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					                    The 2x2 Table




                  The
                2x2 Table



Detectives in the Classroom - Investigation 2-1: The 2x2 Table
    Where are we and where are we going?


1. How is this disease distributed and what
   hypotheses might explain that distribution?

2. Is there an association between the hypothesized
   cause and the disease?

3. Is the association causal?

4. What should individuals and society do when
   preventable causes of disease are found?

5. Did the prevention strategy work?

   Detectives in the Classroom - Investigation 2-1: The 2x2 Table
    Where are we and where are we going?
          1                                                   2
 How is this disease          Hypotheses                  Is there
    distributed?                                      an association?
    What’s                                                  Did the exposure and
my hypothesis?                                               the disease turn up
                                                                  together?
                                    3
                           Is the association
                                 causal?




                                     4
                          What should be done
                         to prevent the disease?




                                    5
                           Did the prevention
                             strategy work?


   Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                         Review


                         How is this disease distributed and
                         what hypotheses might explain that
                         distribution?


                         Health-related conditions and
                         behaviors are not distributed
                         uniformly in a population. Each has a
                         unique descriptive epidemiology that
                         can be discovered by identifying how
                         it is distributed in a population in
                         terms of person, place, and time.
                         Descriptive epidemiology provides
                         clues for formulating hypotheses.


Detectives in the Classroom - Investigation 2-1: The 2x2 Table
               Testing Hypotheses




                        Hypothesis




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                         Epi Talk


                          Analytical Epidemiology

                          A study designed to
                          investigate hypothesized
                          causal relationships.

                          Tries to determine why
                          disease is occurring.

                          Tests hypotheses.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                        Scenario

                                         Acne

                                      Oily skin

                                      Oily skin
                                    causes acne.
         Epidemiologist
                                      AcneMed
              Drug
            Company
                                     AcneMed
                                   prevents acne.


Detectives in the Classroom - Investigation 2-1: The 2x2 Table
          “AcneMed Prevents Acne.”




                                        for 6 months

                                       10 teenagers
                                        had acne.



Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                Epi Log Worksheet




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
             10 Teenagers Had Acne                               1



                                  No
                       Acne      Acne       Total


       AcneMed          10         90        100




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                Express It in Numbers

                             Risk

A measure of how often an event
occurs in a defined population in                 Numerator
a defined period of time. It
consists of a numerator and a                   Denominator
denominator.
The numerator is the number of
people in the population who                         Event
experience the event and the
                                                  Population
denominator is the total number
of people in the population.


  Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                         Epi Talk


                       Risk

                      A measure of how often an
                      event occurs in a defined group
                      of people in a defined period of
                      time.

                      The likelihood of developing a
                      disease.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
             Express It in Numbers                               2




                            No                         Acne
                 Acne      Acne       Total            Risk

                                                 10
 AcneMed          10         90        100              or    10 %
                                                 100




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
          “AcneMed Prevents Acne.”
                                   Inference




   Process of predicting from what is observed
              to what is not observed.
Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                         Epi Talk


                          Inference

                          Process of predicting
                          from what is observed in
                          a sample to what is not
                          observed in a population.

                          To generalize back to the
                          source population.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
               “AcneMed Prevents Acne.”


                                 No                         Acne
                      Acne      Acne       Total            Risk

                                                      10
                                                       0
      AcneMed          10        90         100              or
                                                                   10 %
                                                                    0
                        0        100
                                                      100




    Maybe they had acne when the experiment began.
     Maybe they said they took AcneMed but did not.
Maybe they took AcneMed but not enough to prevent acne.

     Detectives in the Classroom - Investigation 2-1: The 2x2 Table
               “AcneMed Prevents Acne.”


                                 No                         Acne
                      Acne      Acne       Total            Risk

                                                      10
                                                       0
      AcneMed          10        90         100              or
                                                                   10 %
                                                                    0
                        0        100
                                                      100




     Maybe taking AcneMed does not prevent acne.
Maybe those 90 teenagers would not have developed acne
          even if they had not taken AcneMed.

     Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                         Epi Talk


                         Control Group

                         People who participate in
                         a trial, but do not get the
                         treatment.

