Introduction to clinical epidemiology

Hui JinDepartment of Epidemiology and Health StatisticsSchool of Public HealthSoutheast University Email: jinhuimc@163.com

Key content

• Definition of epidemiology• Definition of clinical epidemiology• The purpose of epidemiology• The use of epidemiology• The types of epidemiology

• What-why-where-when-which

Increasing demands on their time are squeezing out opportunities to stay abreast of the literature, much less read it critically. Results of several studies indicate an inverse relation between knowledge of contemporary care and time since graduation from medical school. In many jurisdictions, attendance at a specified number of hours of continuing medical education courses is mandatory to maintain a licence to practise. However, the failure of these courses to improve patient care emphasises the importance of selfdirected learning through reading. Many clinicians in practice, though, report that they feel unqualified to read the medical literature critically.

Scientific illiteracy is a major failing of medical education.

Clinicians today are in a bind.

Grimes DA, Schulz KF. An overview of clinical research: the lay of the land. Lancet 2002; 359:57-61.

Field of Medicine

• Basic medicine ----cell,• Clinical medicine ---organ, system, person• Population medicine ---public health

• Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals.“

2 distinct characteristics: preventive rather than curative

population-level rather than individual-level

Quantitative Methods

• Epidemiology and biostatistics are the basic sciences of public health, as quantitative methods in Public health investigations

• Epidemiology is about the understanding of disease development and the methods used to uncover the etiology, progression, and treatment of the disease

• Information (data) is collected to investigate a question

• The methods and tools of biostatistics are used to analyze the data to aid decision making

What is Epidemiology?What is Epidemiology?

• Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems

Disease

Distribution of people, place and time

Risk factors

Such as, smoking, exercise, fatty foods,

stress

Such as, heart disease, a felling of well-being,

lung cancer

Such as, Men 25-40, children, 7 out of 10 women

What is Epidemiology?What is Epidemiology?

THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK FOR :

• TIME• PLACE• PERSON

TimeTime

• Changing or stable?

• Seasonal variation.

• Clustered (epidemic) or evenly distributed (endemic)?

• Point source or propagated.

TimeTime

TimeTime

14

a single point source exposure mode

15

Multi-points spread - human-to-human transmission mode

PlacePlace

• Geographically restricted or widespread (pandemic)?

 • Relation to water or food

supply.  • Multiple clusters or one?

PlacePlace

PlacePlace

PersonPerson• Age

• Socio-economic status

• Gender

• Ethnicity/Race

• Behavior

Some famous examples in the history of Epidemiology

John Snow- 1830’s cholera epidemic spread from

Asia, >60,000 deaths

- New epidemic 1850’s ~ Great Britain

- John Snow, plotted location of deaths in London

- Postulated association between deaths & provision of water

Photo source of color image: Sukon KanchanaraksaPhoto source of portrait: http://www.ph.ucla.edu/epi/snow/fatherofepidemiology.html. Public Domain

Snow’s Cholera Map, 1854

Cholera example Water Population Deaths from Supplier 1851

Cholera Rate*

•Southwark 167,654 844 5.0& Vauxhall

•Lambeth 19,133 18 0.9

•Both 300,149 652 2.2

*Death rate per 1000

20th CenturySmoking & lung cancer

- Increase in lung cancer deaths in 1940s (x 6 within 30 years) published in BMJ

- Postulated cause?

- Doll & Hill published a case-control study (1950) – smokers at higher risk (interviews with hospitalised men)

- Study design criticised

- Subsequent cohort studies – British Doctors Study - provided firm evidence of relationship- demonstrated reversibility of effect

Sir Richard Doll (1912 – 2005) was a British physiologist who became the foremost epidemiologist of the 20th century. He was a pioneer in research linking smoking to health problems. With Bradford Hill, he was credited with being the first to prove that smoking caused lung cancer and increased the risk of heart disease.

• Sir Austin Hill(1897 - 1991), English epidemiologist and statistician, pioneered the randomized clinical trial and, together with Richard Doll, was the first to demonstrate the connection between cigarette smoking and lung cancer.

