My Descriptive Epidemiology

Descriptive Epidemiology

ABHIJNA RAI

08M4203

Epidemiology- definition

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.

Epidemiological study- types

a. Observational studies:1. Descriptive studies2. Analytical studies

a) Case control studiesb) Cohort studies

b. Experimental / Interventional studies:1. Randomized controlled trials2. Non randomized trials

Introduction

• Descriptive studies are usually the first phase of an epidemiological investigation

• Concerned with distribution and associations of the disease in question

• Eg., Meticulous observations made by Burkitt in Africa led to the eventual incrimination of EBV as the etiological factor for Burkitt’s lymphoma

PROCEDURES IN DESCRIPTIVE STUDIES

1.Defining the population to be studied2.Defining the population under study3.Describing the disease by: (a)Time

(b)Place (c)Person4.Measurement of the disease5.Comparing with known indices 6. Formulation of an aetiological hypothesis

1.DEFINING THE POPULATION:

• The “defined population” can be the whole population in a geographic area or a representative sample taken from it.

• It can also be specially selected groups such as age, sex, occupational groups, hospital patients, school children

• The community chosen should be :Stable Without migration Actively participating

2.DEFINING THE DISEASE UNDER STUDY:

• Operational definition i.e.,definition by which the disease can be identified and measured in the defined population with a degree of accuracy.

• Eg., Tonsillitis... • Clinical definition- inflammation of tonsils caused by

infection, usually by Streptococcus pyogenes.• Operational definition- presence of enlarged red tonsils

with white exudate which on throat swab culture grows predominantly Streptococcus pyogenes.

3.DESCRIBING THE DISEASE:

• Primary objective of descriptive epidemiology• Described with respect to TIME PLACE PERSON

TIME DISTRIBUTION

I.Short term fluctuation II.Periodic fluctuationsIII.Long term fluctuations

I. Short term fluctuation

• The best known short term fluctuation in the occurrence of a disease is an epidemic.

• Epidemic=occurrence of disease in a population clearly in excess of normal expectancy

• Types of Epidemics: A. Common source epidemic (a) single exposure (b) multiple exposure B.Propagated epidemic (a) person to person (b) arthropod vector (c) animal reservoir C.Slow epidemic

II. Periodic fluctuation

(i) Seasonal trend eg., Measles and Varicella- early spring URTI- winter (ii) Cyclic trend eg., Measles (in pre vaccination era)- every 2-3years Rubella- every 6-9 years

III. Long term fluctuation

• Also called “secular trend”• It implies a progressive change in the disease i.e., either a

progressive increase or decrease over a long period of time• Eg., over the past 50 years Coronary heart disease, Lung

cancer, Diabetes- upward trend whereas TB, Typhoid, Diphtheria and Polio- decline

PLACE DISTRIBUTION

a. International variations: Ca stomach- common in Japan, unusual in US. Oral malignancy and Ca cervix- common in India, uncommon in developed countries

b. National variations: Endemic goitre, Lathyrism, Fluorosis, Guinea worm disease, Leprosy, Malaria, Nutritional deficiencies

c. Rural-urban differences: Chronic bronchitis, accidents, lung ca, drug dependence- more common in urban areas Skin and zoonotic diseases, soil transmitted helminths- more common in rural areas.

d. Local distributions

PERSON DISTRIBUTION• Age : Measles in childhood, Cancer in middle age,

Atherosclerosis in old age• Sex : Diabetes, Hypothyroidism, Obesity- women anatomical, genetic, behavioral differences• Marital status: mortality rates are lower among married

males and females.• Race and Ethnicity: TB, HTN, Ca, SCA• Occupation: Silicosis• Social class: upper class- HTN, DM• Behavior: cigarette smoking, sedentary life style, over

eating, drug abuse → cancer, obesity, coronary heart disease

• Stress: susceptibility to diseases, exacerbation of symptoms

• Migration: leprosy, filaria, malaria- from rural to urban

4.MEASUREMENT OF DISEASE:

• It is mandatory to have a clear picture of the disease load in the community.

• This information should be available in terms of mortality, morbidity, disability and so on.

• Measurement of mortality is straightforward, but morbidity has two aspects-

• incidence(longitudinal study)• Prevalence(cross-sectional study)

Prevalence and Incidence

Prevalence

= prevalent cases

Prevalence and Incidence

Old (baseline) prevalence

= prevalent cases = incident cases

New prevalence

Incidence

No cases die or recover

Prevalence and Incidence

= prevalent cases = incident cases = deaths or recoveries

5.COMPARING WITH KNOWN INDICES:

• Making comparisons btw different populations , and subgroups of the same population,it is possible to arrive at clues to disease aetiology.

• We can also identify or define groups who are at an increased risk for certain diseases.

6.Formulation of a hypothesis:

• By studying the distribution of the disease and also using the techniques of descripitive epidemiology, it is often possible to formulate hypothesis relating to the disease.

• An epidemiological hypothesis should specify the following:

a. The populationb. The specific cause being consideredc. The expected outcomed. The dose-response relationshipe. The time-response relationship

USES OF DESCRIPTIVE EPIDEMIOLOGY:

a. Provide data regarding the magnitude of the disease load and types of disease problems in the community.

b. Provide clues to disease aetiology and help in the formulation of an aetiological hypothesis.

c. Provide background data for planning, organizing,and evaluating preventive and curative services.

d. They contribute to research by describing variations in disease occurrence by time, place and person.

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Distribution over Time

Time

Distribution over Place

Place

Distribution over Persons

Person