The CD 10 Polls, in Graphic Form
Margin is narrowing between McCaul and Doherty
By Lee Nichols,
5:20PM, Mon. Oct. 27, 2008
When we interviewed Democratic Congressional District 10 candidate Larry Joe Doherty earlier this month, he told us his campaign was "on track to peak on Nov. 4." The Pollster.com illustration to the right, showing trendlines of the four polls that have been taken in the race since May, seem to indicate the strategy to unseat incumbent Michael McCaul may be working.
After the jump is an explanation from Pollster.com as to how the trendlines are determined. Frankly, it's gobbledygook to me, but I think I'm at least bright enough understand the end product.
The dots represent the actual numbers for the individual polls.
How do regression trend lines differ from simple averages?
Charles Franklin, who created the statistical routines that plot our trend lines, provided the following explanation last year:
Our trend estimate is just that, an estimate of the trends and where the race stands as of the latest data available. It is NOT a simple average of recent polling but a "local regression" estimate of support as of the most recent poll. So if you are trying to [calculate] our trend estimates from just averaging the recent polls, you won't succeed.
Here is a way to think about this: suppose the last 5 polls in a race are 25, 27, 29, 31 and 33. Which is a better estimate of where the race stands today? 29 (the mean) or 33 (the local trend)? Since support has risen by 2 points in each successive poll, our estimator will say the trend is currently 33%, not the 29% the polls averaged over the past 2 or 3 weeks during which the last 5 polls were taken. Of course real data are more noisy than my example, so we have to fit the trend in a more complicated way than the example, but the logic is the same. Our trend estimates are local regression predictions, not simple averaging. If the data have been flat for a while, the trend and the mean will be quite close to each other. But if the polls are moving consistently either up or down, the trend estimate will be a better estimate of opinion as of today while the simple average will be an estimate of where the race was some 3 polls ago (for a 5 poll average-- longer ago as more polls are included in the average.) And that's why we estimate the trends the way we do.