DESCRIPTIVE EPIDEMIOLOGY

How we view the world…..

Pessimist: The glass is half empty.

Optimist: The glass is half full.

Epidemiologist: As compared to what?

EPIDEMIOLOGY

Epi - ABOUT Demi – PEOPLE Logy - STUDY

“

“The study of the distribution and determinants of health-related states in specified populations, and the application of this study to control of health problems”

EPIDEMIOLOGY

- John M. Last (1988)

History

◉ Dates back to 3rd century BC◉ Foundations was laid in 19th century◉ Landmark in epidemiology: Cholera◉ John Snow: Father of epidemiology

The Cholera study (1854)

The Broad Street Pump

STUDY DESIGNS

DESCRIPTIVE EPIDEMIOLOGY

First phase of an epidemiologic investigation

Concerned with: Observing the distribution of disease in population and Identifying the characteristics

associated with it.

Descriptive studies mainly answers:

3Q’s

TIME

PLACEPERSON

5W’s

Who

What

Why

When

Where

So what

STEPS IN DESCRIPTIVE STUDIES◉ Defining the population◉ Defining the disease under study◉ Describing the disease

- Time - Place - Person◉ Measurement of disease◉ Comparing with known indices◉ Formulation of aetiological hypothesis

Defining the population

Define the population in relation to:

1. Number2. Age 3. Gender4. Occupation5. Cultural and other characteristics

The defined population can be:

1. Whole population2. Sample3. Specially selected groups

Defined population should be:

Large enough Stable (no migration) Clear on who belongs to the population Community participation

This step is very important as it forms the basis for all calculations

2. Defining the disease under study• Need for an operational definition

Why is it important?? Epidemiologist needs a definition that is precise and valid to obtain an accurate estimation about the disease

What is an operational definitionA definition by which the

disease or condition can be identified and measured in the defined population with a degree of accuracy….

◉ Gingivitis - Inflammation of the gingiva

◉ Dental caries - Infectious microbial disease affecting the calcified tissues of teeth . . . . .

For example:

OPERATIONAL DEFINITION

GingivitisGingival bleeding in one or more sites after gently probing the gingival sulcus

Dental cariesThe lesion is clinically visible and obvious. Explorer tip can penetrate deep into soft yielding material. There is discoloration or loss of translucency. Explorer tip resists removal after moderate to firm pressure

DEFINITION

3. Describing the disease

TIME PLACE PERSON

Year, season

Month, week

Day, hour

Duration

Climatic zones

Country, region

Urban/rural

Towns, cities, institutions

Age

Gender

Marital state

OccupationSocial statusEducation

Birth order

Family size

Height, weight

BP. Cholesterol, habits

Objective of descriptive epidemiology? This involves systemic collection and analysis

of data.

• Week, month, year• Seasonal in occurrence• Periodic decrease or increase / consistent trend

◉ Gives source of etiology preventive measures

◉ Three types of time trends:

• Short term fluctuations• Periodic fluctuations• Long term/ secular trends

Time distribution

Disease can be described by time of its occurrence

Short-term fluctuations

TypesCommon source epidemics

Single source or point source epidemicContinuous or multiple exposure epidemic

Propagated epidemicsPerson-to-personArthropod vectorAnimal reservoir

Slow or modern epidemics

Epidemic

Number of Casesof Disease

Time

Periodic fluctuations

1) Seasonal trends Related to environmental conditions

2) Cyclic trends Some conditions occur in cycles spread over short period of time

Long-term or secular trendsChanges in occurrence of a disease over long periods of time

(years or decades)

Interpretation of time trends:

1. Disease increasing/decreasing2. Effectiveness of measures3. Formulates etiological hypothesis based on characteristics

observed

Tetanus – by year, USA, 1955-2000During 2000, a total of 35 cases of tetanus were reported. The percentage of cases among persons aged 25-59 years

Has increased in the last decade. Note: A tetanus vaccine was first available in 1933.

0

100

200

300

400

500

600

700

800

900

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Year

b) Place distribution

Geographical comparisons• Differences in disease patterns b/w countries and within countries• Importance of genes versus environment; changes with migration and role of

dietCultures and standard of living and external environment vary in different

parts of world

Helps distinguish role of genetic and environmental factors

Range of studies vary with:

• International variations• National variations• Rural-urban differences• Local distribution

c) Person distribution

• AgeIf equal susceptibility in all ages – no previous immunityProgressive increase with age – persistent exposure to causative agent

• Bimodality: two separate peaksTwo separate sets of causative factors

• Sex:Biological difference b/w two sexesCultural diff. B/w two sexes

Study of persons who develop disease based on host factors. These factors include:

ETHINICITY: difference in disease occurrence in different population subgroups

OCCUPATION: occupational disorders

SOCIAL CLASS:• Higher social class – diabetes, coronary heart diseases• Lower social class – nutritional disorders

BEHAVIOR: cigarette smoking, sedentary life, drug abuse, over-eating

STRESS: affects response to variables• Susceptibility to diseases• Exacerbation of symptoms• Compliance with medical regimen

MIGRATION: eg. malaria from rural to urban areas

4. Measurement of disease

Knowing “ DISEASE LOAD ” is important

Measured based on:• Mortality• Morbidity• DisabilityMortality – straightforwardMorbidity – incidence and prevalenceIncidence – longitudinal studiesPrevalence – cross sectional studies

IncidenceDefinition: incidence is defined as the occurrence of new cases of disease that develop in a population over a specified time period.

Incidence = Total no. Of people affected with caries over a specified time period x 1000

Total population

PrevalenceDefinition: all current cases (old and new) existing at a given point in time, or over a period of time in a given population.

Two types: Point prevalence: proportion of population that is diseased at a single point in time. It is a single snapshot of the population

Period prevalence: proportion of population that is diseased during a specific period of time

Prevalence and Incidence

= prevalent cases = incident cases = deaths or recoveries

5. Comparing with known indices

• Make comparisons• Ask questions

Helps:• Aetiology• Identify groups at increased risk

6. Formulation of etiological hypothesisIt is a supposition, arrived at from observation or reflection

It should specify following:

• Population

• Specific cause being considered

• Expected outcome – the disease

• Dose-response relationship

• Time-response relationship

Uses of descriptive epidemiologyContributes to research by describing variations in disease

occurrence by time, place and person

Clues to disease epidemiology – aetiological hypothesis

Data regarding magnitude of disease load and types of disease problems in community in terms of morbidity and mortality

Background data for planning, organizing and evaluating preventive and curative services

To summarize . . . ◉ Epidemiology

Definition History

◉ Epidemiological methods (Classification)

◉ Descriptive epidemiology• Steps • Defining the population• Defining the disease under study• Describing the disease• Measurement of disease• Comparing with known indices• Formulation of etiological hypothesis• Uses

Any questions?

Thanks!

SUGGESTED READING

◉ 1) Soben Peter. Essentials of Public Health Dentistry. 5th ed.

New Delhi: Arya Publising House; 2013.

◉ 2) Park, Park’s Textbook of Preventive &Social Medicine, 22nd Edition, Jabalpur: Banarsidas Bhanot,2013.