Introduction to epidemiology and study designs with descriptive epidemiology in detail.

1. DESCRIPTIVE EPIDEMIOLOGY

2. How we view the world…..
Pessimist: The glass is half empty.
Optimist: The glass is half full.
Epidemiologist: As compared to what ?

3. EPIDEMIOLOGY
Epi - ABOUT Demi – PEOPLE Logy - STUDY

4. “
“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)

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

6. The Cholera study (1854)
The Broad Street
Pump

7. STUDY DESIGNS

8. DESCRIPTIVE EPIDEMIOLOGY
First phase of an epidemiologic investigation
Concerned with:
Observing the distribution of disease in population and
Identifying the characteristics associated with it.

9. Descriptive studies mainly answers:
3Q’s
TIME
PLACEPERSON
5W’s
Who
What
Why
When
Where
So
what

10. 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

11. Defining the population
Define the population in relation to:
1. Number
2. Age
3. Gender
4. Occupation
5. Cultural and other characteristics
The defined population can be:
1. Whole population
2. Sample
3. Specially selected groups

12. 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

13. 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 definition
A definition by which the disease
or condition can be identified and
measured in the defined population with
a degree of accuracy….

14. ◉Gingivitis -
Inflammation of the
gingiva
◉Dental caries -
Infectious microbial
disease affecting the
calcified tissues of teeth . .
For example:
OPERATIONAL DEFINITION
Gingivitis
Gingival bleeding in one or more sites after
gently probing the gingival sulcus
Dental caries
The 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

15. 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
Occupation
Social status
Education
Birth order
Family size
Height, weight
BP.
Cholesterol,
habits
Objective of descriptive epidemiology?
This involves systemic collection and analysis of data.

16. • 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

17. Short-term fluctuations
Types
Common source epidemics
Single source or point source epidemic
Continuous or multiple exposure epidemic
Propagated epidemics
Person-to-person
Arthropod vector
Animal reservoir
Slow or modern epidemics
Epidemic
Number of
Cases
of Disease
Time

18. Periodic fluctuations
1) Seasonal trends
Related to environmental conditions
2) Cyclic trends
Some conditions occur in cycles spread over short period of time

19. Long-term or secular trends
Changes in occurrence of a disease over long periods of time (years or decades)
Interpretation of time trends:
1. Disease increasing/decreasing
2. Effectiveness of measures
3. Formulates etiological hypothesis based on characteristics observed

20. Tetanus – by year, USA, 1955-2000
During 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

21. 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 diet
Cultures and standard of living and external environment vary in different parts of world

22. Helps distinguish role of genetic and environmental factors
Range of studies vary with:
• International variations
• National variations
• Rural-urban differences
• Local distribution

23. c) Person distribution
• Age
If equal susceptibility in all ages – no previous immunity
Progressive increase with age – persistent exposure to causative agent
• Bimodality: two separate peaks
Two separate sets of causative factors
• Sex:
Biological difference b/w two sexes
Cultural diff. B/w two sexes
Study of persons who develop disease based on host factors. These factors include:

24. 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

25. 4. Measurement of disease
Knowing “ DISEASE LOAD ” is important
Measured based on:
• Mortality
• Morbidity
• Disability
Mortality – straightforward
Morbidity – incidence and prevalence
Incidence – longitudinal studies
Prevalence – cross sectional studies

26. Incidence
Definition: 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

27. Prevalence
Definition: 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

28. Prevalence and Incidence
= prevalent cases = incident cases = deaths or recoveries

29. 5. Comparing with known indices
• Make comparisons
• Ask questions
Helps:
• Aetiology
• Identify groups at increased risk

30. 6. Formulation of etiological hypothesis
It 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

31. Uses of descriptive epidemiology
Contributes 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

32. 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

33. Any questions?
Thanks!

34. 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.