University of North Carolina at Chapel Hill
                          School of Public Health
                         Department of Epidemiology
                   Fundamentals of Epidemiology (EPID 168)

                          Midterm Exam, Fall 1996 
                                  EPID 168

Most of the questions in this examination are based on the article:

     Garry VM, Schreinemachers D, Harkins ME, Griffith J.  Pesticide appliers,
     biocides, and birth defects in rural Minnesota.  Environ Health Perspect

A copy of this article was provided to you before this examination and can be
used in answering the following questions.

    1.  Briefly state the primary study question of this report. Identify the
        main exposure and outcome of interest.  (3 pts) 

    2.  Briefly explain the difference between disease classification based on
        manifestational criteria and disease classification based on causal
        criteria.  What is the logic for analyzing the data in relation to
        categories of anomalies grouped by organ system? (4 pts)


    3.  As discussed in class, epidemiologic studies often have both
        descriptive and analytic characteristics.  State one way in which this
        study is descriptive and one way in which it is analytic? (4 pts)

    4.  The reporting of birth defects was provided in accord with state
        statutes, and grouping of birth defects categories followed the
        National Centers for Health Statistics guidelines (page 394 second
        paragraph - methods).  This reporting of birth defects is an example
        of which of the following types of data collection methods.  Choose
        one best answer. (4 pts)

        A.  Active surveillance
        B.  Ongoing crossectional survey
        C.  Passive surveillance
        D.  Follow up study of dynamic population  

    5.  This study determined exposure and outcomes using data from "a list of
        all members of the agricultural community who were certified to apply
        restricted-use pesticides in 1991" (p. 394-methods) and from "all in-
        wedlock live births recorded in the state for the years 1989 through
        1992" (p. 394-methods). Briefly assess the strength of these data
        sources in establishing the temporal sequence of pesticide exposure
        and birth defects and provide support for your assessment.  (4 pts)

    6.  For each of the following epidemiologic measures, indicate whether it
        is a rate, a proportion, or a ratio that is neither a rate nor a
        proportion, or none of these.  Circle the best answer (4 pts)

A. Population attributable risk (PAR)   rate     proportion   ratio  neither
B. Incidence density (ID)               rate     proportion   ratio  neither
C. Prevalence                           rate     proportion   ratio  neither
D. Relative risk                        rate     proportion   ratio  neither

    7.  The use of the term "rate" is not an infallible guide to the specific
        epidemiologic measure being presented.  Which one of the following
        epidemiologic measures best characterizes the measure that the authors
        refer to as the  "rate of anomalies per 1000 live births"  (Table 2 -
        footnote)? Choose one best answer. (4 pts)

        A.  cumulative incidence (CI)
        B.  incidence density (ID)
        C.  prevalence
        D.  attributable risk proportion

    8.  The authors indicate that table 1 supports their statement...
        "pesticide appliers had significantly more children with an anomaly
        than did nonappliers" (p.395 results first paragraph).    This
        statement is readily understood but not literally correct.  Which one
        of the following state the finding more precisely? Choose one best
        answer. (4 pts)

        A.  pesticide appliers had 1.37 times more births with anomalies than
            did the general population.

        B.  pesticide appliers had more children with birth anomalies than did
            the general population.

        C.  pesticide appliers had a greater proportion of births with
            anomalies as compared to the general population.

        D.  Pesticide appliers accounted for more births with anomalies than
            did the general population.

    9.  Table 1 presents both crude and age-adjusted odds ratios.  In the
        table, the age adjusted odds ratio for gastrointestinal anomalies is
        slightly larger than the crude estimate, as is the case for most of
        the odds ratios presented. If the difference between the crude and
        age-adjusted odds ratios had been large, explain in general terms what
        this would mean regarding the respective ages of the pesticide
        appliers and the general population.  Assume the maternal age
        structure of the combined population was used as the standard. (3 pts)


   10.  Using data in Table 1:

        a.  Compute an estimate of the potential impact of pesticides on birth
            anomalies (in wedlock, all types together) to fathers who are
            certified pesticide appliers. State the assumption required to
            interpret this estimate.  (4 pts)

        b.  Compute an estimate of the potential impact of pesticides on birth
            anomalies (in wedlock, all types together) in the Minnesota
            population as a whole.  (3 pts). 

