<P> A comprehensive discussion of these biases and how they should be understood and mitigated is included in several sources including Dillman and Salant (1994). </P> <P> A widely publicized failure of opinion polling to date in the United States was the prediction that Thomas Dewey would defeat Harry S. Truman in the 1948 US presidential election . Major polling organizations, including Gallup and Roper, indicated a landslide victory for Dewey . </P> <P> It was widely predicted that Donald Trump would lose the 2016 US presidential election to Former US Secretary of State Hillary Clinton; however, Donald Trump was elected the 45th President of the United States and was inaugurated on January 20, 2017 . Many publications, such as FiveThirtyEight, predicted that there was a significant chance of the Trump winning . </P> <P> In the United Kingdom, most polls failed to predict the Conservative election victories of 1970 and 1992, and Labour's victory in 1974 . However, their figures at other elections have been generally accurate . In the 2015 election virtually every poll predicted a hung parliament with Labour and the Conservatives neck and neck when the actual result was a clear Conservative majority . On the other hand, in 2017, the opposite appears to have occurred . Most polls predicted a Conservative landslide, even though in reality the election resulted in a hung parliament with the Conservatives losing their majority . </P>

Which is a main purpose of public opinion polls