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Etienne LeBel

I loved the bears repeating photo!!

Commenter

In my experience, this is really an issue of statistical training. Personality psychologists and social psychologists -- especially from top programs -- tend to learn a great deal about why one should avoid median splits, why one should use latent variables, why one should examine familywise error rates and so on. But they don't get a great deal of training on how to measure multicollinearity, how to deal with endogeneity, and so on. In economics, the inverse seems to be true. So training is idiosyncratic to the discipline.

In addition, sociology and public health seem to train people to think of age as a categorical variable. Students who learn this then become professors in the discipline, and thus this idea becomes institutionalized. Most sociology and public health journals in 2014 will accept articles in which age is treated as a categorical variable, simply because that's now an institutionalized standard and it's not even considered to be problematic. And I don't think they're entirely blameworthy because this is a case where they don't know what they don't know.

(Incidentally, I once emailed the first author of 'Looking Deathworthy' to find out why she used a median split in her analysis. I never got a response. Not sure how that median split got past Psych Science reviewers. Anyway the article has now been cited over 250 times.)

Stuart Buck

A recent dietary study fell prey to this very problem: http://stuartbuck.blogspot.com/2014/03/meat-smoking.html

Dave

As a PhD student, training under a personality psychologist, I was warned about the perils of median splits and "bucketing." However, I recently spoke with someone from a marketing firm. It turns out that they use age as a categorical variable. Hearing this, at first I was shocked, but the rationale is simple. If you have are trying to predict college enrollment, you probably don't predict a linear relationship with age. Using 0-17, 18-22, 23-27, works a lot better. I am not saying that most (or even half) of the cases in which this technique is used are justified this well, but sometimes, it may be the best strategy.

Ryne Sherman

I personally find developmentalists (psychologists or otherwise) to be the worst offenders here, defending that such "buckets" represent "qualitatively different age groups." But that is just my experience.

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