How is Performance Index calculated for a race result?

The Performance Index (PI) for a race result provides a measure of how strong the performance of a given runner was in a race compared to other runners in that race. 

Because some popular races can attract really strong pro fields that can skew winning times, we wanted to avoid anchoring the calculation of the Performance Index on race winners. So instead, the Performance Index is anchored on the median result in a race. 

To understand what the median result represents, imagine a race with 101 race finishers. The median result is the result of the 51st finisher in the race. There will be 50 finishers in the race who did better than the median result and 50 finishers who did worse that the median result. The median result sits in the middle: it's faster than half of the field, and slower than half of the field (note that the median result is taken on race finishes, so it excludes DNS/DNFs from the results count). 

As a quick technical note, notice that we're using the median result and not the mean result. The mean result can still be skewed by very fast or very slow race performances, and we want to avoid that as well.

So how is the Performance Index calculated?

Let's go back to the 101-person race. If your result is the median result (so you finished 51st out of 101 starters - right in the middle), your race result's Performance Index is 100.

If your race result is 20% better than the median result, your race result's Performance Index is 120. If your race result is 20% worse than the median result, your race result's Performance Index is 80.

To make this clearer, let's look at a few concrete examples of how the Performance Index is calculated:

Example 1: The median race result for a 50mi race was 10:00:00 (10h), and your finish time was 9:30:00 (9.5h). 

The Performance Index for your race result is (10:00:00 / 9:30:00) * 100 = (10 / 9.5) * 100 = 105.3. 

Example 2: The median race result for a 50mi race was 10:00:00 (10h), and your finish time was 11:00:00 (11h). 

The Performance Index for your race result is (10:00:00 / 11:00:00) * 100 = (10 / 11) * 100 = 90.91.

Example 3: The median race result for a 24h race was 150km, and your finish distance was 180km.

The Performance Index for your race result is (180 / 150) * 100 = 120.

Example 4: The median race result for a 24h race was 150km, and your finish distance was 120km.

The Performance Index for your race result is (120 / 150) * 100 = 80.

There are some results that do not receive a Performance Index. Backyard races, for example, are excluded from PI calculations, due to the intricacies of the backyard format. Races with fewer than 20 participants are also excluded because of their size. Finally, for fixed time races, only 6h, 12h, 24h, and 48h races are included in PI calculations.