This is an update to an analysis I run each time the marathon world record is broken. If you like this sort of thing, you will like my forthcoming book, Probably Overthinking It, which is available for preorder now.

On October 8, 2023, Kelvin Kiptum ran the Chicago Marathon in 2:00:35, breaking by 34 seconds the record set last year by Eliud Kipchoge — and taking another big step in the progression toward a two-hour marathon.

In a previous article, I noted that the marathon record speed since 1970 has been progressing linearly over time, and I proposed a model that explains why we might expect it to continue.  Based on a linear extrapolation of the data so far, I predicted that someone would break the two hour barrier in 2036, plus or minus five years.

Now it is time to update my predictions in light of the new record.  The following figure shows the progression of world record speed since 1970 (orange dots), a linear fit to the data (green line) and a 90% predictive confidence interval (shaded area).

This model predicts that we will see a two-hour marathon in 2033 plus or minus 6 years.

However, it looks more and more like the slope of the line has changed since 1998. If we consider only data since then, we get the following prediction:

This model predicts a two hour marathon in 2032 plus or minus 5 years. But with the last three points above the long-term trend, and with two active runners knocking on the door, I would bet on the early end of that range.

This analysis is one of the examples in Chapter 17 of Think Bayes; you can read it here, or you can click here to run the code in a Colab notebook.

UPDATE: I was asked if the same analysis works for the women’s marathon world record progression. The short answer is no. Here’s what the data look like:

You might notice that the record speed does not increase monotonically — that’s because there are two records, one in races where women compete separately from men, and another where they are mixed. In a mixed race, women can take advantage of male pacers.

Notably, there have been two long stretches where a record went unbroken. More recently, Paula Radcliffe’s women-only record, set in 2005, stood until 2017, when it was broken by Mary Jepkosgei Keitany in 2017.

After that drought, two new records followed quickly — both set by runners wearing supershoes.