Introduction to Epidemiology: Study Design, Exposure, and Outcome

Running head: INTRODUCTION TO EPIDEMIOLOGY (401076) 1
Introduction to Epidemiology (401076)
Student Name
Institution

INTRODUCTION TO EPIDEMIOLOGY 2
Introduction to Epidemiology (401076)
Question 1: [26]
“What study design does this study employ and identify from the paper methodological features
that support the design you just noted?” [2 mark]
This is a cross-section study design. The participants comprised of 1,099 women of age
18 to 49 who had delivered at least one kid in a period of 2 years from ten districts in Ho Chi
Minh City were. A cross section study analyses a subset of the population and observes them for
a period of time. This is satisfied by the fact that this was done for a sample of 10 districts in a
period of 2 years (Vo & Le, 2019).
(b) “What is/are the main exposure/s and what is/are the main outcome/s of this study in
operational terms (i.e., you answer should include how the measure was defined)?” [4 marks]
The main exposure is violence during pregnancy. The study examines the link between
various kinds of domestic violence such as emotional violence, physical violence and sexual
violence on pregnant term/birth weight (Laelago, Belachew, & Tamrat,2017). The study
concluded that domestic violence during pregnancy has about 1.44 times increased risk of a
pregnant mother delivering a preterm or low birthweight child.
(c) “Do you think the risk of selection bias is high or low? Your answer should be justified by no
more than 60 words!! Too long answer will be penalised ?” [4 marks]
The risk of selection bias is low. The criteria used for selection of participants is
probabilistic (Woodward,2013). Random sampling of districts and the subjects to be involved in
the study was done. The data collection method (questionnaire from WHO) is correct. The study
subjects willingly participated in the study increasing validity and accurateness (García-
Moreno,et al,2015)
(d) “Compute the crude OR for domestic violence (yes/no) and pre-term/low birthweight from
data presented in Table 4 and interpret this in your own words.” [4 mark]
Odds Ratio=Odds that a case was Exposed (A/C)/Odds that a Control was Exposed (B/D)
OR=AD/BC= [54/106]/[203/736]=1.84

INTRODUCTION TO EPIDEMIOLOGY 3
Since OR>1, There is a strong association between domestic violence and the likelihood of
having a preterm/low birth weight child and the association is significant (p<.05).
This means that with the presence of domestic violence, one has 84% likelihood to have a
preterm/low birth weight child.
(e) “Was the crude OR differ from the adjusted and if yes why? if no why?” [4 marks]
The crude OR is not different from the adjusted OR since the presence of any type of
domestic violence was found to have a high association with preterm/low birth weight child. All
the exposure are equally likely to have similar effects on the dependent variable.
(f) “What was the prevalence of physical violence in this sample and what was the adjusted
prevalence odds ratio (POR-Table 5) between physical violence and pre term/low birthweight?
Interpret the association in your own words, could it be a result of some error?” [4 marks, no
more than 100 words]
The POR was found to be 1.44.This is a significant high value (POR>1) which implies
of presence of a strong positive association between domestic violence and preterm/low birth
weight child. The prevalence of physical violence can be computed as 54/257=0.210.This is a
21% prevalence rate.
(g) “What could be a source of measurement bias in this study and did the authors tried to
address this problem. Explain with evidence from the text.” [4 marks]
Recall bias would be one of the causes of bias in the study. This is where the study
subject answer questions about things that happened to them in the past. In such cases, the
accuracy of information depends on the participant’s memory. The selected subjects in this case
were less than 2 years since they gave birth to their child and can remember most of the
occurrences.
Selection bias-This occurs during sampling. To prevent this, probabilistic sampling
method was used in selection of subject such simple random sampling to get districts, use of
random numbers to select subjects (Flegal & Graubard,2017).

INTRODUCTION TO EPIDEMIOLOGY 4
Question 2 [12]
“In a case –control study researchers investigated the association between birth defect and
consuming folic acid supplements during pregnancy. There were 72 infants born with birth
defect and 55 of their mothers did not take folic supplementation during pregnancy. The authors
recruited for each case 3 mothers who gave birth to normal infant as control group. Among the
control group there were 70 mothers who did not use folic supplementation during pregnancy.”
(a) “Construct a 2x2 table with columns and rows headings and calculate an appropriate
measure of the strength of association between birth defect and folic acid supplements during
pregnancy.” [4 marks]
According to the description above, taking the55 mothers who had no folic supplement and
3mothers recruited for each case; 55*3=165-70=95.
FOLIC SUPPLEMENT
YES NO TOTAL
DEFECT 17 55 72
NO DEFECT 95 70 165
TOTAL 112 125 237
To measure the strength of association the relative risk or odd ratio can be used (Wassertheil &
Smoller,2015).
In a case control study, the incidence cannot be calculated .In such case therefore the Odds ratio
is used instead of the relative risk (Oakes & Kaufman,2017)
Odd Ratio= Odds that a case was exposed/Odds that a Control was Exposed
Odds Ratio=Odds that a case was Exposed (A/C)/Odds that a Control was Exposed (B/D)