2. Learning
Objective
Identify the following for an NCD problem:
● the type of study to conduct (descriptive or analytic, case-control
or cohort)
● what data to collect
● the appropriate sources for that data
● the appropriate measure of association for the data
● and the sampling method
Descriptive and Analytic Studies 2
3. Lesson
Overview
Principles of epidemiologic studies
Definition and characteristics of descriptive studies
Definition and characteristics of analytic studies
Methods of sampling
Descriptive and Analytic Studies 3
6. WhyConduct
Studies?
To learn what causes a disease
in order to prevent it
To generate and test
hypotheses in order to learn
about disease cause-effect
relationships
Causes are often called
exposures
Effects are often called
outcomes
Descriptive and Analytic Studies 6
7. Exposures and
Outcomes
Exposure: Factors that increase a person’s risk for an
adverse health condition
Outcome: Effect of exposure (often, adverse health
condition)
Different levels of exposure can be associated with different
outcomes
Descriptive and Analytic Studies 7
Exposure Outcome
8. Research
Questions
1. Does obesity increase the risk for diabetes?
What is the exposure?
What is the outcome?
2. Does diabetes increase the risk for cardiovascular
disease?
What is the exposure?
What is the outcome?
Descriptive and Analytic Studies 8
obesity
diabetes
diabetes
cardiovascular disease
10. Risk Factors
andCauses
Risk factors may not be the same
as causes
Men are at higher risk for lung
cancer, but being male is not a
cause of lung cancer
Sleepiness, intoxication, and
dangerous road conditions are
risk factors for car crashes and
can also be the cause
Descriptive and Analytic Studies 10
11. Establishing
Temporality
Causes must occur before the disease
Causes must occur within a certain time frame
Lung cancer often takes years to develop
Descriptive and Analytic Studies 11
12. Study Designs
toAssess
Cause and
Effect
Two types of epidemiologic studies:
Experimental: Investigators expose study subjects to a
potential risk factor (or cure) and document outcomes
Observational: Investigators observe the effect of
exposures on study subjects
Descriptive and Analytic Studies 12
14. Descriptive or
Analytic
Studies
Epidemiologic studies can be descriptive or analytic.
Descriptive studies
Ask who, where, when, and what
Generate hypotheses
Analytic studies
Test hypotheses
Ask why and how
Descriptive and Analytic Studies 14
15. Research
Question
Your office is interested in assessing the relationship between
number of years spent in school and age at first sexual intercourse.
1. What is the exposure?
2. What is the outcome?
3. Descriptive or analytic study?
4. Experimental or observational study?
Descriptive and Analytic Studies 15
Years of schooling
Age at first intercourse
Descriptive
Observational
18. Descriptive
Studies
●Characterize who, where, and when in relation to what (outcome)
Person
Characteristics (age, sex, occupation) of the individuals affected by the
outcome
Place
Geography (residence, work, hospital) of the affected individuals
Time
When events (diagnosis, reporting; testing) occurred
Descriptive and Analytic Studies 18
20. Cross-
Sectional
Study
Purpose: To learn about the knowledge, attitudes, and practices of
a population at one point in time (like a photo “snap shot”)
Design: No comparison group
Population: All members of a small, defined group or a sample from
a large group
Results: Produces estimates of the prevalence of the outcome of
interest
Descriptive and Analytic Studies 20
21. When to
Conduct a
Cross-
Sectional
Study
To estimate burden of a health
condition in a population
To learn knowledge, attitude
and practices of individuals in
a population
To monitor trends over time
with serial cross-sectional
studies
Descriptive and Analytic Studies 21
25. Example:
Cross-
Sectional
Study
Objective
To estimate the magnitude and patterns of violence against
pregnant women
Study
Population-based, household, cross-sectional study in Mbeya and
Dar es Salaam,Tanzania, 2001-2002
Result
Violence experienced by 7% in Dar es Salaam and 12% in Mbeya
Descriptive and Analytic Studies 25
30. Developing
Hypotheses
A hypothesis is an educated guess
about an association that is testable
in a scientific investigation
Descriptive data (Who?What?