Use of epidemiology

• Disease surveillance• Causation• Natural history of disease• Description of health status in populations• Evaluation of interventions

Use of epidemiology (Disease surveillance)

1. Frequency with which the disease occurs

2. Definition of the disease 3. Size of the population

from which the cases develop

4. Completeness of the reporting of the cases.

Use of epidemiology (Causation)

Good health Ill health

Genetic factors

Environmental factors (including life style)

Natural history of diseaseNatural history of disease

Stage ofsusceptibility

Stage of subclinical

disease

Stage of clinical disease

Stage of recovery,

disability or death

PRIMARY PRIMARY PREVENTIONPREVENTION SECONDARY SECONDARY

PREVENTIONPREVENTION TERTIARY TERTIARY PREVENTIONPREVENTION

Exposure

Pathologicchanges

Onset of symptoms

Usual time of diagnosis

Use of epidemiology(Description of health status in populations)

Good health

Ill health

TimeProportion with ill health,change over time, change with age, etc.

Use of epidemiology(Evaluation of interventions)

Good health Ill health

Health promotionPreventive measuresPublic health services

Two Broad Types of EpidemiologyTwo Broad Types of Epidemiology

Examining the distribution of a disease in a population, and observing the basic features of its distribution in terms of time, place, and person.

Typical study design:

community health survey (approximate synonyms - cross-sectional study, descriptive study)

Hypothesis generating

Testing a specific hypothesis about the relationship of a disease to a putative cause, by conducting an epidemiologic study that relates the exposure of interest to the disease of interest.

Typical study designs: cohort, case-control

Hypothesis testing

DESCRIPTIVE EPIDEMIOLOGY ANALYTIC EPIDEMIOLOGY

Two Broad Types of EpidemiologyTwo Broad Types of Epidemiology

what Hypothesis generating

States what happened

Explains what a theory says

Lists details

why

Hypothesis testingIdentifies the significance

Shows why something is relevant or suitable

Evaluates the relative significance of details

DESCRIPTIVE EPIDEMIOLOGY ANALYTIC EPIDEMIOLOGY

Her skin is very white, a heritage from her mother.

Descriptive Studies• Correlational studies (may be called ecological

studies)– Examine characteristics of entire populations

• Example: Examination of state data on tobacco sales and mortality from CHD

– First step in examination of a disease exposure relationship

– Strength • Quick and inexpensive, can be used as first step

– Limitation• Doesn’t link specific persons’ exposure with specific outcome

Risk of ecological fallacy• Can’t control for potential confounding factors

Descriptive Studies• Case Reports and Case Series

– Describes single patient or group of patients experience

– Most common form of study published in medical journals

– Strengths• May lead to formulation of new hypotheses

• Important link between clinical medicine and epidemiology

– Limitation• Cannot be used to test hypotheses

Descriptive Studies• Cross-Sectional (or prevalence) Survey

– Exposure and disease status are simultaneously assessed in a population

– Strength• Provides information about the frequency and characteristics

of a disease– Useful for public health– Can provide information concerning the prevalence of

disease or other health outcome in special groups (e.g. occupations)

– Limitation• Can’t determine whether exposure preceded or occurred as a

result of the disease

Descriptive Epidemiology Is Descriptive Epidemiology Is A Necessary Antecedent Of A Necessary Antecedent Of

Analytic EpidemiologyAnalytic Epidemiology To undertake an analytic

epidemiologic study you must first:• Know where to look • Know what to control for • Be able to formulate hypotheses

compatible with laboratory evidence

The Basic Triad Of The Basic Triad Of Analytic EpidemiologyAnalytic Epidemiology

THE THREE PHENOMENA ASSESSED IN ANALYTIC EPIDEMIOLOGY ARE:

HOST

ENVIRONMENTAGENT

AgentsAgents

• Nutrients• Poisons • Allergens • Radiation • Physical trauma • Microbes • Psychological experiences

