When I taught research methods and statistics to graduate students at Brooklyn College, if a student submitted a project like the daily and weekly political polls featured during the recent presidential campaign, the student would receive a low grade or fail the course. I would explain to the student that at best his or her poll was what is called a pilot investigation–a work in progress–to identify the issues and obstacles for designing a valid piece of research.
The same can be said about many of the political polls.Their primary flaw was in the critical first link in the chain: the sampling, which refers to how the pollsters selected the people they queried and how many participants were in the final samples from which conclusions were drawn.
If the purpose of a poll was to assess preferences or intentions to vote for Donald Trump or Hillary Clinton in a particular state, the sample should have had a sufficient number that included the diversity of the voting population–by age, religion, ethnicity, political affiliation, education, and income. Many samples did not. For example, according to the Independent Voter Network (IVN), the CNN polls did not have adequate representation of 18-34 year old voters, a demographic of 75 million, the largest living U.S. generation; and Fox News polls notably under-sampled independent voters.