Where?When?) provide
information to develop hypotheses
Hypotheses tend to be broad
initially and are then refined to have
a narrower focus
Descriptive and Analytic Studies 30
31. Developing
Hypotheses
Example
Hypothesis: People who ate at the church picnic were more likely
to become ill
Exposure is eating at the church picnic
Outcome is illness – this would need to be defined, for example, ill
persons are those who have diarrhea and fever
Hypothesis: People who ate the egg salad at the church picnic
were more likely to have laboratory-confirmed Salmonella
Exposure is eating egg salad at the church picnic
Outcome is laboratory confirmation of Salmonella
Descriptive and Analytic Studies 31
33. Analytic
Studies
Analytic studies test
hypotheses about exposure-
outcome relationships
Measure the association
between exposure and
outcome
Include a comparison group
Descriptive and Analytic Studies 33
36. CohortStudies
What is a cohort?
A well-defined group of individuals who
share a common characteristic or
experience
Example: Individuals born in the
same year
What are other examples of cohorts?
Descriptive and Analytic Studies 36
37. CohortStudy
(longitudinal
study, follow-up
study)
● Participants classified according to exposure
status and followed-up over time to ascertain
outcome
● Ensures temporality (exposure occurs before
observed outcome)
Can be used to find multiple outcomes from a single exposure
Appropriate for rare exposures or defined cohorts
Descriptive and Analytic Studies 37
39. Types of
CohortStudies
Prospective cohort studies
Group participants according to past or current exposure and
follow-up into the future to determine if outcome occurs
Retrospective cohort studies
At the time that the study is conducted, potential exposure
and outcomes have already occurred in the past
Descriptive and Analytic Studies 39
42. When to
Conduct a
CohortStudy?
When the exposure is rare and the outcome is common
Agricultural pesticide use and cancer events
To learn about multiple outcomes due to a single exposure
Health effects of a nuclear power plant accident
Descriptive and Analytic Studies 42
43. Analysis of
CohortStudies
Risk:
Quantifies probability of experiencing the outcome of
interest in a given population
Calculation: Number of new occurrences of outcome /
population at risk
Example:
29 new cases of diabetes in a community
100,000 people in the community at risk for diabetes
Descriptive and Analytic Studies 43
What is the risk of diabetes? 29/100,000
44. Risk Ratio
Can also be called Relative Risk or RR
Quantifies a population’s risk of disease from a particular
exposure
Calculation:
= Risk in the exposed group / Risk in the unexposed group
When using Person-Time as denominator
In one group - Rate
Comparison of two rate – Rate ratio
Descriptive and Analytic Studies 44
45. RRStrength
Scales
Descriptive and Analytic Studies 45
RR Strength RR
0.71 – 0.99 Weak 1.01 – 1.50
0.41 – 0.70 Moderate 1.51 – 3.00
0.00 – 0.40 Very strong >3.00
OlecknoWA. Essential epidemiology: principles and applications. Prospect
Heights, IL 2002;108.
46. 2x2Tables
Typical set-up:
Descriptive and Analytic Studies 46
Cases Non-cases
Exposed a b
Not Exposed c d
Exposed Not Exposed
Cases a b
Non-cases c d
Alternate set-up:
RR = a/(a+c)
b/(b+d)
RR = a/(a+b)
c/(c+d)
47. Example:
Risk Ratio
Descriptive and Analytic Studies
500 150,000 200,000
200 780,000 800,000
700 930,000 1,000,000
Risk (Smokers) = 500/200,000 = 0.0025 or 25/10,000
Risk (Non-smokers) = 200/800,000 = 0.00025 or 25/100,000
Risk Ratio = 0.0025/0.00025 = 10
Interpretation: The one-year risk of lung cancer among those who
smoke is 10 times the risk among those who do not smoke.
Smokers Lung Cancer No Lung Cancer Total
Yes
No
Total
47
48. Example:
Risk Ratio
Person time
NHANES – Follow-up Study (male diabetics subset)
Original enrollment 1971- 1975
Follow-up 1982 – 1984
Complete follow-up on:
Mortality Rate Ratio: 100/1414.7 ÷ 811/28,029.8 = 70.7/1000 ÷
28.9/1000= 2.5
Descriptive and Analytic Studies
48
49. Practice:
Risk Ratio
Example: In a given community…
19 new cases of diabetes among obese individuals
10 new cases of diabetes among non-obese
individuals
100,000 people in the community at-risk for
diabetes (no one already has diabetes)
25% of the community are estimated to be obese
Descriptive and Analytic Studies 49
What impact does obesity have on the risk of diabetes?
