Jens Jakob Andersen first noticed it during a race a few years back.
A former competitive runner himself, Andersen felt he had a pretty good idea for how long races and marathons usually took.
But after finishing the Copenhagen Marathon a few years ago, one thought prevailed in his mind over all others: Everyone seemed slower... a lot slower.
At first, Andersen couldn’t tell if his eyes were playing tricks on him or if there was some credence behind what he thought he saw.
That’s when the Danish statistics professor and self-described “data geek” from Copenhagen Business School (CBS) in Denmark decided to find out for himself.
Turns out, he was onto something.
Andersen, with the help of Ivanka Andreeva Nikolova, published his mega study “American Runners Have Never Been Slower” on Tuesday, July 4, which claims that Americans are getting slower across all of the major race distances, genders and ages, excluding the top elite runners.
Andersen, a statistics and forecasting professor for the past two years at CBS, was the lead researcher for the study, while Nikolva, Ph.D. in mathematical analysis, assisted him.
It was one of the most exhaustive studies of running race results in history, with approximately 34,680,750 results analyzed from 28,732 different marathons, half marathons, 10-kilometer races (10K) and five-kilometer races (5K) in the United States.
The initial database included results from 1902 through May of 2017, but Andersen and Nikolova ultimately limited their analysis to U.S citizens who finished races in the 20 years spanning 1996-2016.
But even after shaving 94 years off their research, Andersen and Nikolova still had their hands full compiling all the data, which took nearly four months of around-the-clock work.
“Collecting data, that's always the biggest challenge,” Andersen said in a recent Skype interview. “Getting right data and getting enough data.”
Andersen said that all of the results in his study are at least 99 percent statistically significant, meaning that there's “less than one percent probability that some of the results are not true.”
And while it may seem odd for a Danish professor to conduct a study on American runners more than 7,000 miles away, it actually wasn’t — they’re Andersen’s main clientele in his other job.
Andersen, who in 2014 founded RunRepeat.com, said he chose to focus on American runners because those are the people who "predominantly" use his website, which he created as a tool for running enthusiasts such as himself to learn which shoes are actually best for their athletic needs.
Also, with all the races that occur in the U.S. on a yearly basis, he felt certain he could acquire enough results for statistical significance.
It appears he did.
According to one of the graphs in the study, the average American runner finished a 26.2-mile marathon in around 4:00:21 in 1996, but that time has grown to around 4:39:42 minutes as of last year.
The study also identified a similar time increase in 13.1-mile half marathons (from 1:55:09 in 1996 to 2:25:32 in 2016), 10K races (from 00:56:26 in 1996 to 01:14:29 in 2016), and 5K races (from 00:30:58 in 1996 to 00:38:59 in 2016).
In his study, Andersen said “the year 2016 was the slowest in history,” but this change wasn’t because of one unexplainable spike — it was a gradual development.
“It’s not like we were fast for 20 years and then all of a sudden we’re slow in 2016,” Andersen said. “It's just a steady slow down over time. I was really surprised to see the results being so significant, that year by year we getting slower it.”
But what’s the reason — or reasons — behind the increase in marathons finishing times?
That’s what Andersen hasn’t quite figured out.
As sure as he is that Americans runners have gotten slower over the last two decades, he can’t say with certainty what’s the cause, though he and Nikolova debunked several myths in their research.
For one, they proved a higher number of female participants hasn’t been the issue. In fact, the study states that the rise in female runners has “less effect in the slowing of pace than the decrease in the speed of men.”
The study also proved that the increase in finishing times is not due to people with inappropriate fitness levels who just walk the race, nor is it due to “the slow getting slower.”
So what is making Americans slower?
Andersen and Nikolova found “great correlations” with the slowing pace and four health-related parameters for Americans — adult obesity, teenage obesity, diabetes and hypertension, and average annual medical expenditure.
The source of the data for the health-related parameters that the researchers used was State of Obesity, confirmed by the Center for Disease Control and Prevention.
But despite finding connections between the deteriorating health of Americans and their slowing pace, Andersen stressed that it’s important to note the lack of evidence for a causal relationship.
“We had some really good correlations, but there's a difference between correlation and causation,” Andersen said.
Though he is yet to find the cause for the slowing pace, Andersen is certain that there is indeed a slowing pace.
“If you look at the graphs in this study, you see for every single race distance — marathon, half marathon, 10K and 5K — we just had never been slower,” Andersen said. “It's just so significant, and the trend line and the correlations are so clear. I’ve rarely seen anything as clear as this study.
“For a statistician such as myself, you rarely find a case that is as significant as this one.”
The full analysis is available at: https://runrepeat.com/american-runners-have-never-been-slower-mega-study.