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ChristianKl comments on Notes on Psychopathy - Less Wrong
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I've noticed this elsewhere (looking into ADHD), Psychiatrists seem interested in developing a criteria which seems naturally continuous, and then using a cutoff without arguing for why that's a good idea. I can easily imagine that some conditions are discrete, but many of them must be pretty continuous. It seems like they would lose a lot of statistical power with a cutoff approach.
Is this purely a historical accident? Is it because discrete judgments seem more authoritative? Is there an actual good reason that I can't see? What's going on here? This sort of thing makes me suspicious of the quality of psychiatry research.
Having a cutoff doesn't stop you from running your statistical test on the scores.
You need a cutoff to decide whether to accept an individual into your study. If you want to study whether psychopathy gets reduced by a treatment it makes sense restrict your study to indiduals who's score is over a certain value.
Having all studies use the same cutoff value helps you to compare different treatments.
That's true, but at least in the ADHD research I saw this is not the way cutoffs were used, they were used to categorize people as "have ADHD" or "not have ADHD" and the "have ADHD" were compared to "not have ADHD".
ADHD seems to get diagnosed based on the DSM-IV. It's not binary. There are three types "ADHD, Combined Type", "ADHD, Predominantly Inattentive Type" and "ADHD, Predominantly Hyperactive-Impulsive Type". A DSM-IV diagnosis however doesn't give you are score.
There are huge issues with the DSM IV. In it's philosophy the DSM describes symptoms. As a result it's authors don't see the necessarity to back up their diagnosis with scientific evidence that get's cited in the DSM.
Psychopathy on the other hand gets diagnosed based on a specific PCL-R test that Robert D. Hare developed. The test is result of psychometric work. It's designed to predict recidivism and violence. The DSM-IV doesn't recognize psychopathy but instead uses the category of antisocial personality disorder.
In the case of depression you also have on the one hand the DSM-IV criteria and on the other hand the Hamilton Rating Scale for Depression.
Psychiatry studies that use tests that are optimized to predict something like the PCL-R should probably be trusted more than the DSM-IV. The present debate about the new DSM-V can also give you a good illustration of it's nature.
Sure, it's not binary but it is discrete.
The process used to design the PCL-R test does seem better.