Survey Methods
Results for this Gallup poll are based on telephone interviews conducted July 20-24, 2011, on the Gallup Daily tracking survey, with a random sample of 1,088 Republicans and Republican-leaning independents, aged 18 and older, living in all 50 U.S. states and the District of Columbia.
For results based on the total sample of Republicans, one can say with 95% confidence that the maximum margin of sampling error is ±4 percentage points.
http://www.gallup.com/poll/148664/Romney-Leads-Field-Announced-GOP-Candidates.aspx