The maps that now appear on the front page of Pollster.com and other parts of the site allow you to quickly scan the latest trend poll trend estimates for every state in the Presidential race, as well as races for Senate and Governor (with U.S. House coming soon). The maps also allow you to navigate to our poll charts by clicking on the state.
How do I use the map to see data for Senate and Gubernatorial races?
In the upper left corner of the map, under the heading "Map Chooser, you will see a pull down menu that allows you to change the map to display races for President, Senate and Governor in 2008. The Chooser menu also includes a Find All Polls option that displays a map that takes you to index pages for each state. These index pages automatically update to include all chart pages on Pollster.com, including archived data from 2006.
Where do the numbers come from?
When you hold the mouse pointer over a state, you see a display of the latest "trend estimate" numbers from our charts of all available public polls for that race. The numbers for each candidate correspond to the most recent trend estimate -- that is the end point of the trend line that we draw for each candidate. If you click the state on the map, you will be taken to the page on Pollster.com that displays the chart and table of polls results for that race.
In most cases, the numbers are not an "average" but rather regression based trendlines. The specific methodology depends on the number of polls available.
- If we have at least 8 public polls, we fit a trend line to the dots represented by each poll using a "Loess" iterative locally weighted least squares regression.
- If we have between 4 and 7 polls, we fit a linear regression trend line (a straight line) to best fit the points.
- If we have 3 polls or fewer, we calculate a simple average of the available surveys.
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.
What do the scoreboards represent?
For the presidential race, we add up the electoral votes represented by each category of our classification. So we have allocated all 538 electoral votes as either strongly supportive or leaning to a candidate.
For the race for Senate, we have classified the current standing of the 35 contests and added those numbers to the party affiliations of the 63 Senators not facing an election this year. As such, the large numbers in the middle of the scoreboard should add to 100. The two senators labeled as "other" (Bernie Sanders of Vermont and Joe Lieberman of Connecticut) caucus with the Democrats.
We follow the same procedure for the races for Governor, plotting our classifications for the 11 contests for Governor and adding those numbers to the party affiliation of the remaining 39 governors not up for election this year.
What is the basis of the classification of each race?
Regardless of the number of polls, we calculate a "confidence intervals" around the trend estimate based on the average sample size for the available polls in each state. These intervals reflect the uncertainty in the estimate due to random noise in the polling data.
If a race shows a lead that is outside the 95% confidence interval, then
we classify this as a "strong" lead. If the lead is between the 68% and
95% confidence intervals, then we classify it as a "lean". If the race
is inside the 68% confidence interval, then we classify the race as
"too close to call."
Why are are some states classified even when no polling data is available?
More often than not, public polling data is not yet available in races not considered competitive. So rather than treat such cases as "toss-ups" we default to the standing of each race as determined by our colleagues at the Cook Political Report.
Why aren't all candidates listed?
Our charts and maps are based entirely on public polling data collected and released by other organizations. We have no control over the candidate choices they offer or any other aspect of the design of their surveys. We include on the map candidates that have been offered as a choice on at least half of the last six polls and when the trend estimate for that candidate is at least three percent of the total.
Where did the 2006 maps and polls go?