Introduction: This activity will enable students to investigate the transmission and origin of diseases
using a protocol similar to one used by the Center for Disease Control. Descriptions for a case control
study and a  cohort study are included. Your students may decide which they can to conduct. It is
most relevant to students when you investigate a recent outbreak in your school or area, one that the
students have personal knowledge.

Materials: Paper, pencil, calculator, student absentee rates, and possibly medical records.

Procedure:
1. Confirm the outbreak by comparing the outbreak time period with base line. This can be done
    by checking and comparing attendance records before and after the outbreak.
2.  Identify and count cases.
    a. Create a case definition which includes age range of those affected, positive test results,
    and a location of the incidents.
    b. Develop interview questions for those possibly affected. Students may include questions
    concerning set of symptom, time of incubation, place of residence, occupation, activities and
    sex of the person.
3. Debrief data – What are the commonalities?
    Students may want to orient data as to time, place, and person. This data analysis leads to the
    formulation of a hypothesis concerning the cause of the outbreak..
4. Formulate hypotheses. As a class, formulate several possible causes. Then divide the class
    into groups based on beliefs.
5. Test hypotheses using one type of CDC methodology: Case control or Cohort.

Case Control Study

Definition: In this methodology, you much initially know the outcomes, i.e. which people are sick and which
    are not. You do NOT know if the illness is due to a particular exposure – the cause of your hypothesis.

Determining a control: As the name implies, you must interview controls. Persons selected as controls
    should be similar to those infected in age, location, activities, and classes. In other words, the conditions
    must be the same as those experienced by the sick person. The people you choose for controls need to be
    healthy.

Data table: Set up your data table in the following manner. You must enter data for each person in a
    single box (a, b, c, or d).

                                                                 Outcome (illness)          No outcome (healthy)

         Exposed to hypothetical cause         _______a_______         _______b_________

        Not exposed to hypothetical cause     _______c_______         _______d_________

Calculate odds ratio

  Odds ratio = odds of exposure in cases      =  ad
                      odds of exposure in controls       bc

Analysis of results: The odds ratio tells you the number of times more or less likely the hypothetical cause
    was responsible for the outbreak. If the odds ration is equal to one, then there is a50/50 chance that the
    cause is responsible for the disease. If the odds ratio = 18, then there is an 18% chance that the hypothetical
    agent is responsible for the disease. It the odds ratio = 0.2, then there is virtually no correlation between
    the hypothetical cause and the disease.

Cohort Study

Definition: In this methodology, you must initially know hwo has been exposed and who has not. You
    do NOT know if students developed a disease or not.

Defining controls: Persons selected as controls, should not have been exposed to the condition. They
    should be similar in age, location, activities, and classes as those exposed. Controls chosen may be ill.

Data table: Set up your data in the following manner. You must enter data for each person in a
    single box (a, b, c, or d).

                                                             Outcome (illness)      No outcome (healthy)

         Exposed to condition                   _______a_______     _______b_________

        Not exposed to condition              _______c_______     _______d_________

Calculate the relative risk:

  Relative risk  =    Incidence in exposed       =   a / (a + b)
                            Incidence in unexposed          c / (c + d)

Analysis of results: The risk of contracting an illness of those that were exposed to the condition is that
     many times greater than those that were not exposed.
 

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