The Difference Between Mass Start And Interval Start Races In One Graph

Thanks to some help from Jan at, I’ve been slowly gathering the split time data for World Cup races from this season. Analyzing them is tricky, though, for a variety of reasons.

First, the data quality is poor. There are numerous instances where the live timing data is obviously wrong in a way that I can’t fix by hand, and so a certain number of split times need to be omitted completely. Second, the live timing data itself doesn’t say exactly how far into the race each timing instance is, so I have to infer that from the times and the length of the race. It works pretty well, but I’m sure it’s only accurate to +/- 500 meters or so. Lastly, handicap start races have to be dealt with separately, since the live timing tracks the time from when the first racer starts, not from when each racer starts.

Still, I’m beginning to find some interesting stuff. A little later I’ll show you some graphs on individual skiers that can shed some light on tactics and performance that I think are kind of interesting. For now, though, the obvious:

These are the 10/15km interval start races and the 30/50km mass start races from World Championships this year. Since I’m comparing races of different lengths, the x axis isn’t the number of kilometers into the race, it’s the proportional progress through the race distance. At each split measurement, I’ve calculated each skier’s percent behind the median skier at that point in the race and then plotted this progress as a line for each athlete. I’ve included a little alpha blending since lots of the lines are directly on top of each other, particularly for the mass start races.

Remember that this is tracking relative performance, not absolute performance. So the men’s mass start graph is not necessarily saying that 2/3 of the way through the race the lead pack suddenly accelerated. It’s just that at that point the median skier was finally popped off the back of the lead pack and so either this person slowed down, or the pack accelerated. But you can’t really tell which from this graph.

The differences here are pretty obvious, no?


Related posts:

  1. How I Learned to Start Worrying and Hate the F-Factor (Part 1)
  2. How I Learned To Start Worrying and Hate the F-Factor (Part 2)
  3. La Clusaz Recap: Distance Mass Start
  4. Race Snapshot: IBU WC 8 Men/Women’s Mass Start
  5. Race Snapshot: WBC Mass Start

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6 Responses to “The Difference Between Mass Start And Interval Start Races In One Graph”
  1. WorldofXC says:

    Interesting, interesting! Looking forward to more of these analysis in the off-season period to come! Still some work to do with the visualizations though, I think. I’ll try to think if I can come up with some other way to visualize this stuff better.

  2. xcskier22 says:

    Is that a polygraph test or something!? Haha.

  3. Marc Beitz says:

    Very cool stuff as usual. It would be really interesting to know if, at the World Cup level, mass start races are faster (feel free to interpret “faster” – you’re the expert) or slower than individual start races of the same distance. Is that too hard to isolate? Or have you already done the analysis? Already debunked the question?

    • Joran says:

      That’s pretty tough to tackle without real-time speed data, like what you’d get with GPS. I know that a limited amount of this kind of information exists (some big events use GPS based timing pods), but I haven’t had any luck acquiring it.

      • Marc Beitz says:

        I suppose it would be easier to devise the right analysis if I just threw out my hunch instead of even trying to word it correctly. So hear it is: it seems that the times (forget speed) for mass start races are longer than (or at least barely on par with) those for individual start races of the same distance and technique. I’ve looked at some times for 50K races at WC and Olympics, but lack the data/skills/patience to do a large enough comparison to be interesting. Even complete parity would be an interesting result.

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