Statistical Skier

Race Snapshot: Rybinsk 10/15k Mass Start

Joran — Sat, 04 Feb 2012 22:50:09 +0000

Within season trends: sprinting

Joran — Fri, 03 Feb 2012 14:00:15 +0000

I know the Moscow city sprints were exciting and all, but I’ll have some posts on them and the Rybinsk races next week. For now, as promised we have the follow-up for sprinting from Wednesday’s post.

These are the folks with the largest improvements in performance (within season). As before their results from this season are highlighted in red. I’m most interested in following up on Devjatiarov, since his trend seems so stark.

As for the folks with the largest declines in performance:

Not who you were expecting, eh?

So once again we have some Americans, who possibly do slightly worse late in the season do to increased travel demands. But how do we explain that the others here are some of the best sprinters, ever?

Well, keep in mind that saying that these skiers see the “biggest declines” during a season is a relative claim. So in Bjoergen’s case, she has had some poort sprint races, but never before January, basically. The trend for Matveeva I’m less likely to dismiss as a fluke since the difference in her case is so stark (although I don’t know her injury/health history, which can affect these things).

For Angere, Bjoergen and Pettersen I really want to think of some other explanation. But their bad races are certainly not evenly spread throughout the season.

Race Snapshot: Moscos Sprints

Joran — Thu, 02 Feb 2012 15:30:10 +0000

Some context for Skofterud’s late season troubles

Joran — Wed, 01 Feb 2012 14:00:23 +0000

Since I feel bad picking on Vibeke Skofterud twice now, I thought I’d follow that up with a look at that kind of trend among all skiers. Today we’ll look at distance results, Friday we’ll consider sprinting.

I hauled out my favorite statistical canon for this one, a mixed-effect model that compares each skiers results to the time during the season, estimating a separate slope for each athlete. Then I can quickly sort skiers into those with a large positive slope (get slower as the season progresses) and those with a large negative slope (get better as the season progresses).

Here are the six skiers, active in 2011-2012, with the largest declines in performance (at least, as measured by this model) over the course of a season:

I’ve highlighted each skiers results from this season in red. Also note that while I’m using standardized percent behind the median skier, the scales on each panel are different, for readability’s sake.

Surprised to see Kikkan Randall here? Note that she has had a large number of poor races in Feb/Mar. But just as influential are the current season’s result (which isn’t finished yet). Since they’ve all been rather good, they will tend to pull down the estimate of her early season performance. Once I made this plot, I realized that it would make more sense to fit the model on only results from completed seasons. For many skiers (indeed, most of the ones I’ll show you) it wouldn’t make much of a difference, but it clearly would with Kikkan. Consider this your daily glimpse into the iterative process that is “statistical analysis”.

Notice also that Skofterud isn’t at the top of this list. So perhaps we shouldn’t be quite as hard on her for tailing off as the season progresses.

How about the folks that see the most significant improvements over the course of a season?

These tend to be skiers with considerably more races under their belts. Oxana Jatskaya is an interesting case, as her current season seems to be playing out right on cue: some “normal” early season results, followed by the terrible results mid-season. What will she pull off in February…?

Skofterud’s Early Season Strengths Update

Joran — Mon, 30 Jan 2012 14:00:10 +0000

Back in December I posted a short note about how Vibeke Skofterud has a tendency to ski considerably better in the early part of the season. Since we’re in the midst of a break between WC periods I thought it might be good to check in and see how things are progressing:

Those are her distance results (using standardized percent behind the median skier) with her 2011-2012 results shown in red. As you can see, there was a bit of a gap between her early races and her most recent one, but based on that one race things seem to be following her typical trend. As for her sprint results:

Overall, the backward trend with sprinting isn’t quite as clear, but she appears to be following up on this one as well so far.

Andrew Musgrave at Norwegian Nationals

Joran — Fri, 27 Jan 2012 14:00:02 +0000

First of all, I’ll echo Nat Herz’s big ups to Andrew Musgrave for finishing second at Norwegian Nationals the other day. I’ve always been a bit fascinated by Musgrave, probably because North Americans identify with any reasonably successful, promising skier not from Scandinavia or the usual European nations as fellow “outsiders”. So I’m always kind of pumped when he skis well.

But looking closely at this result, I started getting a little puzzled. The first thing that puzzled me was that I didn’t think that his WC results so far this season had been all that great. So I checked, and…

…generally my impression was correct. His results have been a bit worse than last season. But for a guy putting up resulting on the WC this far behind the median skier, second place at Norwegian Nationals seems out of place. So let’s look more closely at this result.