50. Practice:
Risk Ratio
Descriptive and Analytic Studies
19 24,981 25,000
10 74,990 75,000
29 99,971 100,000
Risk (obese) = 19/25,000 = 0.00076 or 76/100,000
Risk (non-obese) = 10/75,000 = 0.00013 or 13/100,000
Risk Ratio = .00076 / .00013 = 5.6
Interpretation: The risk of diabetes among those who are obese is 5.6
times the risk among those who are not obese.
Obese Diabetes No diabetes Total
Yes
No
Total
50
52. Case-Control
Study
Purpose:
To study rare diseases (when <5% of population experience
the outcome)
To study multiple exposures that may be related to a single
outcome
Study Subjects
Participants selected based on outcome status:
Case-subjects have outcome of interest
Control-subjects do not have outcome of interest
Descriptive and Analytic Studies 52
54. When to
Conduct a
Case-Control
Study
The outcome of interest is rare
Multiple exposures may be associated with a single outcome
Funding or time is limited
Descriptive and Analytic Studies 54
55. Case-Control
Study:
Analysis
Format
Exposure Cases Controls
Yes a b
No c d
Descriptive and Analytic Studies 55
Exposure odds ratio (OR) ≈ RR when disease is rare
Odds of being a case among the exposed = a/b
Odds of being a case among the unexposed = c/d
Exposure odds ratio = (a/b)/(c/d) = (a*d)/(b*c)
(Cross-product ratio)
56. ExampleOdds
Ratio
Hospital birth Fistula No Fistula
Yes 30 150
No 70 250
Descriptive and Analytic Studies 56
Odds of fistula among the exposed = 30/150 = 0.20
Odds of fistula among the unexposed = 70/250 = 0.28
Odds ratio = (30/150)/(70/250) = (30*250)/(70*150) = 0.71
57. Practice
Odds Ratio
Mine Worker Lead Poisoning No Lead
Poisoning
Yes 42 247
No 107 825
Descriptive and Analytic Studies 57
Odds of being a case among the exposed = ?
Odds of being a case among the unexposed = ?
Odds ratio
= (a/b)/(c/d) = ?
58. Practice
Odds Ratio
Mine Worker Lead Poisoning No Lead
Poisoning
Yes 42 247
No 107 825
Descriptive and Analytic Studies 58
Odds of being a case among the exposed = 42/247 = 0.170
Odds of being a case among the unexposed = 107/825 = 0.130
Odds ratio = [(a/b) / (c/d)] = [(42/247) / (107/825)] = 1.3
Interpretation: Mine workers had 1.3 times the odds of developing lead
poisoning than did people who did not work in a mine.
59. Prevalence
Ratio &
Prevalence
Odds Ratio
Chronic disease – date of onset is unknown
Measure prevalence rather than incidence
RR -> PR (prevalence ratio)
OR -> POR (prevalence odds ratio)
60. Prevalence
Ratio
Chronic disease – date of onset is unknown
Measure prevalence rather than incidence
RR -> PR (prevalence ratio)
OR -> POR (prevalence odds ratio)
61. Prevalence
Odds Ratio
Usually from a cross-sectional study
Similar to odds ratio from case control study
Calculated same way as odds ratio:
POR = a*d /c*b
62. Prevalence
Ratio &
Prevalence
Odds Ratio
Prevalence of breast cysts among ever users = 124/3247 = .038
Prevalence of breast cysts among never-users = 77/2644 = .029
Prevalence ratio = .038/.029 = 1.3
Prevalence odds ratio = 124 * 2557 / 3123 * 77
= 1.3
64. Practice
exercise 1
Background:
NCDs such as type 2 diabetes are poorly understood and under-
prioritized in many low-to-middle income countries.
You want to determine the risk of type 2 diabetes associated with
cardiovascular risk factors such as obesity and abdominal fat mass
in your country.
Questions:
What type of study would you conduct and why?
What is the measure of association to calculate for this study?
Descriptive and Analytic Studies 64
65. Practice
exercise 2
Background:
The prevalence of prostate cancer has increased in your country
over the last 5 years.
You want to examine the association between calcium intake and
prostate cancer risk.
You have limited time and funding to conduct this study.
Questions:
What type of study would you conduct and why?
What is the measure of association to calculate for this study?
Descriptive and Analytic Studies 65
66. Practice
exercise 3
Background:
Cardiovascular disease (CVD) is of growing concern; however your
country has no recent data on the burden of this disease.
You want to estimate the burden of cardiovascular disease in the
two main cities in your country.
Questions:
What type of study would you conduct and why?
What is the measure of association to calculate for this study?
Descriptive and Analytic Studies 66
68. Discussion
Question
Why do we use sampling?
