Sunday, February 20, 2011

Use of decimals in polls: a thought exercise

Yesterday on CBC Radio's The House, Allan Gregg of Harris-Decima spoke more about what he has said recently on the At Issue Panel and in last weekend's Canadian Press piece by Joan Bryden on polling. Among other things, he spoke about reporting polls to the first decimal point.

He called pollsters who report in this way (EKOS and Nanos being the only ones who do so) "tremendously naive, or ridiculously deceitful". Things are getting personal. But competitors criticizing each other? Stop the presses!

But does he have a "point"? (snicker)

As Frank Graves of EKOS has mentioned to me, why conduct polls with larger sample sizes if you don't take advantage of their greater precision? Harris-Decima's last poll had a sample of over 3,000 people, but they rounded off their poll results. Yet, Harris-Decima's poll mentioned a margin of error of +/- 1.8 points. That becomes a rather useless number when you round off polling results.

Let's conduct a thought exercise using Harris-Decima's most recent poll, focusing on the Conservative result.Harris-Decima reported the Conservatives being at 37% support. With rounding, however, that could put the Conservatives at anywhere from 36.5% to 37.4%, and with the 1.8 point margin of error that means the Conservatives could be between 34.7% and 39.2%. In effect, it increases the margin of error to 2.3 points, rather than 1.8 points.

So, rounding off poll results gives us a far less precise and even more cloudy picture of the situation. The standard poll has a margin of error of 3.1 points, which gives a rounded off result of 37% a range of between 33.4% and 40.5% (or 33% to 41%, if we're rounding). If that result was reported as 37.0% instead, the range would be 33.9% to 40.1% (or 34% to 40%, with rounding). And that is with the exact same poll result.

Sure, an argument can be made that reporting to the first decimal point might be providing more precision than is necessary. But if polling results are going to be rounded off, perhaps reported margins of error should be as well. It certainly sounds better to say that a poll is accurate within 1.8 points rather than two points, but it would appear to be providing the same "unnecessary" precision.

To paraphrase Nate Silver, more precision is better than less. It is up to us to use that extra precision correctly, and within context.