In Comparison To What?

A shaggy dog story

I love tennis and play as many hours a week as I possibly can. I also love my dog and the time we spend together on long hikes. One would expect that if I were to get injured, it would come either from the long hours I spend on the tennis court or the steep grades of the hiking trails near my house. I have, in fact, hurt myself on multiple occasions while engaging in both activities.

But in the summer of 2023, I suffered a serious knee injury in a completely unexpected way … while standing still. I’m not a dog-park person, more of a hiker, as I said. But while on a vacation where there weren’t any good hiking places nearby, I took my dog to a dog park. Just as I came in, a woman threw a ball right by me, and her dog  – the most compact, muscly bulldog I have ever seen – ran full speed into my right knee.

I fell to the ground in a lot of pain, but shook it off and stood up. When I realized I could walk, I thought, I’ll be okay. It should be fine in a few days. But it was still throbbing over a week later, though not enough to get me to a doctor. I was walking fine and with enough ibuprofen I was still hiking and taking some tennis lessons. 

About ten days after the injury, I decided that I was in good enough shape to play a competitive tennis match. I loaded up on seven Advil (not sure why I thought that was reasonable) and three hot hours later walked off the court with a victory surprisingly pain-free, although I guess 1400 mg of ibuprofen will do that for you.

By the next day I could barely put any weight on my leg without excruciating pain.

Playing instead of resting wasn’t the smartest decision I have ever made by a long shot, but it did finally get me to go to an orthopedist. 

I told the doctor about my run-in with a canine cannonball and described the pain in my right knee. Like any good orthopedist, she began to manipulate my injured knee, looking for signs  of ligament damage.

“This joint is really mobile. I think you might have a torn meniscus or an ACL tear.”

My response? “Could you please check my left knee?”

Why did I ask her that?

Well, because the doctor wasn’t comparing apples-to-apples. 

When she was manipulating my right knee, she was comparing its mobility to all other knees she had manipulated before. Clearly, the mobility of my right knee was out at the tail of that distribution, on the hypermobile end of the scale. That led her to the conclusion that I must have torn a ligament. 

But the more appropriate comparison would have been to my uninjured left knee.

Now, it was easier for me to spot the error because I happen to know something about my own knees, which is that my joints are hypermobile. But I also know a little something about control groups, which is just the group in an experiment that is used as the standard of comparison for the group that is receiving some treatment. The control group allows a scientist to know if the intervention they are testing has an effect on an outcome they are interested in studying.

Control groups are the cornerstone of the scientific method. 

The best standard for figuring out if my right knee was unusually mobile was my uninjured left knee, not all other knees the doctor has seen. My left knee offered a natural control group. It offered a better comparison for figuring out if the injury has caused the hypermobility or if my knee was hypermobile independent of the injury.

When the doctor manipulated my left knee, lo and behold, she remarked, “Wow, your left knee is also really hypermobile. I don’t think you tore anything, but you may have a fracture. We should get you an MRI.”

It turned out that my knee was broken. I had a very severe bone bruise from the dog running into me and that caused a hairline fracture of my fibula because I didn’t rest it.

Here, we get to the core of the problem, which is understanding how you’re supposed to model facts available to you for making decisions. You could be comparing sales quarter over quarter to determine judge performance of you sellers without taking into account the general economic expansion or contraction of your sector. Or you could be comparing starts for two baseball players to figure out who’s better without taking into account the dimensions of their home park (it matters if it’s Yankee Stadium or Fenway!). Or you could be comparing the mobility of my right knee to all other knees you’ve seen before instead of to my left knee (that’s literally sitting next to the right one).

We make these types of faulty comparisons all the time, and they lead us to faulty conclusions. Those conclusions, in turn, drive errors in our decisions.

And that’s why this all really matters. 

“In comparison to what?” might be the most important question you ask whenever you encounter information. Without it, the quality of your decisions will suffer.