The biggest problems with online surveys are the potential for survey fraud and the consequences of that fraud for the business conducting the survey. If people receive rewards for taking surveys, especially if they get money, they will want to take as many surveys as they possibly can. This leads to a lot of cheating by people who repeatedly falsify their demography so that they can become eligible to take the same survey numerous times. They also don't read and consider the survey questions; they just fill out the survey rapidly and haphazardly.
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