   11.  Using the data presented in Table 1, recalculate the crude odds ratio
        for all births with anomalies assuming that all  musculoskeletal birth
        anomalies occurring among those with maternal age greater than 30 and
        the "other" anomalies among maternal age > 35 were later found to
        actually have occurred among persons incorrectly classified as
        appliers.  Explain what implications this new calculation would have
        on the conclusions of the study. (3 pts) 


   12.  It is possible that the pesticides examined in this study might have
        reduced fecundity or increased the proportion of conceptions not
        resulting in live births.  Assume that both of these effects (lower
        fecundity, more spontaneous abortions, and more still births) have in
        fact occurred in the pesticide applier population studied here, so
        that the number of live births to pesticide applier fathers is smaller
        than it would have been in the absence of pesticide exposure. Which of
        the following statements is (are) TRUE and which is (are) FALSE? (2
        pts each)

____ ____   A. Since all births would be affected equally, effects on
               fecundity and spontaneous abortion WOULD NOT have influenced
               the size of the odds ratio presented in this study.  [This
               question is problematic.]

____ ____   B. If pesticides were equally likely to cause fetal loss and birth
               anomalies, then the odds ratios would strongly understate the
               harmful effects of pesticides.

   13.  Table 4 shows the frequency per 1000 births of major anomalies for the
        general population by region.  Which of the following best describes
        the study design from which these data were obtained. (4 pts)

        A.  ecologic study
        B.  prospective cohort study
        C.  retrospective cohort study
        D.  region-specific case control study 

   14.  The authors begin their discussion section by stating that this report
        "is an initial step in the evaluation of the possible relationships
        between the frequency of birth anomalies and pesticide use".  They
        conclude, however by saying that these data "signify a clear-cut need
        for comprehensive examination of the health issues involved".  This
        latter statement seems to indicate that the authors suspect a causal
        relationship.  Identify and describe three criteria for causal
        inference for which at least some information is present in the
        article.   Give specific examples from the article to support your
        selection. (9 pts)


   15.  Suppose that after this publication came out, another study was
        conducted in Illinois to investigate the hypothesis that birth defects
        occurred more often in Illinois  as compared to Minnesota.  However,
        in this new study the authors thought that the type of water consumed
        could be related to birth defects.  They wanted to adjust
        (standardize) the rates of defects in the two states for water type.
        Data from the two studies are compared as below.

                              Births by state and water type                 

                    Minnesota Pesticide Appliers   Illinois Pesticide Appliers
                      Normal     With anomalies     Normal     With anomalies
Water Type              (#)       (#)     rate*       (#)       (#)     rate*

Well water only        3379       93      26.8        100        2      ____
City water only         874       27      30.0        200        6      ____
Bottled water only      206        5      23.7       7293       145     ____
Total                  4456       125     28.0       7593       153     ____

* per 1000 live births

        a.  calculate the crude rate and the water-type specific rates for
            Illinois.  Briefly describe how these two states compare in crude
            rates of birth anomalies. (4 pts)  

        b.  Using the combined number of live births as a standard, calculate
            a standardized rate (standardized for water type) for each of the
            states.  Briefly describe how these standardized rates compare
            with each other and reasons why they may or may not agree with the
            crude rates. (6 pts)

   16.  Would an inference of causality based on the data in Table 4 be
        subject to criticism based on the ecologic fallacy concept.  Briefly
        explain your answer. (2 pts)

   17.  Which of the following statements about the present study are (is)
        TRUE and which are (is) FALSE.  Indicate TRUE or FALSE for each
        statement. (2 pts each)

____ ____   A. Subjects used in the analyses for Table 1 of this study were
               selected on the basis of their exposure status.

____ ____   B. Table 4 in this study supplied dose response evidence to
               support an inference of a causal relationship between
               pesticides and birth defects. 

____ ____   C. The age-adjusted odds ratio for all birth anomalies of 1.41 is
               considered a modest association.

____ ____   D. Since birth defects of these types are rare in the general
               population, a cohort study could be designed to efficiently
               examine further the relationship of pesticides and birth

____ ____   E. Exposure status in this study was randomized resulting in an
               equal distribution of known and unknown confounding variables
               between pesticide appliers and the general population.

____ ____   F. a correlation coefficient is a measure of association but is
               not useful in assessing the dichotomous outcomes measured in
               this study.

____ ____   G. Table 1 used stratified analyses to adjust for a confounding
               effect of maternal age on the association between
               musculoskeletal/integumental anomalies and pesticide exposure.
[question #18 has been removed, 10/7/97]

   19.  Succinctly evaluate whether or not, on the basis of the information in
        the article (including information that the authors cite to other
        work), further measures are warranted now to prevent birth defects
        caused by chlorophenoxy herbicides. (5 pts)

1/22/97, 10/7/97 - wr:vs \ mepid168 \ exams 1996 Midterm exam