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Descriptive Epidemiology
Presented by : Dr. Vini Mehta
MDS 1st Year
1
Contents
• Introduction
• Classification of Epidemiological studies
• Descriptive Epidemiology
• Uses of Descriptive Epide...
Introduction
• The best study of mankind is man.
• Descriptive studies are usually the first phase of an
epidemiological i...
EPIDEMIOLOGICAL
STUDIES
OBSERVATIONAL
STUDIES
DESCRIPTIVE
STUDIES
ANALYTICAL
STUDIES
ECOLOGICAL
STUDIES
CROSS-
SECTIONAL
S...
Cross Sectional Survey
• Takes place at a single point in time
• Measure disease and exposure simultaneously
in a well-def...
Longitudinal Survey
• Observations are repeated in the same population or
different populations over a period of time thro...
• Limitations
1) Difficult to organize and more time
consuming
2) Attrition is more common as time goes by
7
8
Descriptive epidemiology
• When is the disease occurring?
Time distribution
• Where is it occurring ?
Place distribution...
Procedures in Descriptive Studies
1. Defining the population to be studied
2. Defining the disease under study
3. Describi...
1.Defining the population to be studied
• Investigations of populations and not an individual
• Define “population base”.
...
• The concept of “defined population”(or population at risk) is
crucial in epidemiological studies. It provides the denomi...
2.Defining the disease under study
• Epidemiologist needs a precise and valid definition
• Operational definition -
a defi...
3.Describing the disease
• Describing the disease by
- Time
- Place
- Person
13
Characteristics frequently examined in
descriptive studies
Time Place Person
Year, Season Climatic zones Age Birth order
M...
Time distribution
15
Types
Short Term
Common
source
Single
exposure
continuous
Propagated
Person-
person Arthropod
Slow
Animal
16
I. Short term fluctuations
• Best short term fluctuation is epidemic.
• According to the modern concepts an epidemic is de...
Epidemic curve
It is a histogram that displays the course of an outbreak or
epidemic by plotting the number of cases accor...
It suggests:
• A time relationship with exposure to a suspected
source.
• A cyclic or seasonal pattern suggestive of a par...
A) Common Source Epidemic
• Common source ,single exposure epidemics
 Known as ‘point source’ epidemic.
 Exposure to the...
Common source epidemics are frequently, but not always, due to
exposure to an infectious agent.
E.g. : Food poisoning, Bho...
Common source, continuous or multiple exposure
• Exposure from the source may be prolonged – continuous,
repeated or inter...
E.g. : Water from contaminated well, Nationally distributed
brand of vaccine or food could result in similar out breaks.
23
B) Propagated epidemic
• Most often of infectious origin and results from person to
person transmission of the agent.
• Ep...
• Transmission continues until the number of susceptible is
depleted or susceptible individuals are no longer exposed to
i...
II. Periodic Fluctuations
a) Seasonal trend –
Seasonal variation is a well known characteristic of many
communicable disea...
b) Cyclic trend –
Some diseases occur in cycles spread over short periods of
time which may be days, weeks, months or year...
III. Long term or secular trends
It implies changes in the occurrence of disease over a long
period of time( i.e., a progr...
• Interpretation of time trends :
- Helps us to know which diseases are increasing and which
are the emerging health probl...
Place Distribution
International variations
National variations
Rural / Urban variations
Local variations
30
• International Variations :
E.g. : Japan – High rates of death from
stomach cancer but unusual in US
31
• National variations :
E.g. : Endemic goiter, Fluorosis etc.
• Rural – Urban variations :
E.g. : Rural areas - Skin and z...
Person Distribution
Some characteristics of a person have been identified as risk
factor for a disease. These variables in...
Marital status
Occupation
Social class
Behaviour
Stress
Migration
34
4.Measurement of disease
• - Measurement of mortality and Morbidity
- Morbidity has two aspects – Incidence and prevalence...
5.Comparing with known Indices
• By making comparisons between different populations and
sub groups of the same population...
6.Formulation of etiological hypothesis
• Hypothesis is a supposition, arrived at from
observation or reflection.
• An epi...
a) The population – The characteristics of the persons to
whom the hypothesis applies
b) The specific cause being consider...
An improved formulation :
“ The smoking of 30-40 cigarettes per day causes lung cancer in
10% of smokers after 20 years of...
Uses of Descriptive epidemiology
• To provide data regarding the magnitude of the disease load
and types of disease proble...
• Provides background data for planning, organizing and
evaluating preventive and curative services
• They contribute to r...
