Page 2 of 4 Debate Effects As Bush and Kerry Prepare for Their First Match-Up, What’s at Stake? Continued 
Note that when change does occur, it isn't necessarily directly debate-related.
Candidate support sometimes shifts with no debate to pin it on. If it happens around a debate, that's said to be the reason — causality is assumed, like someone guessing what moved the stock market.
Change
Measurable effects show that debates — at least in immediate reactions — mainly tend to reinforce preconceived notions rather than change them; most of each candidates' supporters say it's their guy who won.
Typical was our first debate-reaction poll in 2000: Seventy-nine percent of Al Gore supporters said Gore won; 70 percent of George W. Bush's supporters said Bush won. And 93 percent of each candidate's supporters stuck with their pre-debate vote preference. Among debate watchers, Gore's support was 45 percent before the debate and 45 percent after it; Bush's went from 48 percent to 49 percent.
Still, while the debates rarely prompt much change, post-debate evaluations can. In 1992, immediately after the first debate, 24 percent of debate watchers said Perot had won. By the very next night, however, that perception had grown to 37 percent among people who either had watched it, or heard or read about it. And as noted, Perot's support did advance, from 6 percent before the debates to 17 percent after them.
Post-Debate Polls
The post-debate polls we've been describing are full-population surveys that capture the state of the overall race. We'll have one of those soon enough. First, though, we will have an immediate debate-reaction poll tonight.
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