How To Read These Charts

This chart plots a "polling trend" line that shows the trend over time in the average of the five most recent polls where respondents are asked whether they would vote for "the Democratic candidate or the Republican candidate in your district." The blue line plots the difference between respondents' answers. Values above zero mean more respondents picked a Democrat than Republicans. Values below zero show the preference for Republicans. The red horizontal line marks the border between Democratic leads above and Republican leads below.

You can click the check boxes in the "Key" legend to make dots appear showing the specific results for each poll. Blue dots in this chart indicate the percentage for the Democrat minus the percentage for the Republican on the generic ballot question.

The "Key" option called "Confidence Interval" displays a band of color that indicates the approximate margin of sampling around the results of each individual survey. Why go to this trouble? Political coverage often focuses on the raw numbers and the spread between them, but polling is not that precise. The bands help put the variation inherent polling in perspective. Since no pollster can survey the entire voting population, they choose a random sample in the state and accept that their findings will be approximate. The "margin for error," I think, attempts to quantify that random imprecision. The bands don't capture even bigger errors that can result from differences in question language or other pollster methodology, but they are our effort to display the sense of perspective required when viewing political polls.