Wednesday, August 7, 2013

Making the Grade

The author after a long day of grading.

I lost my TA, so I'm stuck doing the grading. I'm always amused that students, despite the bold-faced, ALL CAPS instructions to round all calculations to the nearest kilocalorie (kcal), still give me values like 338.58 kcal of protein, as if they actually had the ability to measure that precisely.

Such excessive, and in some students' work seemingly obsessive, expression of precision reminds me of the fallacy of misplaced precision, also called false precision, among other things, best illustrated, I think, by the perhaps apocryphal story of the scientists who, in a study of human gestation lengths, reported their subjects' pregnancies to be, on average, 38.001 weeks, implying that the scientists were on the scene to make their initial observations within 10 minutes of conception. Awkward!

The assignment in question includes a one-day food diary. One student observed that the difference between his estimated energy needs and actual intake could have been due to being more conscious of what he was eating throughout the day, causing him to eat smaller portions and healthier foods.

This is a great example of the Hawthorne effect, so-called because of the famous study at the Hawthorne Works electric factory, where subjects modify their behavior because it is being observed. This is a problem for anyone conducting nutrition research.

And here's a bonus fallacy that popped up in my Feedly: a case of statistical special pleading.

(Image credit: Language Arts Blog.)

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