Conclusion
For epidemiologists, the identification of descriptive
characteristics frequently constitutes an important
step...
References
• Park K. Textbook of Preventive and Social Medicine. 23rd ed.
Bhanot Publishers. Pg no.62-73
• Peter S. Essent...
• http://www.thelancet.com/journals/lancet/ar
ticle/PIIS0140-6736%2802%2907373-
7/fulltext
44
45
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Descriptive epidemiology

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Descriptive epidemiology

  1. 1. Descriptive Epidemiology Presented by : Dr. Vini Mehta MDS 1st Year 1
  2. 2. Contents • Introduction • Classification of Epidemiological studies • Descriptive Epidemiology • Uses of Descriptive Epidemiology • Conclusion • References 2
  3. 3. Introduction • The best study of mankind is man. • Descriptive studies are usually the first phase of an epidemiological investigation. • These studies are concerned with observing the distribution of disease or health related characteristics in human populations and identifying the characteristics in which the disease in question seems to be associated. 3
  4. 4. EPIDEMIOLOGICAL STUDIES OBSERVATIONAL STUDIES DESCRIPTIVE STUDIES ANALYTICAL STUDIES ECOLOGICAL STUDIES CROSS- SECTIONAL STUDIES CASE CONTROL STUDIES COHORT STUDIES EXPERIMENTAL STUDIES RANDOMIZED CONTROLLED TRIALS FIELD TRIALS COMMUNITY TRIALS 4
  5. 5. Cross Sectional Survey • Takes place at a single point in time • Measure disease and exposure simultaneously in a well-defined population • Often used to look at the prevalence of something in a given population 5
  6. 6. Longitudinal Survey • Observations are repeated in the same population or different populations over a period of time through follow up examinations. Advantages 1) To study the natural history of disease and its future outcome 2) For identifying risk factors 3) Rate of occurrence of new cases of disease 6
  7. 7. • Limitations 1) Difficult to organize and more time consuming 2) Attrition is more common as time goes by 7
  8. 8. 8 Descriptive epidemiology • When is the disease occurring? Time distribution • Where is it occurring ? Place distribution • Who is getting the disease? Person distribution “I keep six honest serving Men. They taught me all I Know. Their names are – what, why, when, how, where and who.” Rudyard Kipling 1903
  9. 9. Procedures in Descriptive Studies 1. Defining the population to be studied 2. Defining the disease under study 3. Describing the disease under study 4. Measurement of disease 5. Comparing with known indices 6. Formulation of an etiological hypothesis
  10. 10. 1.Defining the population to be studied • Investigations of populations and not an individual • Define “population base”. • Defined population -Whole population: (geographic area) - Representative sample • Large enough and stable 10
  11. 11. • The concept of “defined population”(or population at risk) is crucial in epidemiological studies. It provides the denominator for calculating rates, which are essential to measure the frequency of disease and study its distribution and determinants. • Epidemiologists have been labeled as men in search of a denominator 11
  12. 12. 2.Defining the disease under study • Epidemiologist needs a precise and valid definition • Operational definition - a definition by which the disease or condition can be identified and measured in the defined population with a degree of accuracy. 12
  13. 13. 3.Describing the disease • Describing the disease by - Time - Place - Person 13
  14. 14. Characteristics frequently examined in descriptive studies Time Place Person Year, Season Climatic zones Age Birth order Month, week Country, region Sex Family size Day, hour of onset Urban/Rural Local community Marital status Height/Weight Duration Towns cities Institutions Occupation Social status Education Blood Pressure Blood Cholesterol Personal habits 14
  15. 15. Time distribution 15
  16. 16. Types Short Term Common source Single exposure continuous Propagated Person- person Arthropod Slow Animal 16
  17. 17. I. Short term fluctuations • Best short term fluctuation is epidemic. • According to the modern concepts an epidemic is defined as “ the occurence in a community or region of cases of an illness or other health related events clearly in excess of normal expectancy” 17
  18. 18. Epidemic curve It is a histogram that displays the course of an outbreak or epidemic by plotting the number of cases according to time of onset. 18
  19. 19. It suggests: • A time relationship with exposure to a suspected source. • A cyclic or seasonal pattern suggestive of a particular infection 19
  20. 20. A) Common Source Epidemic • Common source ,single exposure epidemics  Known as ‘point source’ epidemic.  Exposure to the agent is brief and essentially simultaneous, the resultant cases all develop within one incubation period of the disease. 20
  21. 21. Common source epidemics are frequently, but not always, due to exposure to an infectious agent. E.g. : Food poisoning, Bhopal gas tragedy 21