The first thing to note is that Martin Johnsrud Sundby won the race handily, beating Musgrave by 67 seconds or so over 15k. A quick glance at Johnsrud Sundby’s WC distance results this season reveals that they have (with one exception) also been significantly better than Musgrave’s. So maybe the field at Norwegian Nationals was somewhat weaker than usual? (Case in point, Alsgaard in 3rd.) Well, scanning down the results sheet reveals several other Norwegian WC regulars, so maybe not.

Let’s do this more systematically by comparing Musgrave’s performance in this race to his performance against the skiers in this field in the past:

Higher is better in this plot (better for Musgrave, that is). The blue points are from this particular 15k from Norwegian Nationals. Note the one blue point way off by itself; that’s Johnsrud Sundby. The red line and shaded ares represent the general trend for Musgrave against the skiers in this field over time.

Clearly this race is at the upper end of Musgrave’s past performance level (red dot). And the overall trend has been moving steadily in Musgrave’s direction over the past few seasons. This suggests that Musgrave has been getting steadily better, on average, against the particular skiers in this field over time. So much so that I’d basically expect him to beat nearly all of them at this point on a regular basis.

Nearly all, of course, is not the same as all. I mean, I don’t put much stock in Petter Northug’s 40th place result in this race. So my read on this is that this field consists mostly of skiers that Musgrave has already shown that he’s as good or better than, and the remaining WC level skiers either crushed him (Sundby) or weren’t taking the race very seriously (Northug).

At the end of the day, though, Musgrave can still say he finished second at Norwegian Nationals, and no amount of complicated analysis on my part can change that.

Noah Hoffman’s Pacing

Joran — Wed, 25 Jan 2012 14:00:22 +0000

I was interested (rather than offended) to read about Noah Hoffman’s pacing strategies in Sunday’s classic WC race. I have a limited supply of data on split times (what I do have is thanks to Jan over at worldofxc.com, though) so the following data is definitely incomplete.

Hoffman seemed determined to not start too fast, something that apparently he does quite often. Since pacing interval start races is so much different than mass start or pursuits, we’ll only look at his splits for interval start races. I only have split times from a total of eight WC-level interval start races for him (of varying lengths). Here’s a simple graph showing them all together, with Sunday’s race highlighted with the black dashed line:

This is a very crude representation of split times, where I’ve simply plotted how fast Hoffman skied each timed section compared to the field. So, for example, the y-axis means he had the 20th fastest, 40th fastest, etc. split time on that section.

In order to compare races of different length I’ve converted the x-axis from raw kilometers to a percentage of the total race distance. (This may be dubious, since pacing strategies will be markedly different in a 15k versus a 30k. However, these data consist of two 10k’s, five 15k’s and only one 30k, so I think we’re on fairly safe ground.)

Certainly Noah’s first split was slower than his subsequent splits on Sunday. And it was the 3rd slowest initial split of the eight I have. But it seems to me like he proceeded to ski the rest of the race fairly consistently, rather than gradually accelerating. At least, until he faded a bit on the last section.

This is in contrast to several of the other lines here that begin with fairly quick initial splits, but by mid-race he’s clocking only ~60th fastest time on each section or so. So whatever he did seemed to work.

I should say that I’m fairly cautious about my ability to analyze split times. I’m sure the coaches are keeping more detailed data on this sort of thing than I have access to. But it’s interesting, nonetheless.

Otepää Sprint Recap

Joran — Mon, 23 Jan 2012 14:00:29 +0000

I think I’ve been remiss in mentioning this, but as always thanks go to Jan at worldofxc.com for helping to provide the heat times for these sprint races. Let’s see how the men’s finalists fared first:

There were apparently some crashes in this race, as evidenced by the outlying slow times. But in general I’m noticing that each round was progressively faster until the finals, when things backed off, but only slightly. And Cologna really was just skiing faster than everyone. It’s interesting that he outpaced everyone by so much in qualification. I wonder if that was intentional, or if he could do it over again he’d back off a bit. Here’s a look at the difference between the two semifinals:

Not too much of a difference there, may be a second or so (not counting the crashes). As for the women:

Even more so than the men, the times here just kept getting faster with each round. The finals didn’t actually get faster, but they didn’t really see the drop-off in times that the men’s final did. As for these semifinals:

Again, not much of a difference, overall, although semifinal 2 was nominally faster.

Race Snapshot: Otepää 10/15km Classic

Joran — Sun, 22 Jan 2012 18:25:47 +0000

Race Snapshot: Otepää Classic Sprint

Joran — Sat, 21 Jan 2012 17:48:08 +0000