The CNN / ORC Poll was done between October 29th and November 4th of a random sample of Iowa Democrats (498) and Republicans (548). The margin of error for Democrats is 4.5 points, and for Republicans is 4.0 points.
The Gravis Marketing Poll, released on November 5th, queried a random sample of 273 Iowa Democrats and 336 Iowa Republicans. The margin of error for Democrats is 6 points, and for Republicans is 5 points.