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Your Mileage May Vary
by Lionel E. Deimel


On televised automobile commercials, advertisers sometimes state EPA mileage estimates, followed by statements like “your mileage may vary.” It is, of course, a virtual certainty that your mileage will vary. It is in the nature of automobiles and driving for it to do so, but that is not exactly what the advertiser intends to tell you. What is meant is that your mileage, varying as mileage is wont to do, will almost certainly not average out to either of the EPA figures. Moreover, if you buy the advertised vehicle and test it using EPA procedures, you will, with almost absolute certainty, get different numbers.

Family driving in carThe commercials, I think, are confused about the words “differ” and “vary.” Two examples hint at the differences between the two verbs:

The amount of gasoline in my tank when I decide to visit the service station varies.

The amount of gasoline in my tank differs from the amount in the tank when I last decided to visit the service station.

In the first instance, we are making a statement about—permit me to get mathematical here—many data points, whereas, in the second instance, we are making a statement about just two. Statisticians speak of measurements of some parameter varying over a range of values representing something else. In the first example, we are saying that, if we look at all times when I decide to visit the service station (the range of values of something else), the quantity of gasoline in my tank (the parameter) is not always the same. This is a more complex mathematical notion than that expressed by the latter sentence.

How does this discussion relate to the automobile commercials? The EPA measure of, say, city mileage is a single number, a statistic representing—to use a mathematical phrase again—a measure of central tendency. One's own mileage for city driving will, in general, be different each time it is calculated, but we said this is not what the advertiser is talking about. What he really refers to is the relationship of the EPA mileage and my measured mileage, however derived. We have, in other words, two numbers to compare. My mileage varies—in a trivial way—over all the measurements I take of it, but the important point being made is that it will most likely differ from the EPA value.

The advertiser could rescue his reputation for linguistic imprecision with another argument. The “your” in “your mileage may vary” might really be second personal plural, not second person singular. Over all car owners, mileage will indeed vary from the single EPA value. Of course, this argument makes the commercial rather less personal, and I doubt that the plural pronoun is the intended one. Whatever the statement says, I doubt any prospective buyers learn anything new from it.

Another commonly encountered confusion of “vary” and “differ” also is on television. A network, at the end of a promotion for one show or another, often ends with something like the following: “Sunday at 9, 8 Central. Local times may vary.” It is true that the time of the show, taken over all the network stations airing it, may indeed vary, though I doubt that the network believes the average viewer is interested in the fact. What the view really needs to know is that his local outlet my choose to air the program at a time other than the announced one. In other words, “your local time may differ” or “local times may differ.” Because of the use of “times” in the standard phrasing, it is easier to assert that the usual words are technically correct, but it is a stretch. The emphasis is simply wrong.

— LED, 2/5/2000

This essay had a small influence on a limerick called “Diversity” written several months later.

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