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 the 2004 Democratic convention, Rasmussen Reports polled 1500 likely voters at random and found that 47% favored John Kerry. Just afterward, they took another random sample of 1500 likely voters and found that 49% favored Kerry. That's a two percentage point increase, but the pollsters claimed that there was no bounce. Explain.