Random-digit-dialing (RDD) is a popular method for selecting people to be in polls.  So, the population from which a sample is drawn is anyone with a telephone (because RDD dials both listed and unlisted numbers).  A problem with RDD is that of bias as a result of nonresponse.

A study by Pew Research compared polling results using random-digit-dialing to the use of registered voter files.  In this study, it was found that in 56 of 65 survey questions, the two polling methods resulted in estimates that were statistically insignificant.

Have your students read the article and discuss issues such as frames, nonresponse bias, under-representation, and the overall challenges in finding a sample that is representative of the population being studied.