descriptive study- epidemiology

1. Descriptive Epidemiology
Maj Dr Naveen Phuyal
MBBS, MD

2. 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.

3. 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.

4. • 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.

5. 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

6. Steps
1. Defining the population to be studied
2. Defining the disease under study
3. Describing the disease by
• Time
• Place
• Person
4. Measurement of disease
5. Comparing with known indices
6. Formulation of etiological hypothesis

7. 1. Defining the Population to be
Studied
Descriptive study is the study of populations not individuals

8. 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

10. 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

11. 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

12. 2. Defining the disease under study

13. 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

14. 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

15. 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

16. 3. Describing the disease

17. Describing the disease
• Time distribution
– When is the disease occurring?
• Place distribution
– Where is it occurring?
• Person distribution
– Who is getting the disease?

18. 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

19. 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

20. Short term fluctuations
• Epidemic is the best known short term
fluctuation.

21. Types of epidemics
A. Common-source epidemics
A. Single exposure or ‘point- source’ epidemics
B. Continuous or multiple exposure epidemics
B. Propagated epidemics
A. Person to person
B. Arthropod vector
C. Animal reservoir
C. Slow or modern epidemics

23. Periodic fluctuations
• Seasonal trend
Eg. Measles, Varicella, Cerebro-spinal meningitis,
Upper respiratory infections, malaria etc.
Sunstroke, Hay fever, Snakebite.
• Cyclic trend
Measles, Inflenza pandemics, Automobile accidents

25. 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

27. Steps
1. Defining the population to be studied
2. Defining the disease under study
3. Describing the disease by
• Time
• Place
• Person
4. Measurement of disease
5. Comparing with known indices
6. Formulation of etiological hypothesis

28. Place Distribution
(Geographical comparisons)
• Geographical pathology is important
dimensions of descriptive epidemiology
• International Variations
• National Variations
• Rural- urban variations
• Local distributions

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

30. National variations
• Endemic goitre
• Lathyrism
• Fluorosis
• Leprosy
• Malaria
• Nutritional deficiency

31. • 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

32. Local distributions
• Best studied by spot maps or shaded maps
• “Clustering of cases”: common source
• John Snow

33. 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

34. Person distribution
• Age
• Sex
• Ethnicity
• Marital Status
• Occupation
• Social Class
• Behavior
• Stress
• Migration

35. 4. Measurement of disease

36. Measurement of disease
• Disease load in terms of
– Mortality
– Morbidity
• Incidence: longitudinal studies
• Prevalence: Cross-sectional studies
– Disability

37. 5. Comparing with known indices

38. 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

39. Formulation of a hypothesis
• A hypothesis is a supposition arrived at from
observation
• It can be accepted or rejected using
techniques of analytical epidemiology

40. An hypothesis should specify the
following
• The population
• The specific cause being considered
• The expected outcome
• The dose-relationship
• The time-response relationship

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

42. 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

43. Thank You