Descriptive Epidemiology

Maj Dr Naveen PhuyalMBBS, MD

Introduction• A descriptive study can be defined as one in which only

one group of subjects is studied, without any comparison group, for describing the outcome in terms of its frequency and its distribution according to selected variables related to person, place and time.

• It gives a tentative guess (hypothesis) about possible role of certain factors in the outcome of interest, but does not confirm the role because of absence of a comparison group.

Characteristics• No comparison group.

• The main objective is to describe:

Incidence or prevalence of disease Describe the natural history of disease Describe the distribution of the disease according to variables

related to person, place and time.

• Makes suggestion or hypothesis about certain “cause and effect” relationship which can be further tested by analytical studies.

• First phase of an epidemiological investigation.

• These are concerned with observing the distribution of the disease or health-related characteristics in human population and identifying the characteristics with which the disease in question seems to be associated.

Descriptive studies answer 3 basic questions

• When is the disease occurring? Time distribution

• Where is it occurring? Place distribution

• Who is getting the disease? Person distribution

Steps

1. Defining the population to be studied2. Defining the disease under study3. Describing the disease by

• Time• Place• Person

4. Measurement of disease5. Comparing with known indices6. Formulation of etiological hypothesis

1. Defining the Population to be Studied

Descriptive study is the study of populations not individuals

Defining the population to be studied

• Define population base in terms of • Age • Gender • Occupation• Cultural characters etc

• Defined population can also be specially selected group• Age and Sex groups• Occupational Groups• School children• Small communities

Defining the population to be studied

• Defined population needs to be large enough

• The community should be stable

• Clear on who belong or does not

• Community should not be different from others in the region

• Health facility should be close enough

Framingham Heart Study: All above criteria were followed

Why do we need “defined population?”

• To provide denominators for calculation of rates.

• Rates are required for measurement of frequency of the disease and study of distribution and determinants.

Epidemiologist: Men in search for a denominator

2. Defining the disease under study

Defining the disease under study

• Needs of clinician and epidemiologist may diverge

• Precise and valid definition is required by epidemiologist

• Operational definition by which a disease or condition can be identified and measured in defined population with a degree of accuracy

Measles

Clinical Case definition: An illness characterized by following:

(1) Generalized rash lasting ≥ to 3 ds

(2) Temperature ≥ 1010 F

(3) Cough, Coryza or Conjunctivitis

Epidemiologist definition

• Suspected case: Any febrile illness accompanied by rash

• Probable case: A case that meets clinical case definition, has noncontributory or no serologic or virologic testing and is not epidemiologically linked to a confirmed case.

• Confirmed case: a case that is laboratory confirmed or that meets the clinical case definition and is epidemiologically linked to a confirmed case

3. Describing the disease

Describing the disease

• Time distribution– When is the disease occurring?

• Place distribution– Where is it occurring?

• Person distribution– Who is getting the disease?

Time Place Person

Year, Season Climatic zones Age Birth Order

Month, Week Country, region Sex Family size

Day, Hour of onset Urban/Rural Marital state Height

Duration Local Community Occupation Weight

Towns Social status Blood Pressure

Cities Education Blood Cholesterol

Institutions Personal Habits

Time distribution• The disease pattern may be described by its time of occurrence ie.

By week, month, year, day of weak, hour of onset etc.

• This explains weather the disease is seasonal, weather it shows periodic increase or decrease, weather it follows a consistent time trend.

• Epidemiologist have identified three different trends or fluctuations– Short term fluctuation– Periodic Fluctuation– Long term or secular trends

Short term fluctuations

• Epidemic is the best known short term fluctuation.

Types of epidemics

A. Common-source epidemicsA. Single exposure or ‘point- source’ epidemicsB. Continuous or multiple exposure epidemics

B. Propagated epidemicsA. Person to personB. Arthropod vectorC. Animal reservoir

C. Slow or modern epidemics

Periodic fluctuations

• Seasonal trendEg. Measles, Varicella, Cerebro-spinal meningitis, Upper respiratory infections, malaria etc.

Sunstroke, Hay fever, Snakebite.

• Cyclic trendMeasles, Inflenza pandemics, Automobile accidents

Long term or secular trends

• Secular means progressive increase or decrease over a long period of time

• Consistent tendency to change in a particular direction or a definite movement in one direction

• Eg CHD, Lung Cancer, DM: upward trend

• Tuberculosis, typhoid fever, diptheria, polio: downward trend

Steps

1. Defining the population to be studied2. Defining the disease under study3. Describing the disease by

• Time• Place• Person

4. Measurement of disease5. Comparing with known indices6. Formulation of etiological hypothesis

Place Distribution (Geographical comparisons)

• Geographical pathology is important dimensions of descriptive epidemiology

• International Variations• National Variations• Rural- urban variations• Local distributions

International Variation

• Pattern of disease is not same everywhere.• Ca stomach• Oral Cancer and Ca Cervix• Breast Cancer

National variations

• Endemic goitre• Lathyrism• Fluorosis• Leprosy• Malaria• Nutritional deficiency

• Chronic bronchitis• Accidents• Lung cancer• Cardiovascular disease• Mental illness and drug

dependence

• Hand, skin and zoonotic disease

• Soil transmitted helminths

• Death rates: Infant mortality, maternal mortality

Rural-urban variation

Local distributions

• Best studied by spot maps or shaded maps

• “Clustering of cases”: common source

• John Snow

Migration studies

• Comparison of disease and death rates of migrants with those of their kin who stayed at home

• Comparison of migrants with local population of the host country

Person distribution

• Age• Sex• Ethnicity• Marital Status

• Occupation• Social Class• Behavior• Stress• Migration

4. Measurement of disease

Measurement of disease

• Disease load in terms of– Mortality– Morbidity

• Incidence: longitudinal studies• Prevalence: Cross-sectional studies

– Disability

5. Comparing with known indices

Comparing with known indices

• Making comparisons and asking questions• Making comparison between different

population• Making comparison in subgroups of same

population• We can arrive at clues to disease etiology• We can also identify or define groups who are

at increased risk for certain disease

Formulation of a hypothesis

• A hypothesis is a supposition arrived at from observation

• It can be accepted or rejected using techniques of analytical epidemiology

An hypothesis should specify the following

• The population

• The specific cause being considered

• The expected outcome

• The dose-relationship

• The time-response relationship

Cigarette smoking causes lung cancer

Smoking of 30-40 cigarettes per day causes lung cancer in 10 percent of smokers after 20 years exposure

Uses of descriptive epidemiology• Provide date: magnitude of disease problems in community in

terms of morbidity and mortality rates and ratios

• Provide clues to disease etiology and help in formulation of etiological hypothesis

• Provide background for planning, organizing and evaluating preventive and curative services

• Contribute to research by describing variation of disease occurrence by time, place and person

Thank You