The first problem with the Center for Security policy poll has to do with methodology. It was an online, opt-in survey, which tend to produce less reliable samples because respondents choose to participate. In traditional polling methods, everyone in a population has a chance of being selected for the survey, meaning the results generally reflect the country’s demographics.
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There may be further problems with the poll, including the reality that many American Muslims are immigrants and not fluent in English, and that the survey asked leading questions with limited response choices, according to a critique by the Washington Post’s Philip Bump.
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Trump is referring to a poll conducted by the Center for Security Policy. However, polling experts raise numerous questions about the validity of the poll’s results, including its "opt-in" methodology and the dubiously large percentages of respondents who said they were unaware of ISIS or al-Qaida. Moreover, an official with the Center for Security Policy cautioned against generalizing the poll results to the entire Muslim-American community.
Another survey, which experts consider credible, found levels about half as high as what the Center for Security Policy poll found.