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Explanation from MSNBC:

"As you know, the sample for the March 24-25 poll on race included an “oversample” of 100 African American voters. There has been some confusion as to exactly how these extra interviews were integrated into the survey; we hope this memorandum will clear up any misconceptions."

"The main sample for the survey was a cross section of 700 registered voters nationally. As is the case with all of our usual polls, this sample is statistically representative of voters across the country, accurately reflecting the gender, age, educational, geographical, and racial makeup of the electorate. The column in the topline document labeled “All Voters”, as well as nearly all of the subgroups listed in the survey crosstabs, are among these cross section of 700. Eleven percent (11%) of these interviews -- or 77 interviews -- were with African Americans, which accurately reflects African Americans’ proportion of the electorate. Thus, African Americans are NOT over-represented in our national sample."

In addition to this national cross section, we interviewed an extra 100 African Americans to analyze the opinions of this group with a greater degree of statistical reliability. We combined these 100 only with the 77 African Americans that naturally fell into our national sample, for a total of 177 interviews with African Americans; these extra interviews were not combined with the full national sample of 700. The column in the topline document labeled “African Americans” shows the responses of these 177 respondents, as do the subgroups in the crosstabs for African Americans, African-American men, and African-American women.

The table below shows the margins of error for the three groups whose responses are shown in the topline document:

National cross section of voters: 700 interviews, +/- 3.7%
White voters: 520 interviews, +/- 4.3%
African-American voters: 177 interviews, +/- 7.4%