CHARLESTON, South Carolina (Reuters) - Barack Obama has a 13-point lead on rival Hillary Clinton but his support has eroded slightly on the eve of South Carolina’s Democratic presidential primary, according to a Reuters/C-SPAN/Zogby poll released on Friday.
Obama’s edge on Clinton slipped by two points overnight but remained in double digits, 38 percent to 25 percent, in the rolling poll, with John Edwards gaining two points to climb to 21 percent and inch closer to second place.
The poll has a margin of error of 3.4 percentage points.

can you really say there’s a gain if the change is within the margin of error? technically, the poll measured no change at all.
persistent ignorance of what a margin of error means is one of my pet peeves.
He’s up seven points in one week, relative to the other candidates. Make of it what you will.
How did the fundraising blitz turn out? Even I chipped in a few bucks…
He’s up seven points in one week, relative to the other candidates. Make of it what you will.
if it exceeds the MOE, it’s probably measuring something real.
if the poll you’re referring to has the same MOE, then it’s probably measuring something. you effectively have to double the MOE to tell if you’re talking about the difference between two candidates (as each candidate’s score could wobble within the MOE. so obama’s “true” score could be as much as 3.4% lower and edwards’ could be as much as 3.4% higher. which means an apparent spread of as much as 6.8% could be nothing but polling error).
If either Clinton or Obama gained 7 points in a week, the media would be howling it from the rafters. Edwards gains and all we hear is crickets chirping.