American University of
Antigua College of
Medicine
Epidemiology & Biostatistics 1
Preclinical Science
October 2023
Juan Manuel Lozano, MD, MSc
Division of Medical and Population Health Sciences Educations and Research
Herbert Wertheim College of Medicine, Florida International University
Introduction to Epidemiology.
Main Types of Questions in Health
Research.
Juan Manuel Lozano, MD, MSc
Division of Medical and Population Health Sciences Research
Department of Medical Education
Herbert Wertheim College of Medicine
Florida International University
Objectives
RU1.EB1.1 Understand the definitions of epidemiology and health research, and their
role in generation of new information, health maintenance, and disease prevention.
Given a clinical or research scenario, graph or table, students should be able to:
 RU1.EB1.1.1 Differentiate the types of health research: basic, clinical,
epidemiological, systems, and services.
 RU1.EB1.1.2 Describe the objectives of health research: description, comparison, and
prediction.
 RU1.EB1.1.3 Apply the principles of the scientific method to propose answers to
specific questions.
 RU1.EB1.1.4 Contrast the scientific method with other forms of knowledge generation.
 RU1.EB1.1.5 Characterize the roles played by epidemiology and biostatistics in health
research projects.
Objectives
RU1.EB1.2 Understand the main components of health research with emphasis
on clinical research.
Given a clinical or research scenario, graph or table, students should be able to:
 RU1.EB1.2.1 Correlate the main components (the anatomy) with their function
(the physiology) in a health research project.
 RU1.EB1.2.2 Determine the roles of subjects, exposure, and outcome
assessment in health research projects.
 RU1.EB1.2.3 Contrast research questions and research hypotheses.
 RU1.EB1.2.4 Differentiate between research and statistical hypotheses.
 RU1.EB1.2.5 Define a variable as it relates to health research.
Smoking and health
Today: smoking
cigarettes increases
the risk of cancer, lung
and cardiovascular
disease, low birth
weight babies, and
many more bad
outcomes
How did we discover “the truth”?
Health Research
“Generation of new knowledge, using the scientific
method, to identify and control health problems”
Commission on Health Research for Development. Health Research: Essential Link to
Equity in Development. Oxford: Oxford University Press; 1990
“Re-search: the never-ending search for truth, which is
elusive and rarely definitive.”
Chen L, et al. Lancet 2014;384:e33
Main purposes in health research
1. Describe: Describe the existing distribution of variables.
 Three key features: person, place, time.
 Five basic “W”: who, what, why, when, and where.
 Studies: case reports, case-series, cross-sectional (prevalence),
surveillance, ecological.
2. Compare: Assess associations between variables.
 Key feature: compare two or more groups.
 Studies:
 Experimental: randomized and non-randomized controlled trial.
 Observational: cohort, case-control, cross-sectional.
Forms of acquiring knowledge
Tradition
 Authority
 Intuition
 Trial and error
 Logic reasoning
 Scientific method

Forms of acquiring knowledge
Method
Tradition
Concepts are accepted as truth because:
They have always been considered true.
Forms of acquiring knowledge
Method
Tradition
Authority
Concepts are accepted as truth because:
They have always been considered true.
“Experts” say so.
Forms of acquiring knowledge
Method
Tradition
Authority
Intuition
Concepts are accepted as truth because:
They have always been considered true.
“Experts” say so.
It is felt they are so (“hunch”).
Common limitations of all the above:
 No test for the veracity of “the truth”.
 No assessment of alternative explanations (potentially
better).
 Limit searching for new information.
Trial and error
Try one of the options and assess
the results:
 Not so good: try other option
 Good: adopt the option, stop trying
others
Limitations:
 Long and unsystematic process
 One untried option could be better
(but remains unknown)
 It does not consider chance as
explanation for the results
Logical reasoning

Induction: inference of a
generalized conclusion from
observations.
"The street has been wet every time it has
rained. Therefore, when it rains, the street
gets wet.“