Can’t get information on everyone in a population
Efficiently gets information on a large population
Obtains a representative sample of a population
68
Descriptive and Analytic Studies
69. Sampling
Methods
Two main types of sampling methods:
Probability sampling
Non-probability sampling
Descriptive and Analytic Studies 69
70. Probability
Sampling
What are types of probability-based samples?
Simple random sampling
Systematic random sampling
Stratified random sampling
Cluster sampling
Descriptive and Analytic Studies 70
71. Simple
Random
Sample
Principle
Equal chance/probability of drawing each unit
Procedure
List all units (persons) in a population
Assign a number to each unit
Randomly select units
71
Descriptive and Analytic Studies
73. Example:
Simple
Random
Sample
Example: Calculate the prevalence of tooth decay among
1200 children attending a school
(sample size =100)
List all children attending the school
Each child assigned a number from 1 to 1200
Randomly select 100 numbers between 1 and 1200
Descriptive and Analytic Studies 73
74. Advantages &
Disadvantages:
Simple Random
Sample
Advantages
Simple!
Disadvantages
Need complete list of units
Units may be scattered and poorly accessible
74
Descriptive and Analytic Studies
75. Systematic
Random
Sample
Principle
Select sample at regular intervals based on sampling
fraction
Procedure
List all units (persons) in a population
Assign a number to each unit
Calculate sampling fraction (population size ÷ sample
size)
Select first unit at random based on sampling fraction
Subsequent units are chosen at equal intervals
75
Descriptive and Analytic Studies
76. Example:
Systematic
Random
Sample
Example: Calculate the prevalence of tooth decay among
1200 children attending a school
(sample size =100)
List all children attending the school
Each child assigned a number from 1 to 1200
Sampling fraction =1200/100 = 12
Randomly select a number between 1 and 12
Example: 8
Select every 12th child, starting with child #8
Example: 8, 20, 32, 44…
76
Descriptive and Analytic Studies
78. Stratified
Random
Sample
Principle
Select random samples from within homogeneous
subgroups (strata)
Procedure
List all units (persons) in a population
Divide the units into groups (called strata)
Assign a number to each unit within each stratum
Select a random sample from each stratum
Combine the strata samples to form the full sample
78
Descriptive and Analytic Studies
79. Method:
Stratified
Random
Sample
Sampling frame divided
into groups (age, sex,
socioeconomic status)
Units in each group
have the same
probability of selection,
but probability differs
between groups
Descriptive and Analytic Studies 79
Men Women
Probability:1/32 Probability: 1/16
80. Example:
Stratified
Random
Sample
Example: Calculate the prevalence of tooth decay among
1200 children attending a school, with equal representation of
males and females
(sample size =100)
List all children attending the school
Divide the children into two groups
540 males and 660 females
Assign each child a number
Males: 1 to 540
Females 1 to 660
Randomly select 50 males and 50 females
Descriptive and Analytic Studies 80
81. Advantages &
Disadvantages:
Stratified
Random
Sample
Advantages
Can get separate estimates from the whole population
and from individual strata
Precision increased if less variability within strata than
between strata
Disadvantages
Can be difficult to identify strata
Descriptive and Analytic Studies 81
82. Class
Discussion
Question
What are some examples of strata that you might sample
within?
Race/ethnicity
Age group
Gender
Geographic location
Socioeconomic status
Smoking status
Occupation
Education
Many possibilities!
Descriptive and Analytic Studies 82
83. ClusterSample
Principle
Select all units within randomly selected geographic
clusters
Procedure
Divide population into geographic groups (clusters)
Assign a number to each cluster
Randomly select clusters
Sample all units within selected clusters OR select a
random sample of units within selected clusters
Descriptive and Analytic Studies 83
85. Advantages &
Disadvantages:
ClusterSample
Advantages
List of sampling units not required
More efficient for face-to-face interviews when units
are dispersed over a large area
Disadvantages
Loss of precision due to correlation within clusters
This correlation needs to be taken into account in
sample size calculations and analysis (“design effect”)
Descriptive and Analytic Studies 85
86. Non-
probability
Sampling
Probability of selection is unknown or zero
Inexpensive
Results not generalizable
Results often biased
Common types of non-probability sampling:
Convenience sampling
Snowball sampling
Voluntary sampling
Descriptive and Analytic Studies 86
88. Choosing a
Sampling
Method
Consider:
Population to be studied
Size/geographic distribution
Availability of list of units
Heterogeneity with respect to variable
Level of precision required
Resources available
Descriptive and Analytic Studies 88
89. Practice
exercise 4
Background: You will choose a sampling method for each of the
following studies.