Host FactorsHost Factors

• Genetic endowment

• Immunologic state

• Age

• Personal behavior

EnvironmentEnvironment

• Crowding

• Atmosphere• Modes of communication –

phenomena in the environment that bring host and agent together, such as:– Vector– Vehicle

Analytic Studies• Explicit comparison of exposure and disease

• Groups are assembled to determine whether risk is different for exposed and unexposed

• Appropriate comparison group

• Hypothesis testing

• Two types– Observational - natural course of events

– Intervention - investigator allocates exposure and follows subjects

Observational Studies• Case-Control

– Persons with disease

– Comparison group

• Cohort– Subjects classified on basis of exposure of a

factor

– Follow-up to determine presence of disease• Prospective vs. retrospective

Intervention Studies• Experimental studies - clinical trials

– Provides most reliable evidence

– Randomization• Controls for known risk factors

• Controls for unknown risk factors

– Useful for studying small to moderate effects

– Ethical considerations• Human rights review

• Data monitoring – “DSMB”

Assign exposures?

Experimental study Observational study

Random allocation? Comparison group?

Randomized controlled

trial

Non-randomized controlled

trial

Analyticalstudy

Descriptivestudy

Direction?

Cohort study

Case-control study

Cross-sectional

study

Yes No

Yes YesNo No

Exposure Outcome Exposure Outcome Exposure and Outcome at the same time

Epidemiologists are required to have some knowledge of the disciplines of public health, clinical medicine, pathophysiology, statistics, and the social sciences.

• public health, because of the emphasis on disease prevention.

• clinical medicine, because of the emphasis on disease classification and diagnosis. 

• pathophysiology, because of the need to understand basic biological mechanisms in disease.

• statistics, because of the need to quantify disease frequency and its relationships to antecedents.

• social sciences, because of the need to understand the social context in which disease occurs and presents.

Purposes Of EpidemiologyPurposes Of Epidemiology

1. Identify causes and risk factors for disease.

2. Determine the extent of disease in the community.

3. Study natural history and prognosis of disease.

4. Evaluate preventive and therapeutic measures

5. Provide foundation for public policy

Men Women

Children

Total

1st class 67% 3% 0 38%

2nd class 92% 14% 0 59%

3rd class 84% 54% 66% 62%

Total 82% 26% 48% 62%

EVERY HEALTH OUTCOME HAS SOME INTERESTING AND USEFUL EPIDEMIOLOGIC

CHARACTERISTIC

DEATH RATES BY SOCIAL CLASS FROM A CERTAIN CAUSE AMONG 1,316 PEOPLE

WHAT CAUSE OF DEATH IS THIS?

The previous slide shows death The previous slide shows death rates by class of ticket on the rates by class of ticket on the

Titanic, a large ocean liner that Titanic, a large ocean liner that sank after colliding with an iceberg sank after colliding with an iceberg

in 1912 in 1912

Clinical Epidemiology

Epidemiology is used in clinical medicine to:

• evaluate diagnostic testing • evaluate prognosis • The goal

- by improving diagnosis and treatment of disease

- by improving the prognosis of patients not to prevent the disease, just to promote clinicial

decisions.

[Greenberg RS (ed.) Medical Epidemiology, 1993]

1 MacMahon B and Pugh TF. Epidemiology Principles and Methods. Boston: Little, Brown and Company. 1970.

2 Lilienfeld AM and Lilienfeld DE. Foundations of Epidemiology (2nd ed.). New York: Oxford University Press. 1980

3 Last JM. A dictionary of epidemiology. Oxford University Press. 1983.

4 Haynes RB, Sackett DL, Guyatt GH, Tugwell P. Clinical Epidemiology: How to Do Clinical Practice Research, 3rd edition. Philadelphia: Lippincott Williams & Wilkins, 2005.

5 Rothman KJ. Modern Epidemiology, 3rd. Lippincott Williams & Wilkins. 2008

Classic BooksClassic Books

Question?

Assigned readings, session 1:• Grimes DA, Schulz KF. An overview of clinical

research: the lay of the land. Lancet 2002; 359:57-61.

What is epidemiology?

What is the limitation of epidemiology?