Deduction: the conclusion comes
from general or universal premises
(or theories).
"When it rains, the street gets wet. It rains.
Therefore, the street should be wet."
 The most rigorous form of
generating knowledge in health
 Basic assumptions:


Nature is ordered and regular
Natural events (including health
and illness):
oDo not occur at random
oTheir causes are potentially
identifiable
oAre predictable (within some limits)
Scientific method
Observations generate
hypotheses (induction)
Hypotheses are tested empirically
Theories generate
new hypotheses
(deduction)
Verified hypotheses
integrate in theories that
pretend to explain natural
phenomena
Scientific method features
Empirical
Uses direct
observations
to obtain data
supporting the
knowledge
generated
Systematic
Sense of order
that aims to
guarantee
validity and
reproducibility
Control
Of factors not
directly related
with the
variables of
interest but
that can
influence them
Critical exam
Findings are
open to public
scrutiny and
empirical
retesting
Scientific knowledge is not definitive, it is temporary
and refutable
The Scope of Health Research
Basic
Clinical
Epidemiology
Health services
Sequence of
biological
events in health
and disease
Clinical
conditions
and
outcomes
Health and
disease in
human
populations
Non-biological
factors that
affect health
services
Molecules,
genes, cells,
animals
Patients
Populations
Health care
systems
Translational Research

NIH and IOM: Two areas of translation:
o Applying discoveries from laboratory and preclinical studies to studies
in humans.
o Enhancing the adoption of best practices in the community.


National Cancer Institute: Transforms scientific discoveries
from laboratory, clinical, or population studies into clinical
applications.
Rubio et al: Multidirectional integration of research (basic,
patient-oriented, population-based) to improve the health of the
public.
Rubio D, et al. Acad Med. 2010;85:470-5
Rubio D, et al. Acad Med. 2010;85:470-5
Activity 1. Identifying the area of research
You are going to receive a short description of 4 published
studies. Please identify the research area of each study:
A.
B.
C.
D.
Basic sciences
Clinical
Epidemiological
Health services & systems
Up to 10’: about 2.5’ per study
What type of research is study 1 – Causes of
stillbirth according to age and race.
A.
B.
C.
D.
Basic sciences
Clinical
Epidemiological
Health services & systems
What type of research is study 2 – NV1FGF in
critical limb ischemia.
A.
B.
C.
D.
Basic sciences
Clinical
Epidemiological
Health services & systems
What type of research is study 3 – Patient
mediated interventions to improve adherence to
CPG.
A.
B.
C.
D.
Basic sciences
Clinical
Epidemiological
Health services & systems
What type of research is study 4 – Women’s
lifestyle and obesity among their offspring.
A.
B.
C.
D.
Basic sciences
Clinical
Epidemiological
Health services & systems
Activity 1 - Study 1
Stillbirth affects 1 in 160 pregnancies in the US. This study aimed to ascertain