  22. 22. Common source, continuous or multiple exposure • Exposure from the source may be prolonged – continuous, repeated or intermittent- not necessarily at the same time or place. • A variation may be that an epidemic may be initiated from a common source and then continue as a propagated epidemic. 22
  23. 23. E.g. : Water from contaminated well, Nationally distributed brand of vaccine or food could result in similar out breaks. 23
  24. 24. B) Propagated epidemic • Most often of infectious origin and results from person to person transmission of the agent. • Epidemic shows gradual rise and tails of over a much longer period of time. • E.g.: Epidemics of Hepatitis A 24
  25. 25. • Transmission continues until the number of susceptible is depleted or susceptible individuals are no longer exposed to infected persons or intermediary vectors. • Speed of spread depends on - Herd immunity - Opportunities for contact 25
  26. 26. II. Periodic Fluctuations a) Seasonal trend – Seasonal variation is a well known characteristic of many communicable diseases like measles, varicella, sunstroke , upper respiratory infections 26
  27. 27. b) Cyclic trend – Some diseases occur in cycles spread over short periods of time which may be days, weeks, months or years. E.g.- Measles appeared in cycles with major peaks every 2-3 years and rubella every 6-9 years. 27
  28. 28. III. Long term or secular trends It implies changes in the occurrence of disease over a long period of time( i.e., a progressive increase or decrease) generally several years or decades. E.G : Coronary heart disease, lung cancer, diabetes have shown a consistent upward trend and diseases like Tuberculosis , typhoid fever, diphtheria & polio have shown a downward trend. 28
  29. 29. • Interpretation of time trends : - Helps us to know which diseases are increasing and which are the emerging health problem and measures to control the old problems. - By studying the time trends, the epidemiologist seeks to provide guidelines to the health administrator in matters of prevention or control of diseases. 29
  30. 30. Place Distribution International variations National variations Rural / Urban variations Local variations 30
  31. 31. • International Variations : E.g. : Japan – High rates of death from stomach cancer but unusual in US 31
  32. 32. • National variations : E.g. : Endemic goiter, Fluorosis etc. • Rural – Urban variations : E.g. : Rural areas - Skin and zoonotic infections and nutritional deficiencies. Urban areas – Chronic Bronchitis, Accidents, C-V diseases 32
  33. 33. Person Distribution Some characteristics of a person have been identified as risk factor for a disease. These variables include : Age Sex Ethinicity 33
  34. 34. Marital status Occupation Social class Behaviour Stress Migration 34
  35. 35. 4.Measurement of disease • - Measurement of mortality and Morbidity - Morbidity has two aspects – Incidence and prevalence. Incidence can be obtained from Longitudinal studies. Prevalence can be obtained from Cross sectional studies. • Cross sectional studies are more useful for the study of chronic diseases than short lived. • Longitudinal studies are difficult to organize and more time- consuming. 35
  36. 36. 5.Comparing with known Indices • By making comparisons between different populations and sub groups of the same population, it is possible to arrive at clues to disease etiology. 36
  37. 37. 6.Formulation of etiological hypothesis • Hypothesis is a supposition, arrived at from observation or reflection. • An epidemiological Hypothesis should specify the following: 37
  38. 38. a) The population – The characteristics of the persons to whom the hypothesis applies b) The specific cause being considered c) The expected outcome – the disease d) The dose response – relationship e) The time response – relationship 38
  39. 39. An improved formulation : “ The smoking of 30-40 cigarettes per day causes lung cancer in 10% of smokers after 20 years of exposure.” 39
  40. 40. Uses of Descriptive epidemiology • To provide data regarding the magnitude of the disease load and types of disease problems in the community in terms of morbidity and mortality rates and ratios. • Provides clues to disease etiology and helps in the formulation of etiological hypothesis. 40
  41. 41. • Provides background data for planning, organizing and evaluating preventive and curative services • They contribute to research by describing the variations in disease occurrence by time, place and person 41
  42. 42. Conclusion For epidemiologists, the identification of descriptive characteristics frequently constitutes an important step in search for determinants or risk factors that can be altered or eliminated to reduce or prevent disease. 42
  43. 43. References • Park K. Textbook of Preventive and Social Medicine. 23rd ed. Bhanot Publishers. Pg no.62-73 • Peter S. Essentials of Preventive and Community Dentistry. 4th ed. Arya Medical Publishers Pg no.92-104. 43
  44. 44. • http://www.thelancet.com/journals/lancet/ar ticle/PIIS0140-6736%2802%2907373- 7/fulltext 44
  45. 45. 45

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