Friday, February 27, 2015

(The Lack of) Methodological Problems in High-Rise Syndrome Cat Studies

There's a popular fact that gets thrown around a lot: cats can survive falling from great heights, and actually survive more often when dropped from a greater height (above seven stories).

There's also a popular rebuttal to this: the study which came to this conclusion was only looking at cats that came into the emergency room, and has what's called a survivorship bias. What they mean is that it could be that there are (hypothetically) a whole bunch of cats who fall from above seven stories which die before ever getting to the emergency room. The study therefore has a methodological flaw, and no valid conclusions can be drawn from it.

Here's the rebuttal to the rebuttal: that's a misreading of the study. The study in question is almost always the 1987 Whitney and Mehlhaff study, which went viral pretty quickly and was then just repeated ad nauseum since forever. What the study actually showed was that the severity of injuries to cats was curvilinear with height - injuries increase as you add height, until a certain point where they stop increasing and start decreasing with height. Specifically, limb fractures begin to decrease while thoracic injuries remain more or less constant (or with a mild increase).

So when a cat falls from a large height, we can divide them up into three groups.
  • Dead cats, which don't go to the vet
  • Injured cats, which go to the vet
  • Uninjured cats, which don't go to the vet
In any study of cats coming into the emergency room, we're only going to be seeing cats in the middle group. We can further divide the injured cats that we're seeing into three groups.
  • Severely injured cats
  • Moderately injured cats
  • Mildly injured cats
Now, if mortality were increasing linearly with height, we would also expect the severity of injuries to increase with height. This is not what we see happening. What we instead see happening is the severity of injuries decreases with height. If you want to maintain that there's a survivorship bias going on here, that bias needs to explain why we see fewer injuries among injured cats.

I've had this debate online a large number of times. In fact, that's why I'm writing this blog post. The generalized argument among the counter-study people is this: most of the cats that fell from high floors simply died. When I ask what plausible hypothesis explains why those that don't die don't see increased injuries, I usually hear back an answer that doesn't make sense. It's usually some variant on "they were lucky", which as you can imagine, I find scientifically lacking.

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