the causes of stillbirth in a population that is diverse by race/ethnicity and
geography. This was a population-based study from Mar 2006 to Sep 2008 with
surveillance for all stillbirths at 20 weeks or later in 59 tertiary care and
community hospitals in 5 catchment areas. Termination of a live fetus was
excluded. Standardized evaluations (medical history, fetal postmortem and
placental pathology, karyotype, other laboratory tests, systematic assignment of
causes of death) were performed at delivery. Of 663 women with stillbirth
enrolled, 500 consented to complete postmortem examinations of 512 neonates.
A probable / possible cause of death was found in 312 stillbirths (60.9%). The
most common causes were obstetric conditions (29.3%), placental abnormalities
(23.6%), fetal genetic/structural abnormalities (13.7%), infection (12.9%),
umbilical cord abnormalities (10.4%), hypertensive disorders (9.2%), and other
maternal medical conditions (7.8%).
Activity 1 - Study 2
Peripheral artery disease affects up to 20 million individuals in
North America and Europe, with 2–5% developing the most
severe form of the disease, critical limb ischaemia. The main
objective of this study was to assess the clinical benefit of
NV1FGF (Non-viral 1 (NV1) FGF (riferminogene pecaplasmid),
a naked DNA plasmid that includes the gene encoding for
human FGF1) in delay of the time to major amputation or death
in patients with clinical limb ischaemia with non-healing
ischaemic skin lesions, in whom revascularisation was not
possible.
Activity 1 - Study 3
Healthcare professionals are important contributors to healthcare quality
and patient safety, but their performance does not always follow
recommended clinical practice. There are many approaches to
influencing practice among healthcare professionals. These include audit
and feedback, reminders, educational materials, educational outreach
visits, educational meetings or conferences, use of local opinion leaders,
patient-mediated interventions, financial incentives, and organisational
interventions. In this review, we evaluated the effectiveness of patientmediated interventions (aimed at changing the performance of healthcare
professionals through interactions with patients, or through information
provided by or to patients) on healthcare professionals' performance
(adherence to clinical practice guidelines or recommendations for clinical
practice).
Activity 2 - Study 4
In women, adhering to an overall healthy lifestyle is associated
with a substantially reduced risk of type 2 diabetes, coronary
heart disease, and mortality. Whether such a healthy lifestyle
in women exerts health effects among offspring, possibly
through modulating the living environment and lifestyle of
children, deserves examination. This study examined the
association between an overall maternal healthy lifestyle
(characterized by a healthy body mass index, high quality diet,
regular exercise, no smoking, and light to moderate alcohol
intake) and the risk of developing obesity in offspring.
Take home messages




