I just looked at the detailed info on the survey....there were more focused questions on 9/11, which surprise, surprise, have much lower proportions overall (in the 15-6% range). The question that asked whether respondents agreed with the statement that the "attack was carried out by 19 hijackers who were members of the al-Qaeda terrorist organization, led by Osama bin Laden" resulted in 12% disagreeing with the statement. However, this question was bundled with a statement about the 9/11 Commission, which may have promoted the "not sure" response because (1) most Americans probably are unfamiliar with the specific content of the 9/11 Commission report and (2) many Americans are cautious about accepting any official report at face value. A question that asked simply "Do you believe that the 9/11 attack was carried out by 19 hijackers who were members of the al-Qaeda terrorist organization led by Osama bin Laden?" would likely have elicted different results than the actual question.
The other questions inquired on whether the collapse of the WTC was a controlled demolition (15% in favor), whether United 93 was shot down (15% in favor), whether no airplane crashed into the Pentagon (13%), and whether no planes crashed into the WTC, such that the television images to the contrary were faked (6% in favor). However in this question, the respondents were not asked whether they believed these claims to be true. They were only asked if these claims are to be deemed as "credible". This is a significant difference. One could have no opinion whatsoever on a matter but still be asked to judge whether a claim sounds credible or not. So those who actually believe these claims is probably a smaller proportion than those who think they are "credible".
The relation between the TV fakery question and the controlled demolition question is interesting, since the no-planers who actually believe in TV fakery (which would have involved bombs in the buildings to simulate airplane impacts) would necessarily believe that the WTC destruction was a controlled demolition. The proportion of those who assented to the TV fakery question is 40% of that of those who assented to the question on controlled demolition. Online CTs regularly who do not agree with TV fakery claims depict the no-planers as "fringe", who do not really represent the common views of those who believe that 9/11 was an "inside job". But either this survey suggests that this is not the case, or there is a problem of representation in the survey methodology (as suggested above, that "I think it is credible" does not mean "I believe that it is so").
