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Investigation 3-3


Students explore the second of five possible explanations for finding an association: chance.  Students are given a deck of 100 cards, with 25 cards each labeled a, b, c, and d, representing the number of people in the population who would be in cells a, b, c, and d if a hypothesis were tested.  Students mix the cards up in a paper bag, take a sample of 20 cards from the bag, count the number of a, b, c, and d cards, construct a 2x2 table, calculate the relative risk, and uncover how an association can be found by chance.

Investigation Preparation Checklist

Check alignment with standards.

National or State file

Review Epidemiology Background for Module 3.

Word or PDF file

Review teacher's notes and slides:  [en espaņol:]

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    Copy Investigation 3-3: Epi Log Worksheet for each student.  [en espaņol:]

    Word or PDF file

    Prepare a deck of 100 cards with 25 labeled with an a, b, c, and d for each Epi Team.

    Word or PDF file

    Get 1 paper bag for each Epi Team.


    Estimated Time / Time Management

    Approximately one class period. Time can be managed by increasing or decreasing the number of 2X2 rate and risk calculations or cell identifications.

    Look For Students To:

    Explain how picking 20 of 100 cards is analogous to selecting a sample from a population.

    Explain how the relative risks calculated for each sample of 20 cards is analogous to finding an association by chance.

    Explain how sample size affects the accuracy of a relative risk.

    Consider the possibility of finding an association due to chance.