Health research using the scientific method is the
best-known way to generate new knowledge.
Other ways of generating knowledge have important
limitations.
Scientific knowledge is not definitive, it is temporary
and refutable.
Health research includes basic, clinical, epidemiological,
services and systems, and translational research.
Introduction to Epidemiology.
Main Types of Questions in Health
Research.
Juan Manuel Lozano, MD, MSc
Division of Medical and Population Health Sciences Research
Department of Medical Education
Herbert Wertheim College of Medicine
Florida International University
Research question & research
hypothesis
 Critical role as part of the scientific
method
 They must:
 Be posed in advance (before starting
the study)
 State key variables and how they are
related
 Be clear: posed in unequivocal (no
ambiguous) terms
 Be consistent with each other
Main objectives in health research
1. To describe the frequency or distribution of a problem or
finding (descriptive studies)
2. To compare (analytical studies):
 Harm: exposed vs. non exposed to some risk factors
 Interventions: treated one form vs. treated other form
 Diagnosis: a new test vs. a “gold standard”
 Prognosis: exposed vs. non exposed to some prognostic
factors
Elements of a research question
Type of question Elements of the question
Descriptive
P. Population or type of patients
O. Outcome variable(s)
Elements of a research question
Type of question Elements of the question
Descriptive
Analytical
P. Population or type of patients
O. Outcome variable(s)
P. Population or type of patients
I/E. Intervention or exposure
C. Comparator
O. Outcome variable(s)
The PICO format for research questions
Population / type of patients
Independent
variable:
Exposure
Dependent
variable:
Outcome
Yes
Yes
No
No
Type of question P*
Harm
Individuals
without a
condition / illness
Intervention
Individuals
without or with
one condition /
illness
I/E
Exposure to one
or more risk
factors
Receiving a new
prophylactic or
therapeutic
method
Diagnosis
New diagnostic
method
Individuals
suspected to have
one condition /
illness
Individuals with a
condition / illness
C
O
No exposure to the Developing the
risk factor(s)
condition / illness
Not receiving the
new prophylactic
or therapeutic
method, or
receiving another
The “best” method
available to
diagnose the
condition
No exposure to the
prognostic factor(s)
Preventing the
condition / illness or
improving its clinical
course
Accuracy of the new
method
Improving /
worsening the clinical
course of the
condition / illness
* P: population or type of patients; I/E: intervention or exposure; C: comparator; O: outcome(s)
Prognosis
Exposure to one
or more
prognostic factors
Type of question P*
Harm
Individuals
without a
condition / illness
Intervention
Individuals
without or with
one condition /
illness
I/E
Exposure to one
or more risk
factors
Receiving a new
prophylactic or
therapeutic
method
Diagnosis
New diagnostic
method
Individuals
suspected to have
one condition /
illness
Individuals with a
condition / illness
C
O
No exposure to the Developing the
risk factor(s)
condition / illness
Not receiving the
new prophylactic
or therapeutic
method, or
receiving another
The “best” method
available to
diagnose the
condition
No exposure to the
prognostic factor(s)
Preventing the
condition / illness or
improving its clinical
course
Accuracy of the new
method
Improving /
worsening the clinical
course of the
condition / illness
* P: population or type of patients; I/E: intervention or exposure; C: comparator; O: outcome(s)
Prognosis
Exposure to one
or more
prognostic factors
Type of question P*
Harm
Individuals
without a
condition / illness
Intervention
Individuals
without or with
one condition /
illness
I/E
Exposure to one
or more risk
factors
Receiving a new
prophylactic or
therapeutic
method
Diagnosis
New diagnostic
method
Individuals
suspected to have
one condition /
illness
Individuals with a
condition / illness
C
O
No exposure to the Developing the
risk factor(s)
condition / illness
Not receiving the
new prophylactic
or therapeutic
method, or
receiving another
The “best” method
available to
diagnose the
condition
No exposure to the
prognostic factor(s)
Preventing the
condition / illness or
improving its clinical
course
Accuracy of the new
method
Improving /
worsening the clinical
course of the
condition / illness
* P: population or type of patients; I/E: intervention or exposure; C: comparator; O: outcome(s)
Prognosis
Exposure to one
or more
prognostic factors
Type of question P*
Harm
Individuals
without a
condition / illness
Intervention
Individuals
without or with
one condition /
illness
I/E
Exposure to one
or more risk
factors
Receiving a new
prophylactic or
therapeutic
method
Diagnosis
New diagnostic
method
Individuals
suspected to have
one condition /
illness
Individuals with a
condition / illness
C
O
No exposure to the Developing the
risk factor(s)
condition / illness
Not receiving the
new prophylactic
or therapeutic
method, or
receiving another
The “best” method
available to
diagnose the
condition
No exposure to the
prognostic factor(s)
Preventing the
condition / illness or
improving its clinical
course
Accuracy of the new
method
Improving /
worsening the clinical
course of the
condition / illness
* P: population or type of patients; I/E: intervention or exposure; C: comparator; O: outcome(s)
Prognosis
Exposure to one
or more
prognostic factors
Research question
In children and adolescents is
chewing gum, as compared to not
chewing gum, associated with a
higher risk of dental caries?
A. Is this a descriptive or a
comparative question?
B. If comparative, what specific type
(harm, intervention, diagnosis,
prognosis)?