Questions:
What sampling method would you use for:
1. The cross-sectional study on CVD described in Practice Exercise
#3?Why?
2. A one-time survey of citizens’ attitudes toward smoking and
second-hand smoke in response to proposed legislation to
impose a ban on smoking in restaurants.Why?
3. Serosurvey of blood lead levels (or urinary arsenic levels) of
prisoners entering the nation’s largest prison (or pregnant
women entering the nation’s largest maternity ward) to
determine average level of exposure in the population.
Descriptive and Analytic Studies 89
91. Descriptive vs.
Analytic
Epidemiology
Descriptive epidemiology deals with the questions:Who,
What,When, andWhere
Types of studies
Aggregate: Ecological study.
Individual: Case study, case series, cross-sectional
study
Analytic epidemiology deals with the remaining questions:
Why and How
Types of studies
Experimental: Randomized control trial
Observational: Cohort, case-control, cross-sectional
Descriptive and Analytic Studies 91
92. Cohort vs.
Case-Control
Studies
Descriptive and Analytic Studies 92
Cohort Study Case-Control Study
Preferred Study Design
When…
Members are easily
identifiable
Members are easily accessible
Exposure is rare
There may be multiple
diseases involved
Identifying entire cohort
would be too costly or time
consuming
Accessing entire cohort would
be too costly or time
consuming
Illness is rare
There may be multiple
exposures involved
Study Group Exposed persons Persons with illness (cases)
Comparison Group Unexposed persons Persons without illness
(controls)
93. Sampling
Advantages
and
Disadvantages
Probability Sampling
Advantages
Results are
generalizable
Representative
Disadvantages
Expensive
Logistically difficult
Time-intensive
Non-Probability Sampling
Advantages
Easy
Quick access to
certain groups
Disadvantages
Pertains only to those
in the study
Results are not
generalizable
Descriptive and Analytic Studies 93
95. Review
1. What are 3 types of descriptive studies?
Answer: Case study, case series, and cross-sectional.
Ecologic studies are also descriptive.
2. What are 3 analytic study designs?
Answer: Cohort, case-control, and cross-sectional (if
appropriately designed). Randomized controlled trials are
also analytic.
Descriptive and Analytic Studies 95
96. Review
3. In a cross-sectional study, information about the exposure is
collected _________ information is collected about the
outcome.
a. before
b. after
c. at the same as
Descriptive and Analytic Studies 96
97. Review
4. In a prospective cohort study design , information about the
exposure is collected _________ information is collected about
the outcome.
a. before
b. after
c. at the same as
Descriptive and Analytic Studies 97
98. Review
5. In a case-control study, participants are selected to participate
in the study on the basis of ______.
Answer: Outcome (or disease) status
6. A ________ sample is a selection of the larger population at
risk, chosen to efficiently represent the disease and exposure
experience of the larger population.The goal of this type of
sampling is to minimize bias; however this sample can take
more time and resources to conduct.
Answer: random
Descriptive and Analytic Studies 98
99. Review
7. If you conduct a case-control study, which ratio measure of
association can you calculate with the data?
Answer:The odds ratio, which is the odds of the outcome in
the exposed group divided by the odds of the outcome in
the unexposed group.
8. What are the units for risk ratio?
Answer:There are no units.This is a ratio measure of
association and divides the risk of the outcome among the
exposed over the risk of the outcome among the
unexposed.
Descriptive and Analytic Studies 99
100. Group activity
#1
What is the burden of hypertension in Riyadh or KSA?
1. Do a literature review to find out the latest study published
for this question.
2. Describe the study methods and sampling method used
3. Could this study have been conducted in different or better
way?
Descriptive and Analytic Studies 100
101. Group activity
#2
What are the risk factors of breast cancer in Riyadh or KSA?
1. Do a literature review to find out the latest study published
for this question.
2. Describe the study methods and sampling adopted used
3. Could this study have been conducted in different or better
way?
Descriptive and Analytic Studies 101
102. Group activity
#3
What is prevalence and factors associated with COPD in
Riyadh or KSA?
1. Do a literature review to find out the latest study published
for this question.
2. Describe the study methods and sampling adopted used
3. Could this study have been conducted in different or better
way?
Descriptive and Analytic Studies 102