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
1996;104:394-399.
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)
TRUE FALSE
____ ____ 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)
TRUE FALSE
____ ____ 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
anomalies.
____ ____ 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