Students struggled with…

  • Explaining if the sample proportions from biased surveys were higher or lower than expected.  They were often contradictory in their responses by mixing up the sample proportion and the population proportion.
  • Selecting strata that would decrease variability.  They sometimes chose strata based on location when it would not help to yield more representative results.
  • The difference between nonresponse and voluntary response.

Remedies…

  • Have students underline what the question is asking for.  They often had the correct thinking but mixed up if they were referring to the sample or population.
  • When choosing strata, ask yourself, “Would this variable influence how people respond?” If the answer is yes, it is probably a good choice of strata. For example, if you are looking at favorite television shows, could gender or age impact how people respond? Probably.  Would the city they live in? Probably not.
  • Give examples of situations where nonresponse is the issue and examples when voluntary response is the issue.  Ask students to identify how the examples are different and similar.  The goal is that they realize that nonresponse is an issue that occurs after the sample has already been chosen and voluntary response is a sampling issue.

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