Boston Marathon Winning Time Progression

Let’s take a look at a chart of the winning times in the Boston Marathon for the Open and Wheelchair divisions:

 

Note how as fields get larger and more experienced, much of the variability from year to year goes away. The trends match the trends for the marathon community as a whole, represented below by the lines representing the marathon world record:

 

When we take a closer look at the modern era (from 1966 on), we can see that the year-to-year variability tracks across divisions, moreso as race field size and quality stabilize:

 

Clearly common external factors (mostly weather, I assume) have a visible effect.