitting catchers and middle infielders, but it seems at least as likely that this not cyclical and that the trend towards better hitting middle infielders will continue. It may be that baseball is changing and that one of the results of the offensive explosion of the last twenty years is that we will have new expectations for what shortstops, catchers and second baseman can do at the plate. This will force us to rethink quite a bit about what we know, and how we analyze baseball.