Dietary Innumeracy

The U.S. Department of Agriculture announced that high prices are not the reason people don't eat enough vegetables. They are probably right, but for the wrong reasons. (Note that the actual USDA study is not available on the internet, but that many newspapers picked it up, including this one.).

If you dig through the math, the point they make is that the recommended 3 servings of vegetables per day can be had for about 12% of the typical household food budget.

This conclusion is nonsense for three reasons: one basic, one statistical, and one economic.

The basic problem is that the food pyramid recommends different numbers of servings from different food groups (to correspond to people of different sizes). How then can their be one budget number for food, when there isn't one recommendation? Obviously the budget numbers must be an average, but this begs the question of whether this is an average across only those who follow the food pyramid, or across the whole population. If it is the latter, is the percent of the budget spent on the 3 servings of vegetables that they probably don't buy in the first place relevent to anyone? If it is the former, why should we care about a conclusion that isn't relevent to the population as a whole?

The statistical part is based on the food pyramid, which recommends 3-5 servings of vegetables per day. So, they have calculated this based on the minimum number of servings. Then this is combined with the 6 to 11 recommended servings of grains, the 2 to 4 of fruits, and the 2 to 3 servings (each) from the dairy and meats groups. But, while it is clear that they are assuming that people are eating the minimum number of vegetable servings (which is rather odd when the point of the press release was to push vegetables), it is not clear how many servings from the other groups they are assuming. One possibility is a person eating the minimum of everything, in which case the 3 servings of vegetables is 20% of their daily food intake (3 of 15 servings). Another possibility is someone who eats the maximum of everything else with the minimum of vegetables, leading to vegetables being 12.5% of their diet (3 of 24 servings).

Put this together and the Department of Agriculture is telling us that: 1) for someone who is small and eats the right proportion of vegetables that those vegetables are relatively cheap (20% of their diet costs 12% of their budget), and 2) for someone who is larger and could stand to eat more vegetables that they are neither cheap or expensive (12.5% of their diet for 12% of their budget). So, the implication is that people who should eat more vegetables have no economic incentive to, and those that eat enough should shift towards what is relatively cheap for them to consume (carotinitis here we come). In fact, there may even be a perverse income effect possible in that case.

The economic problem is that vegetables often require more preparation time than other foods. But, the claim that vegetables are cheap relies only on prices of unprocessed raw vegetables. It does not include the opportunity costs of preparation. That means that using the store prices overstates the cost of preprepared foods and understates that of unprepared ones. Which means that all of these budget numbers are understated. This furthers the point that the USDA is providing information to people who need to eat more vegetables that they are cheap, when in fact they are relatively expensive. One more snow job from the federal government.

To me, this is starting to sound an awful lot like a reason not to eat more vegetables. I don't need any more excuses than I already have.

Also, note that the USDA isn't the only one making these inane pronouncements. Check out this earlier post about Hollywood's innumeracy regarding the summer movie season.

Comments

[Jacqueline Mackie Paisley Passey]

What I'd like to read more about is the health and nutritional value of the foods that are most heavily subsidized. I vaguely recall an article that said the subsidies mostly went to grains, sugar, and dairy products, and that there were very few subsidies for fresh fruits, vegetables, and fish. Do you know where I could find info on agricultural subsidies broken down by food item? Thanks.