Faculty of Medicine Introduction to Community Medicine Course

(31505201)

Unit 4 Epidemiology

Introduction to Epidemiology

EPIDEMIOLOGICAL STUDY METHODS

By

Hatim Jaber MD MPH JBCM PhD

6+8 12- 2016

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Presentation outline

Time

Introduction and Definitions 12:00 to 12:10

CLASSIFICATION

12:10 to 12:20

STUDY DESIGNS

12:20 to 12:30

VARIOUS DESIGNS

12:30 to 12:50

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Invisible and silent killers

Invisible killer AIR POLLUTION

Silent killer

Important

“What you cant measure you cant control!”

epidemiological study methods are used to study your health and my health and its determinants, as we join hands to ensure a healthier us.

INTRODUCTION

• The science of epidemiology has matured significantly from the times of Hippocrates and john snow [physician] that the techniques for analyzing data vary depending on the type of disease being monitored but each study will have similarities. environmental factors can influence the occurrence of the diseases.

• epidemiology study of what is upon the people

• Derived from the greek terms epi=upon,among. demos=people, district. Logos study, word.

DEFINITION OF EPIDEMIOLOGY

• Epidemiology is the study of distribution and determinant of health related state or event in a specified human population and the application of this study to the control of health problem.

EPIDEMIOLOGY [DEFINITION OF KEY TERMS]

• Distribution : Frequency (including rates & risks) & pattern of health events(person, place, time)

• Determinants : factors or events that are capable of bringing about a change in health

• Human population : Epidemiology examines health events among population groups rather than individuals.

EPIDEMIOLOGY [DEFINITION OF KEY TERMS]

• Health related states: infections, chronic diseases & physiological events &various states of health such as disability, injury, mortality

• Health related events : immunization, hospital attendance, bed occupancy

• Application : basis for directing interventions

CLASSIFICATION

CLASSIFICATION(CONT.)

CORRELATIONAL

Epidemiologic Study Designs

Aims of Epidemiologic Research

1. Describe the health status of a population

2. To assess the public health importance of diseases

3. To describe the natural history of disease,

4. Explain the etiology of disease

5. Predict the disease occurrence

6. To evaluate the prevention and control of disease

7. Control the disease distribution

Descriptive epidemiology

Analytic epidemiology

Applied epidemiology

Descriptive and Analytical Epidemiology

1. Descriptive epidemiology • Describes the occurrence of disease (cross-

sectional)

2. Analytic epidemiology: • Observational (cohort, case control, cross-

sectional, ecologic study) – researcher observes association between exposure and disease, estimates and tests it

• Experimental (RCT, quasi experiment) – researcher assigns intervention (treatment), and estimates and tests its effect on health outcome

OBSERVATIONAL VS EXPERIMENTAL STUDIES

• Observational studies

Allow nature to take its cause; the investigator measures but does not intervene

• Descriptive study: focuses on the description of the occurrence of a disease in a population

• Analytical study analyses relationships between health status and other variables

OBSERVATIONAL VS EXPERIMENTAL STUDIES

• Experimental or interventional studies: involve an active attempt to change a disease determinant(e.g an exposure or a behaviour) or the progress of a disaese (through treatment)

• The studies are based on a group which has had the experience compared with control group which has not had the experience.

PURPOSE OF DESCRIPTIVE EPIDEMIOLOGY

• To generate hypothesis

• To permit evaluation of trends in health & disease and comparisons among countries and subgroups within countries.

• To provide a basis for planning, provision and evaluation of health services

• To identify problems to be studied by analytical methods and to suggest areas that may be fruitful for investigation

CASE STUDIES(CASE SERIES)

• Case reports:documents unusual medical occurrence and can represent the first clues to the formulation of hypothesis, generally report a new or unique findings and previous undescribed disease.

• Case series: collection of individual case reports which may occur within a fairly short time, and experience of a group of patients with similar diagnosis.

Case Series

Advantages Useful for hypothesis generation

Informative for very rare disease with few established risk factors

Usually of short duration.