                         People whose results are
                         compared to the group that
                         was treated.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
          “AcneMed Prevents Acne.”
                                   Inference




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                      Epi Teams




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
               Experimental Group                                3



                            No                         Acne
                 Acne      Acne       Total            Risk

                                                 10
 AcneMed          10         90        100              or   10 %
                                                 100




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                   Control Group                                  3



                            No                          Acne
                 Acne      Acne       Total             Risk

                                                 10
 AcneMed          10         90        100               or    10 %
                                                 100

   No                                              ?
                                       100               or    ?%
 AcneMed                                          100




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                         Epi Talk


                         Exposures

                         Factors with which
                         individuals come in contact.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                         Epi Talk


                         Outcomes

                         All the possible results that
                         occur because of exposure
                         to a causal factor.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
           2 Rows, 2 Columns, 4 Cells

                             Outcome / Disease


                                             No
                                 Acne       Acne      Total


                 AcneMed           a
                                   10        b
                                             90        100
Exposure
                    No
                 AcneMed
                                   c          d        100




 Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                       2x2 Table                                 4



                                  No
                      Disease   Disease     Total


       Exposed           a         b
                             a b
                             c d
     Not Exposed




              People who are exposed


Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                       2x2 Table                                 5



                                  No
                      Disease   Disease     Total


       Exposed           a
                             a b
                             c d
     Not Exposed




People who are exposed and have the disease


Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                            2x2 Table                                 6



                                       No
                           Disease   Disease     Total


            Exposed
                                  a b
                                  c d
          Not Exposed
                                        d



People who are not exposed and do not have the disease


     Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                           2x2 Table                                 7



                                      No
                          Disease   Disease     Total


           Exposed
                                 a b
                                 c d
         Not Exposed
                                       d



People who do not have the disease and are not exposed


    Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                       2x2 Table                                 8



                                  No
                      Disease   Disease     Total


       Exposed
                             a b
                             c d
     Not Exposed
                         c         d



            People who are not exposed


Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                       2x2 Table                                 9



                                  No
                      Disease   Disease     Total


       Exposed                     b
                             a b
                             c d
     Not Exposed
                                   d



       People who do not have the disease


Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                         2x2 Table                                 10



                                    No
                        Disease   Disease     Total


         Exposed                     b
                               a b
                               c d
       Not Exposed




People who are exposed and do not have the disease


  Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                       2x2 Table                                 11



                                  No
                     Disease    Disease     Total


       Exposed          a
                            a b
                            c d
     Not Exposed
                        c



           People who have the disease


Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                        2x2 Table                                 12



                                   No
                       Disease   Disease     Total


        Exposed
                              a b
                              c d
      Not Exposed
                          c



People who are not exposed and have the disease


 Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                         Review


                        Epidemiology

                        The study of how and why
                        diseases or other health-
                        related conditions are
                        distributed in a population
                        the way they are; in other
                        words, why some people
                        get sick and others do not.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                    The 2x2 Table


                                                 Epidemiology
                            No
                 Acne      Acne       Total       The study of
                                                 how disease is
 AcneMed          10         90        100        distributed in
                                                   populations
                                                    and of the
    No
                                       100       exposures that
 AcneMed
                                                   cause that
                                                  distribution.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                    The 2x2 Table


                                                 Epidemiology
                            No
                 Acne      Acne       Total       The study of
                                                 how disease is
 AcneMed          10         90        100        distributed in
                                                  populations
                                                    and of the
    No
                                       100       exposures that
 AcneMed
                                                   cause that
                                                   distribution.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                    The 2x2 Table

                    Disease
                                                 Epidemiology
                            No
                Disease
                 Acne     Disease
                           Acne       Total       The study of
                                                  how disease
 AcneMed          10         90        100       is distributed
                                                 in populations
                                                    and of the
    No
                                       100       exposures that
 AcneMed
                                                   cause that
                                                   distribution.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                                The 2x2 Table

                                Disease
                                                             Epidemiology
                                        No
                            Disease   Disease     Total      The study of
                                                            how disease is
Exposures




             AcneMed
              Exposed         10         90        100       distributed in
                                                            populations and
                                                                 of the
                 Not
                No
                                                   100      exposures that
              Exposed
             AcneMed
                                                              cause that
                                                             distribution.




            Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                    The 2x2 Table


                                                 Epidemiology
                            No
                 Acne      Acne       Total       The study of
                                                 how disease is
 AcneMed           a         b                    distributed in
                                                   populations
                                                    and of the
    No
                   c         d                   exposures that
 AcneMed
                                                   cause that
                                                  distribution.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table
            How Is Acne Distributed?



When you can measure what you are speaking
about, and express it in numbers, you know
something about it. But when you cannot measure it,
when you cannot express it in numbers, your
knowledge is of a meager and unsatisfactory kind.
                         Lord Kelvin




 Detectives in the Classroom - Investigation 2-1: The 2x2 Table
                    The 2x2 Table

           Investigation
                2-1
            has ended.




Detectives in the Classroom - Investigation 2-1: The 2x2 Table

				
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