In the latest issue of Free Inquiry (journal of the Council for Secular Humanism), they charge that there's a persistent pro-religious bias on the part of religion surveyors.
MORE: http://marksilk.religionnews.com/2013/06/19/when-push-polling-comes-to-religion/#sthash.y4eUKZeF.dpuf
See also...
Cheating or Leveling the Playing Field? Rethinking How We Ask Questions About Religion in the United States
http://www.secularhumanism.org/index.php?section=fi&page=cragun_kosmin_33_4
