We often see examples of insufficient precision, as when thirteen workers are killed in a building collapse, and a news report says that “more than a dozen workers died.” We can see that they’d want to hedge if there were still uncertainty about the number of deaths, but sometimes they just seem to think it sounds better this way. And if the number is high, it often makes more sense to report “more than six hundred” than to use the more precise “six hundred fourteen.” But when I hear “more than twenty,” I want to ask, “What does that mean? Twenty-one? Twenty-three?” If they’re uncertain, “at least twenty” makes that clearer.
On the other hand, there are times when there’s no value in more precision, and getting it just puzzles us. The Boston Globe’s excellent The Big Picture series has one of those in its 2009 in photos set. The caption to photo 32 of part 3 says this:
A girl dives in a lake 100 km (62 miles) west of St. Petersburg, Russia, in this Aug. 7, 2009 photo.
Now, it’s unlikely that the lake is exactly 100 km from St Petersburg. It might actually be, say, 98 km away, or 103... or even 110. “100” is the sort of approximation we expect in a place such as this, a nice, round number that’s close enough for the purpose. So when someone punches that into a converter and gets 62.137 out in miles, we’d rather expect that to turn into “60 miles”. It would jar us to see the figure given to three decimal places, but it’s still kind of weird to see the the precision of “62”, given that the accuracy of the original “100” wasn’t good enough to warrant that. (And, conversely, seeing “62” might lead us to interpret “100” as being more accurate than it is.)
I saw a similar thing in an American airplane recently, where writing on an exit door said, “Caution: Door weighs 35.3 pounds.” That’s clearly a rounding of 16 kilograms (35.274 pounds) up to the next tenth of a pound, but why pick that particular precision? The point, here, is to warn someone who might have to manipulate the exit door about how heavy it is. “35 pounds” would be just fine for that — a quarter of a pound won’t make any difference, and if they think that matters then “36 pounds” would do as well. Is the door really 16 kg, and not, say, 15.8 or 16.3?
Think about what you’re stating (and converting), and use a precision that’s appropriate to the purpose. And if you’re worried about nitpickers, qualifiers such as “about” or “at least” are your friends.