The odds are very good that shortly after President Obama completes his health care address this evening, at least two television networks will release results from "instant reaction" surveys. Others will likely stage focus groups in which selected participants react to what they see and hear. The odds are almost as good that pundits and partisans will grossly misread or spin whatever those surveys and focus groups produce. Here is a primer for those hoping to make sense out of whatever new poll data we see over the next 24 hours.
1) Instant response polls measure only speech-watchers. While the methodologies vary, the most important thing to remember that these surveys aim to sample only those who watch the speech and, as such, are are not intended to represent the views of all Americans. The pollsters will hopefully provide some before-and-after comparisons of the speech audience -- showing how viewers felt about health care reform before and after the speech -- but those comparisons will involve only the sample of speech viewers. Thus, no one should take any of the numbers they see tonight and make comparisons to full-sample results from previously surveys of all adults or all "likely voters."
2) The audience is usually skewed toward the president's fans. Remember, not all Americans watch presidential addresses. Out of roughly 113 million television households in the U.S., between 52 and 63 million watched the debates last fall and roughly 53 million watched President Bush's address on the economic crisis last September. Those are huge audiences, but plenty of Americans still tune out.