A restatement of the research question
Explains what is expected to happen
It should:
Be clear and understandable
 Be testable / measurable
 Contain the exposure and the outcome

When is a hypothesis needed?
Type of study
Research question
Descriptive
P. Population or type of patients
O. Outcome variable(s)
P. Population or type of patients
I/E. Intervention or exposure
C. Comparator
O. Outcome variable(s)
Analytical
When is a hypothesis needed?
Type of study
Research question
Descriptive
P. Population or type of patients Not needed
O. Outcome variable(s)
P. Population or type of patients Required
I/E. Intervention or exposure
C. Comparator
O. Outcome variable(s)
Analytical
Research hypothesis
Research question &
research hypothesis
Question: In children and adolescents
is chewing gum, as compared with not
chewing gum, associated with a
higher risk of dental caries?
Hypothesis: Children and
adolescents who chew gum have a
risk of caries that is different (higher)
than in those who do not chew gum.
Research
question
Research
hypothesis
Statistical
hypotheses
Activity 2. Descriptive or analytical? Type of
clinical question?
1. Description of three studies.
2. Select the type of question for
each study:
A.
B.
C.
D.
E.
Comparative – Intervention
Comparative – Harm
Comparative – Diagnosis
Comparative – Prognosis
Non-comparative - Descriptive
Up to 9’: about 3’ per study
What type of research question did study 1
(features of pregnant women with ZIKV infection)
address?
A. Comparative – Intervention
B. Comparative – Harm
C. Comparative – Diagnosis
D. Comparative – Prognosis
E. Descriptive – Non comparative
What type of research question did study 2
(women’s lifestyle and their offspring risk of
obesity) address?
A. Comparative – Intervention
B. Comparative – Harm
C. Comparative – Diagnosis
D. Comparative – Prognosis
E. Descriptive – Non comparative
What type of research question did study 3 (longterm outcomes in children with epilepsy)
address?
A. Comparative – Intervention
B. Comparative – Harm
C. Comparative – Diagnosis
D. Comparative – Prognosis
E. Descriptive – Non comparative
Activity 2 - Study 1
Zika virus (ZIKV) was first identified in Brazil in 2015 from patients
who presented with a dengue like illness. Soon thereafter, local
transmission of ZIKV was reported. We have been conducting
active surveillance for dengue infection in the general population of
Rio de Janeiro since 2007. In 2015, we noted an increase in cases
of a dengue like illness. Most cases were originally reported to
surveillance systems as dengue; however, ZIKV was eventually
identified. To identify ZIKV cases in our population, we enrolled
women at any week of gestation who presented with a rash. Here
we report demographic, clinical, laboratory, and gestational
ultrasonographic findings in the cohort of pregnant women enrolled
in our ZIKV study to date.
Activity 2 - Study 2
In women, adhering to an overall healthy lifestyle is associated
with a substantially reduced risk of type 2 diabetes, coronary
heart disease, and mortality. Whether such a healthy lifestyle
in women exerts health effects among offspring, possibly
through modulating the living environment and lifestyle of
children, deserves examination. This study examined the
association between an overall maternal healthy lifestyle
(characterized by a healthy body mass index, high quality diet,
regular exercise, no smoking, and light to moderate alcohol
intake) and the risk of developing obesity in offspring.
Activity 2 - Study 3
There are few studies on long-term outcomes in children with epilepsy. A
population-based cohort of 245 children with epilepsy (122 with idiopathic or
cryptogenic epilepsy, and 123 with remote symptomatic epilepsy - i.e., with a
major neurologic impairment or insult) was followed for 40 years. Sixty subjects
(24%) died during follow-up; this rate is three times as high as the age-adjusted
expected mortality. Death rates were 1.5 per 1000 person-years in subjects in
remission (≥5 years seizure-free) not receiving medication, 11.8 per 1000 personyears in subjects in remission receiving medication, and 15.9 per 1000 personyears in subjects not in remission (P<0.001). A remote symptomatic cause of
epilepsy was associated with an increased risk of death as compared with an
idiopathic or cryptogenic cause (37% vs. 12%, P<0.001).
Take home messages


Health research using the scientific method is
the best-known way to generate new knowledge
Research studies can be:
 Descriptive:
do not test a hypothesis
 Analytical: test a hypothesis

