Sunday, March 23, 2008

Death of the Diet: Doubting the Data

I wish I had known last Tuesday was “Death of the Diet Day.” I probably would have celebrated it. It sounds like a really fun day.

But why March 18th? That’s supposedly “the day when more commitments will fall by the wayside than on any other day in the calendar” (in the U.K.). The article implies that the commitments in question are New Year’s resolutions. So in other words, DoD Day is defined as the modal day of commitment breaking. Why not report the median or mean day?

It’s easy enough to dismiss the mean. If there are enough people who successfully keep their commitments indefinitely, then it’s mathematically impossible to calculate the mean, because the largest observations are excluded.

But what about the median? That would seem to be more useful than the mode; it would tell the day by which exactly 50% of commitments have been broken. So why not tell us that? I have a hypothesis, though I’d need to see the underlying data to be sure. If the distribution of diet-breaking dates is positively skewed, meaning we have a relatively large number of people keeping their commitments a long time (or indefinitely), then the median is probably a good bit later than the mode. That is, the situation probably looks something like this:

So reporting the median would mean waiting longer to announce the failure of our collective willpower, and that would not be consistent with the ongoing campaign to convince Westerners that we’re all too fat and need somebody to come to our rescue.

I’m also skeptical about the very notion of a diet being broken on a specific day. I’ve never had a successful diet that didn’t allow for the occasional indulgence. To construe any one indulgence as “breaking” the diet is to miss the point badly. Having treats every once in a while is part of a good diet. The demise of a diet is not revealed by what happens on any single day, but by a pattern of behavior over time.

1 comment:

Lasker said...

Is it really believable that a an arbitrary day like March 18th has more broken commitments than January 2nd? (Or January 1st). I'd bet those are the true modes.