Political pundits talk about the "bounce" that a presidential candidate gets after his party's convention. in the past 40 years, it has averaged about 6 percentage points. just before a recent convention, a random poll of 1500 likely voters found that 56% favored a particular candidate. just afterward, another random sample of 1500 likely voters found that 59% favored that candidate. that's a three percentage point increase, but the pollsters claimed that there was no bounce. explain.