As of this morning we have made some small changes to the algorithm that classifies leaders in each state on the maps we display for the Presidential, Senate and Gubernatorial races. As quite a few readers noticed earlier in the week, there were some odd inconsistencies in the way the margins separating the candidates translated into "toss-up" or "lean" status. So we have changed the classification slightly to make the process more consistent and intuitive.
The bottom line is that the new criteria shifts three states into the yellow toss-up designation, Virginia, North Carolina and Arizona (more about that last one below). Also, two states that had been designated strong Obama (Michigan and Iowa) are now lean, while Georgia moves from strong to lean McCain.
The inconsistency between states was partly technical (having to do with the way we calculated the margin between the candidates) and partly a function of calculating a confidence interval around the trend line. That process meant that states with more polls in their chart were producing much narrower "margins of error" than polls with few.
So we have changed to using a confidence interval based on the average sample size for the available polls in each state (essentially the same approach we used during 2006). That means that the margin necessary to be classified a leader will be slightly smaller in states where pollsters tend to conduct more interviews (such as Pennsylvania, Connecticut and Texas). But overall, things will be more consistent.
This choice may tend to overstate the uncertainty about a leader in some cases, especially where we have a huge number of polls (such as the National trend). Given all of real-world variability (and volatility) built into opinion surveys, particularly the difficulty of approximating the "likely" electorate at this stage in the contest, we believe it is better to err on the side of less certainty rather than more.
We are also using a totally subjective standard which will on occasion produces some strange results. We believe the "smoothed" loess regression trend lines that appear in the charts provide the best visualization of the trends that underlie available data. It also has the very helpful feature of essentially ignoring "outlier" values in most instances. One poll in ten, no matter how out of line with the trend, rarely budges the trend line.
But there is one circumstance that can produce squirrelly results: When a new "outlier" poll appears in a state with relative few polls, especially if several weeks or months have passed since the last poll.
That, unfortunately, is exactly what happened in Arizona, not once but twice in late June. First, the ASU/Cronkite released a survey based on 175 interviews showing 34% of registered voters as undecided and producing much lower than usual percentages for both McCain and Obama. Next, the always shaky Zogby Internet Panel survey showed a three point Obama lead (and included Barr at 7%). The combination has narrowed the McCain's trend-line margin to just 3.5 percent, just enough to edge below the "toss-up" line.
If we throw out the Zogby poll, the trend lines would now show McCain leading by 10 percentage points. If we left the Zogby poll in, but threw the ASU/Cronkite poll out, McCain would lead by more than 6 points. Either margin would easily classify Arizona as lean McCain. The next poll out in Arizona will most likely have the same effect.
And as long as we are on the subject of housekeeping, please note that by popular demand we added back a "blog roll" of links to all 50 state charts and all of the charts with available data in race. That collection of links now appears on our front page and everywhere else on Pollster.com. So if you are not a fan of the maps, or just prefer to jump from chart to chart with one click, the links are back in place.