Analytical studies have:
 Research
question (PICO format)
 Research hypothesis
Type of question:
• Intervention
• Harm
• Diagnosis
• Prognosis
Introduction to Research Designs.
Juan Manuel Lozano, MD, MSc
Division of Medical and Population Health Sciences Research
Department of Medical Education
Herbert Wertheim College of Medicine
Florida International University
Objectives
RU1.EB1.3 Identify the main characteristics used to differentiate epidemiological
research designs.
Given a clinical or research scenario, graph or table, students should be able to:
RU1.EB1.3.1 Describe the main similarities and differences between descriptive
and comparative (analytical) studies.
RU1.EB1.3.2 Describe the main features of observational and experimental
studies.
RU1.EB1.3.3 Describe the main features of the following studies: cohort, casecontrol, cross-sectional, randomized controlled trials.
RU1.EB1.3.4 Recognize the designs of analytical studies based on their
characteristics, including directionality, and timing.
P: Patients or
Population
I/E: Intervention
or exposure
C: Comparator
O: Outcome(s)
In adults,
Does the consumption of fish / fish oil
supplements,
As compared to not consuming fish / fish
oil supplements,
Modifies the risk of coronary artery
disease (CAD)?
The PICO question
Population / type of patients: adults
Independent
variable:
Exposure
Dependent
variable:
Outcome
Yes fish
Yes CAD
No fish
No CAD
Let’s see how we would answer the research question using
different designs (RCT, cohort, case-control, cross-sectional):
a) How would we select participants for the study?
b) How would we allocate participants to or assess the
exposure?
c) Directionality: is the association assessed from Exp →
Outcome, or vice versa?
d) Timing: does we assess the facts concurrently (“prospectively”)
or historically (“retrospectively”)?
e) Need follow- up?
Randomized controlled trial (RCT)
Citation: Chapter 4 What Is the Question?, Guyatt G, Rennie D, Meade MO, Cook DJ. Users' Guides to the Medical Literature: A Manual for Evidence-Based
Clinical Practice, 3rd ed; 2015. Available at: https://jamaevidence.mhmedical.com/content.aspx?bookid=847&sectionid=69031460 Accessed: October 03, 2023
Copyright © 2023 American Medical Association. All rights reserved
Feature
Participants
Allocation of
exposure
Directionality
Timing
Follow-up
Description
Individuals without the outcome, but who can develop it
By investigator, using a random process
From exposure to outcome
Concurrent
Yes, to ascertain who develops the outcome
Cohort study
Citation: Chapter 4 What Is the Question?, Guyatt G, Rennie D, Meade MO, Cook DJ. Users' Guides to the Medical Literature: A Manual for Evidence-Based
Clinical Practice, 3rd ed; 2015. Available at: https://jamaevidence.mhmedical.com/content.aspx?bookid=847&sectionid=69031460 Accessed: October 03, 2023
Copyright © 2023 American Medical Association. All rights reserved
Feature
Participants
Allocation of
exposure
Directionality
Timing
Follow-up
Description
Individuals without the outcome, but who can develop it
Clinicians’ or participants’ choice (not investigator’s)
From exposure to outcome
Concurrent (prospective”) or historical (“retrospective”)
Yes, to ascertain who develops the outcome
Concurrent vs historical cohort studies
“Prospective” (concurrent) cohort study
Sample of
individuals
without outcome
Exposed
to factor
Unexposed
to factor
Outcome
No outcome
Outcome
No outcome
“Retrospective” (historical) cohort study
Sample of
individuals
without
outcome
Exposed
to factor
Unexposed
to factor
Outcome
No outcome
Outcome
No outcome
Follow
Trace
Past time
Present time
Future time
Case-control study
Exposed
to factor
Sample of individuals
without outcome
Unexposed
to factor
Exposed
to factor
Sample of individuals
with outcome
Unexposed
to factor
Follow
Trace
Past time
Present time
Future time
Exposed
to factor
Unexposed
to factor
Exposed
to factor
Unexposed
to factor
Feature
Participants
Allocation of
exposure
Directionality
Timing
Follow-up
Sample of individuals
without outcome
Sample of individuals
with outcome
Description
Individuals with and without the outcome (cases and controls)
Clinicians’ or participants’ choice (not investigator’s)
From outcome to exposure
Historical (“retrospective”)
No, the outcome already occurred
Cross-sectional study
Sample of
individuals
Exposed /
Without