Disadvantages Cannot study cause and effect relationships

Cannot assess disease frequency

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CROSS-SECTIONAL STUDY

• It is also called epidemiologic study or prevalence study

• It analyses (describes)data collected on a group of subjects at one point in time rather than over a period of time. i.e they survey exposure and disease at a single point in time.

• Both exposure and outcome variables are been evaluated at the same point in time(without any inbuilt directionality)

• Most sophisticated descriptive study • It answers the question “WHAT IS HAPPENING RIGHT

NOW?”

QUESTION: “WHAT IS HAPPENING?” NO DIRECTION OF INQUIRY

o onset time end

subjects

With outcome

Without outcome

CROSS-SECTIONAL STUDY

ADV

• Best for determining the status quo(prevalence)

• Quick

• Relatively inexpensive

DISADV

• Only a snapshot at a time leading to a misinformation

• Response rate may be low ,with result not representative of the population

Cross-sectional studies

Disadvantages Weakest observational design,

(it measures prevalence, not incidence of disease). Prevalent cases are survivors

The temporal sequence of exposure and effect may be difficult or impossible to determine

Usually don’t know when disease occurred

Rare events a problem. Quickly emerging diseases a problem

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CORRELATIONAL STUDY DESIGN

• A study comparing incidence/prevalence of one event against another on a global scale

• Measures that represent characteristics of entire populations are used to describe the disease in relation to some factor of interest (such as age, calendar time, food consumption, drug use and utilization of health services)

CORRELATIONAL STUDY DESIGN

ADV

• Compares events among nations

DISADV

• Doesn’t compare individuals, so it might lead to overgeneralization.

• Attack rate is a Cumulative Incidence; it shows the risk (probability) of disease to occur in a population

• In regard to risk, measles is the most important disease to public health while rubella being the least

Hypothetical Data

Measles Chickenpox Rubella

Children exposed

Children ill

Attack rate

251

201

0.80

238

172

0.72

218

82

0.38

Attack rate = Number of Ill persons (new cases) Population at risk exposed

Which Disease if More Important to Public Health? Measure of Disease Occurence

Description of Disease Distribution in the Population

Disease affects

mostly people

under five years of

age

Disease affects

people living

alongside the river

Disease reaches

its peak in

frequency in Week

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ANALYTICAL STUDIES

Two basic designs:

• Case – control or retrospective study

• Cohort or prospective

NOTE

• There must be a comparison group

• No control No conclusion(NCNC)

CASE CONTROL OR CASE HISTORY STUDY

• A group of affected people is compared to unaffected people(the control)

• It’s a LONGITUDNAL STUDY (like cohort study) because it’s a study over a period of time.

• Subjects are selected based on a particular outcome and a study backwards in time to try to detect the causes or risk factors that may have earlier been reported in a descriptive study

• Subjects are then matched and assigned into the two groups. Subject selected on the basis of disease[e.g lung cancer].

• Sometimes called a retrospective study because of the direction of study

CASE CONTROL OR CASE HISTORY STUDY

Advantages of case control

• It is relatively easy to carry out bcos we go back to existing records in the hospital

• It is also rapid and inexpensive

• It requires comparatively few subjects

• It can assist one in studying different etiological factors

• One does not need an ethical clearance

• There is no risk to the subject

Disadvantages of case control

• It introduces bias

• To select an appropriate control could be difficult

• It may be difficult to distinguish between the cause of a disease and an associated factor

COHORT STUDY

• A cohort is a group of people who have something in common and remain part of a group over an extended time

• A group of people exposed to a suspected etiological agent are compared with a matched control who have not been similarly exposed. Subject selected on the basis of exposure [etiological factor; cigarette smoking]

• Follow-up over a period to compare the outcome

• Also a longitudinal study or prospective study

time

Study begins here

Study

population

free of

disease

Factor

present

Factor

absent

disease

no disease

disease

no disease

present

future

ADVANTAGES OF COHORT

• There is no bias

• The risk can be calculated -the incidence can be calculated

• It is effective for studying rare exposures

• It allows the study of the natural history of the disease

• It assists in determining the temporal relationship between the etiological factor & the disease

