A couple of posts last week by the fine writers over at Me and Pedro got me thinking about stolen bases. Through the 90s and the first half of this decade, a combination of sabermetric influence and an increase in power has lead to a greater awareness of the negative consequences of stolen base attempts (getting caught). Over the last couple years, it seems to me that teams are starting to run a bit more, but have also gotten smarter and aren't making as many foolish outs on the basepaths. Is it true? I decided to take a look at the trend for two stats over time: stolen base attempts per game and stolen base %. I looked at all the data going back to 1951, the first year that MLB kept track of times "caught stealing."

Here's the average stolen base attempts per game per team across MLB:

The blue dotted line is the average for every season from 1951-2007. The purple line is a five year trailing average, which helps smooth out some of the random year-to-year fluctuations, so we can focus on the long-term trends. Basically, there was a steady, slow growth in the 50s and 60s, followed by a huge increase in the 70s. I won't try to speculate on what the cause was. The rate remained high through the 80s, but then fell slowly through the 90s, followed by a steeper drop from 2000-2005. Over the last two seasons, stolen base attempts increased slightly, possibly signaling an end to the long decline phase. That makes sense because overall offensive levels are down a little.

This next chart tracks the success rate of stolen base attempts over the same time period:

Interesting. Generally, it's considered that the break-even success rate in today's game is around 70%, but it varies a little based on the offensive level at the time (in other words, if homeruns are common, the cost of losing a baserunner is higher). This chart shows a surprisingly consistent upward trend over the last 50+ years. The 5-year average takes a few dips, but the general trend has been steadily ticking upward since 1951. Very interesting. One question this raises is whether this can continue to climb indefinitely. The rate has been pretty steady since the late 80s, at around 69-70%. In 2006, a new record was set at 71.4%. But, 2007 was almost literally off the chart, with a 74.4% rate. That's a huge increase! So far this year, the rate is 72.7%. It's a little early in the season to draw any conclusions, but it seems likely that there will be a bit of a contraction this year.

I think the most interesting thing about these two graphs is that they don't seem to have any relationship. It seems natural to assume that there's a relationship between how often players try to steal and how often they'll get caught. This certainly seems true on the individual level. Carlos Beltran has always had a high success rate, but that's because he chooses his spots wisely. I'm pretty sure that if you asked Beltran to double his steals, he'd respond that he could, but he'd get caught much more often. But, on the macro level, this hasn't happened. SB% has consistently risen whether teams are attempting steals more often or less often. From 1971-1976, there was a huge 64% increase in stolen base attempts, but those attempts were apparently no more risky than they'd been in the past - the success rate actually went up. I guess the explanation is that the change in attempts is mostly explained by the abilities of the players at the time, not by a shift in strategy. It seems that the players were just faster (or the catchers had worse arms), so they were able to steal more bases, while maintaining the desired success rate.

That doesn't answer the question of why the success rate has consistently gone up over time. I'm not sure what the answer is.