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The sample size of black participants in Iowa is not statistically significant. Remember "margin of error" that comes with every poll (even exit polls). That's because the poll doesn't question EACH AND EVERY member of a group. They question just a very few people. Depending on the sample size and the ESTIMATE of the number of total participants of a given group that you want to characterize, you also have to calculate the margin of error. Given the total population of African Americans in Iowa and the sample size used to determine that groups preference. But let's say the exit poll snags, say, 2000 people to interview (you'd be surprised at how small the samples really are, I doubt anywhere near 2000 people were interviewed). Given that Iowa has something like 3 percent African American population, that's like 60 people. Oops. That group is way too small to determine anything about racial preference. They could have easily gotten 30 people that voted for Hillary or 0 people that voted for Hillary. Sample size of African Americans (given that they select 2000 people again) is going to be, what 1000, 1200? Now you can get a sample size that means something.
But to determine a trend based on that is bogus.
If you want trends, you need to sample the same demographics over a period of time. Not DIFFERENT demographics over a period of time.
One could just as easily make up different trends not based on race but on occupation or income or just about anything. But because you are using widely differing demographics, those trends are simply bogus as well.
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