This radio story by Mara Liasson (reporting for NPR) discusses the surprising primary loss of former Republican House Majority Leader Eric Cantor. It was surprising because internal polling conducted by Cantor's team gave him an easy win, but he lost out to a Tea Party favorite, David Brat. The story goes on to describe why it is becoming increasingly difficult to conduct accurate voter polling via telephone and the internet.
Some specific points from this story that teach students about sampling techniques:
1) Sample versus population: One limitation of polling data is the fact that many telephone call-based sampling techniques include landlines and ignore the growing population of people who only have cell phones.
2) Response rates for political polling are on a decline, making the validity of the available sample shrink.
3) Robocalls, while less expensive, have no way of validating that an actual registered voter is responding to the questions. Additionally, restrictions on placing robocalls to cell phones (but not landlines) create more difficulties for pollsters.
I used this example as a way of introducing sampling error and, eventually, the distribution of the sampling mean/standard error.
I emphasized that Cantor's polling had been conducted by high-end polling firm McLaughlin and Associates (source). I also explained just how powerful Eric Cantor was and if an extraordinarily rich and powerful person using a very expensive polling firm has to contend with sampling error, then what does that say for the rest of us?
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