Look around the web, and you’ll see lots of discussion of Hillary Rodham Clinton’s “10-point win” over Barack Obama.

I think it was my sixth-grade teacher who told me always to round after the calculation rather than rounding the inputs to the calculation. Too bad she doesn’t work for any of the big media outfits.

According to CNN, Clinton has 1,258,000 of 2,300,000 votes. That comes to just under 54.7%, which means that she beat Obama by 54.7 – 45.3 = 9.4 percentage points. If you weren’t interested in too much precision, you might round it down to 9 points.

But instead, CNN rounds 54.7% up to 55%, and 45.3% down to 45%, giving Clinton an apparent 10-point win. Everyone else does the same.

And that will still be apparently true even if, as I estimate, the final margin after those last Delaware County and Philadelphia precincts come in is 54.5+ to 45.4+.

So a margin of about 9.1 points gets translated into a margin of 10 points: a single-digit win into double-digit win. And all because the networks don’t think their viewers can stand three significant figures, and the pundits can’t be bothered to do their own arithmetic.

Update A reader points out that the rule “calculate, then round off” applies only when you’re rounding off just to save space. If the rounding reflects the limits of precision of the underlying measurements, it’s better to get rid of the noise first.