Disadv of cohort study

• It takes a long time

• It is expensive

• Large number of subjects are needed

• There could be changes in the standard methods or diagnostic criteria

EXPERIMENTAL STUDIES

• Studies in which 1 group is deliberately subjected to an experience compared with a control group with no similar experience

• The gold standard in medicine -it proves causality

• Can be controlled or uncontrolled

UNCONTROLLED EXPERIMENTAL STUDIES

• Intervention is not compared with a control

• The aim is to confirm that the Intervention made a difference

CONTROLLED EXPERIMENTAL STUDIES

• In this study, a drug or procedure is compared to:

1. Another drug

2. Procedure

3. Placebo

4. Previously accepted tx

• The aim is to proove the difference due to tx

CONTROLLED EXPERIMENTAL STUDIES

• Blind trial-single or double

• Control could be:

A. METHODOLOGY

1. Concurrent or parallel: randomized or non- randomized(quasi)

2. Sequential control: self controlled or cross over

3. External control

B. STUDY POPULATION

1. Clinical trials

2. Field trials

3. Community trials

EXPERIMENTAL STUDIES

ADV

• Best study type

• Greatest prove of causality

• Gold standard for other design

• Least bias

• Proves best treatment or procedure efficacy

DISADV

• Greatest expense

• Long duration

• Unproven facts adopted by community can hinder study acceptance

overview

Study Design and Its Strength of Evidence

1. Systematic review, meta-analysis: secondary data analysis

2. Randomized Controlled Trials (RCT)

3. Cohort: prospective or retrospective

Quasi experiment

4. Case control: prospective or retrospective

5. Cross sectional

6. Case Reports / Case Series

Strongest

evidence

Weakest

evidence

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Epidemiologic Measures of Association

• Compute & Interpret Relative risk (RR) & Odds ratio (OR) as a measure of association between exposure and Disease

• Understand when OR approximates RR

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Definitions Association

• A statistical relationship between two or more variables

Risk • Probability conditional or unconditional of

the occurrence of some event in time

• Probability of an individual developing a disease or change in health status over a fixed time interval, conditional on the individual not dying during the same time period

Absolute risk

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Association between exposure & Disease

• Question:

Is there an excess risk associated with a given exposure?

• Objective:

To determine whether certain exposure is

associated with a given disease

• Methodology:

Use one of the epidemiologic study designs

Cohort

Case-control

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Cohort Study

• Assess the cumulative incidence (CIE+) of disease in an exposed group (absolute Risk)

Assess the cumulative incidence (CIE-) of disease in unexposed group (absolute Risk)

e.g. Coronary Heart Disease (CHD) Risk among Smokers

1-year risk of CHD among smokers (CIE+)*

CHD

Yes No Total

Smokers 84 2916 3000

CIE+ = 84/3000 = 28/1000/yr (1-risk of CHD among smokers)

Cont.

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CHD Risk among non-smokers

• 1-year risk of CHD among non-smokers (CIE-)

CHD

Yes No

• Non-smokers 87 4913 5000

CIE-= 87/5000=17.4/1000/yr (1-yr risk of CHD among non-smokers)

Cont.

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Assessment of Excess Risk (Two methods)

a. Ratio RR (Ratio of two risks; Risk Ratio; Relative Risk)

CIE+ / CIE- = 28/17.4 = 1.6 Interpretation of RR Smokers were 1.6 times as likely to develop CHD as

were non-smokers

b. Difference Difference of two risks (Risk Difference)* CIE+- CIE- = 28.0 – 17.4 = 10.6

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OR (Odds Ratio, Relative Odds) • In case-control study (CCS), we cannot calculate the CI or IR, therefore, cannot calculate the RR “directly” • OR as a measure of association between exposure & disease is used when data are collected in case-control study • OR can be obtained however, from a cohort as well as a case-control study and can be used instead of RR.