outcome
Unexposed
/ Without
outcome
Exposed /
With
outcome
Unexposed
/ With
outcome
Follow
Trace
Past time
Present time
Future time
Sample of individuals
Feature
Participants
Allocation of
exposure
Directionality
Timing
Follow-up
Exposed /
Without
outcome
Unexposed /
Without
outcome
Exposed / With
outcome
Unexposed /
With outcome
Description
Mixed sample of individuals (exposed, unexposed, with
outcome, without outcome)
Clinicians’ or participants’ choice (not investigator’s)
None: exposure and outcome assessed simultaneously
NA
No
Activity 3. Identifying the research design
You are going to receive a short description of 3 published
studies. Please identify the research design of each study:
A.
B.
C.
D.
E.
Randomized controlled trial.
Concurrent (“prospective”) cohort study.
Historical (“retrospective”) cohort study.
Case-control study.
Cross-sectional study.
Up to 10’: about 2.5’ per study
What type of research design is study 1 – Highflow vs standard oxygen therapy in children with
acute respiratory failure.
A. Randomized controlled trial.
B. Concurrent (“prospective”) cohort study.
C. Historical (“retrospective”) cohort study.
D. Case-control study.
E. Cross-sectional study.
What type of research design is study 2 – Effect
of air travel on the risk of venous thrombosis.
A. Randomized controlled trial.
B. Concurrent (“prospective”) cohort study.
C. Historical (“retrospective”) cohort study.
D. Case-control study.
E. Cross-sectional study.
What type of research design is study 3 – Link
between the MMR vaccine and autism.
A. Randomized controlled trial.
B. Concurrent (“prospective”) cohort study.
C. Historical (“retrospective”) cohort study.
D. Case-control study.
E. Cross-sectional study.
Activity 3 - Study 1
The objective of this study was to determine the effect of early
high-flow oxygen therapy vs standard oxygen therapy in
children with acute hypoxemic respiratory failure. A
multicenter study was conducted at 14 metropolitan and
tertiary hospitals in Australia and New Zealand, including 1567
children aged 1 to 4 years requiring hospital admission for
acute hypoxemic respiratory failure. Enrolled children were
randomly allocated 1:1 to high-flow oxygen therapy or standard
oxygen therapy. The primary outcome was length of hospital
stay with the hypothesis that high-flow oxygen therapy reduces
length of stay.
Activity 3 - Study 2
We studied the effect of various modes and duration of air travel on
the risk of venous thrombosis. Consecutive patients younger than
70 y with a first venous thrombosis were invited to participate in the
study, along with their partners who did not have venous
thrombosis. All participants filled in a detailed standardized
questionnaire on general demographic and anthropomorphic
characteristics, as well as on risk factors for venous thrombosis,
including air travelling. The questionnaire was sent to all
participants within a few weeks after the event and covered the
period of 1 y prior to the date of the thrombotic event. When the
participant was unable to fill in the questionnaire, we asked
questions by phone.
Activity 3 - Study 3
The hypothesized link between the measles, mumps, rubella (MMR)
vaccine and autism continues to cause concern and challenge
vaccine uptake. This study evaluates whether the MMR vaccine
increases the risk for autism in children. Participants were 657 461
children born in Denmark from 1999 through 31 December 2010, with
follow-up from 1 year of age and through 31 August 2013. Danish
population and health registries were used to link information on MMR
vaccination, autism diagnoses, other childhood vaccines, sibling
history of autism, and autism risk factors to all children included in the
study.
“Prospective” (concurrent) cohort study
Case-control study
Exposed
to factor
Exposed
to factor
Sample of
individuals
without outcome
Unexposed
to factor
Exposed
to factor
Unexposed
to factor
Unexposed
to factor
Outcome
No outcome
Outcome
Sample of
individuals
without outcome
No outcome
No outcome
R
Exposed
to factor
Unexposed
to factor
Outcome
No outcome
Outcome
No outcome
Follow
Trace
Past time
Outcome
Randomized controlled trial
“Retrospective” (historical) cohort study
Sample of
individuals
without
outcome
No outcome
Sample of
individuals with
outcome
Unexposed
to factor
Exposed
to factor
Outcome
Present time
Future time
Take home messages


The main analytical designs are RCT, cohort study,
case-control study and cross-sectional study.
Directionality:
 RCTs
and cohort: from exposure to outcome
 Case-control: from outcome to exposure
 Cross-sectional: exposure and outcome simultaneously


Timing: cohort studies can be concurrent (“prospective”)
or historical (“retrospective”).
More on specific designs on future sessions.
Introduction to Populations and
Samples.
Juan Manuel Lozano, MD, MSc
Division of Medical and Population Health Sciences Research
Department of Medical Education
Herbert Wertheim College of Medicine
Florida International University
Objectives
Goal RU1.EB1.4 Understand basic principles of populations and samples
in health research
Given a clinical or research scenario, graph or table, students should be
able to:
RU1.EB1.4.1 Define population and sample in epidemiological research.
RU1.EB1.4.2 Contrast the use of samples and populations in health
research.
RU1.EB1.4.3 Apply the principles, advantages, and limitations of
probabilistic and non-probabilistic sampling methods in examples of
epidemiological research.
 All subjects in a
population:
o can rarely be studied.
o do not need to be
studied.
o should not be studied.
 We can work on a
sample of those subjects
(from the population) by
design and then
generalize to the
population (inference).
Samples in health research
Samples in health research
Target or reference population: group to
which the researcher intends to generalize
findings.
e.g., all children with learning disabilities in
the US.
Samples in health research
Target or reference population: group to
which the researcher intends to generalize
findings.
e.g., all children with learning disabilities in
the US.
Accessible population: portion of the target
population that has a chance of being
selected.
e.g., children with learning disabilities in
Miami's school system.
Samples in health research
Target or reference population: group to
which the researcher intends to generalize
findings.
e.g., all children with learning disabilities in
the US.
Accessible population: portion of the target
population that has a chance of being
selected.
e.g., children with learning disabilities in
Miami's school system.
Sample: subset of the accessible population
that is selected.
e.g., children with learning disabilities who
attended school X and Y at a given date.
Steps for acquiring study subjects
1. Selection criteria: characteristics of the target
population that serve the research question well.
 Inclusion:
requirements to get subjects in.
 Exclusion: reasons to leave subjects out.
2. Sampling: selecting a subset of individuals from the
accessible population.
Selection criteria

Inclusion: requirements to get in the study (relevant for
research question, efficient for the study):
 Demographic
(age, gender, etc.).
 Clinical (specific condition, severity, comorbidities, etc.).
 Geographic: place and setting.
 Temporal: time frame of the study.

Exclusion criteria: subset of those who fulfill the inclusion
criteria that need to be excluded because:
 High
likelihood of being lost to follow-up.
 Inability to provide good data.
 High risk of potential side effects.

Probability (random):
 Driven
by probabilistic
methods (like a raffle).
 Participants have a
known (but not
necessarily equal)
probability of being
selected.

Non-Probability:
 Driven
by nonprobabilistic methods
(“cherry picking”).
Types of samples (some)

Probability (random):
 Simple
 Systematic
 Stratified


Proportional
Disproportional
 Cluster
 Multistage

Non-Probability:
 Convenience
 Purposive
 Quota
 Snowball
Types of samples (some)
Characteristics of a good sample
1. Representative of the target population:
 More
easily achieved using probabilistic sampling methods.
 Particularly critical for descriptive studies like surveys.
 Main sources of bias:


Voluntary bias.
Nonresponse.
2. Adequate size:
 Large enough to control random sources of error.
 Not too large (avoid wasting resources).
Common objectives in health research
Objective
Test of
Sampling?
hypotheses?
Most often
1. To describe (descriptive studies): No
• The frequency / distribution of a
probabilistic
problem or finding
2. To compare (analytical studies):
•
•
•
•
Causality: exposed vs. non exposed
Interventions: treated vs. untreated
Diagnosis: a test vs. a “gold standard”
Prognosis: exposed vs. non exposed
Yes (always)
Most often
convenience
Take home messages



Most health research is based on the study of samples.
Advantage: efficiency: examining a subset of subjects reduces
cost, time and effort.
Disadvantage: error may be introduced if the sample:
not represent the target population (findings may not generalize).
 Is not large enough (poor precision on estimates).
 Does

The appropriate sample depends on the research question:
 Descriptive:
Random sample is usually more appropriate.
 Analytical: Convenience sample is more often used.