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OR in case-control and cohort studies • Cohort study Ratio of the proportion of exposed subjects

who developed the disease to the proportion of non-exposed subjects who developed the disease

• Case-control study Ratio of the proportion of cases who were

exposed to the proportion of controls who were non-exposed

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Odds Ratio

• Odds are ratio of two probabilities i.e. Probability that event occurs / 1-Probability

that event does not occur

• Odds refer to single entity

• If an event has the probability P, then the odds of the same event are P/1-P

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Derivation of OR in Cohort study P D

+|E+ = (exposed developed the disease) = a/(a+b)

P D

-|E+ = (exposed did not develop the disease) = b/(a+b)

Odds of developing disease among exposed = D+|E+/1-P D-|E+ = a/(a+b) b/(a+b) = a/b P D

+|E- = (non-exposed developed the disease) = c/(c + d)

P D

-|E- = (non-exposed did not develop the disease)= d/(c + d)

Odds of developing disease among non-exposed = = PD+|E

-/1-P D+|E- = c/(c+d)

d/(c + d) = c/d

Odds ratio a/b : c/d = ad/bc

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OR in case-control study

In case-control study RR cannot be

calculated directly to determine the

association between exposure and

disease.

Don’t know the risk of disease

among exposed and un-exposed since we

start recruiting cases and controls.

Can use OR as measure of

association between exposure and

disease in a case control study.

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OR in case-control Study

Probability of case being exposed = Pcase

Probability of case being non-exposed =1-Pcase

Odds of case being exposed = Pcase/1- Pcase

Probability of control being exposed = Pcontrol

Probability of case being non-exposed =1-Pcontrol

Odds of control being exposed = Pcontrol/ 1-Pcontrol

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Derivation of OR in case-control Study

Probability of being exposed among cases = a /(a + c)

Probability of being non-exposed among cases) = c /(a + c)

Odds of being exposed among cases = a/c

Probability of being exposed among controls = b/(b + d)

Probability of being unexposed among controls = d/(b + d)

Odds of being exposed among controls = b/d

OR = ad/bc

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• Past surgery HCV status

HCV+ HCV-

• Yes 59 168

• No 54 48

» 113 216

Example OR in case-control Study

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Odds of Past surgery among HCV+

P1 (Surgery among HCV+) = 59/113

1-P1 (No surgery among HCV+) = 54/113

Odds of surgery among HCV+ ) = 59/54 =

1.09

Odds of Past surgery among HCV-

P2 (Surgery among HCV-) = 168/216

1-P2 (No surgery among HCV-) = 48/216

Odds of surgery among HCV- = 168/48 =

3.5

OR = 3.50/1.09 = 3.21

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When is the OR a good estimate of RR? In CCS, only OR can be calculated as measure

of association

In Cohort study, either RR or OR is a valid

measure of association

When a RR can be calculated from case control

study?

*When exposure prevalence among studied

cases in similar and nearly similar to that of

disease subjects in the population from which

cases are taken.

*Prevalence of exposure among studied

controls is similar to that of non-diseased

population from cases were drawn.

*Rare disease (CI < 0.1)

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Matched case-control study

Matching: In a matched case-control

study each case is matched to a control

according to variables that are known to be

related to disease risk i.e. age, sex, race

Data are analyzed in terms of case-

control pairs rather than for individual

subjects

Four types of case-control

combinations are possible in regard to

exposure history.

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Concordant pairs are ignored since

they don’t contribute in calculation of

effect estimate (i.e. OR)

Disconcordant pairs of cases and

controls are used to calculate the

matched OR.

Matched OR = Ratio of discordant

pairs = b /c

i.e. # of pairs in which cases exposed

/ # of pairs in which controls were

exposed

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Example:

Risk factors for brain tumors in children.

Hypothesis = children with higher birth

weights are at increased risk for certain

childhood cancers.

Cases = Children with brain

tumors

Controls = Normal children

Exposure = Birth weight > 8 lbs.

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8 18

7 38

8 + 1b

Case

s

<8 1b

Total

26

45

15 56 71

8+

1b

<8 1b

Total

Normal Controls

Odds Ratio 18/7 = 2.57

χ2 = 4.00; P = 0.046

Interpretation the is same as before

Example