1. Epidemiologic basis of public health
Epidemiology MPH 531 Types Study Designs in Epidemiology (DESCRIPTIVE STUDIES)
Mpundu MKC
MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN
Detels - PH Sept 2009

2. Learning Objectives Define descriptive Epidemiology
Describe the variables in Descriptive Epidemiology; Time, Place and person
Understand the importance of three variables in Descriptive Epidemiology
To understand why epidemiological studies are conducted
To understand the concepts of different epidemiological study designs

3. Types of Studies Epidemiology
1. Descriptive Epidemiology
Study of occurrence and distribution of disease
2. Analytic Epidemiology
Studies to determine the validity of hypothesis concerning the occurrence of disease
3. Experimental Epidemiology
Deliberate manipulation of the cause is predictably followed by an alteration in effect

4. Overview of Epidemiologic Studies
1. Descriptive Studies
Population
Individual
Case report
Case series
Cross sectional studies
2. Analytic Studies
Observational
Case control
Cohort
Retrospective
Prospective

5. Overview of Descriptive Studies
3. Interventional/Experimental Studies
Randomised Controlled Trial
Field trial
Clinical trial

6. Descriptive Epidemiology
Definition
Descriptive epidemiology the basic type of epidemiology, which is concerned with describing the frequency and distribution of diseases and other health related conditions by time, place and person.

7. Descriptive Epidemiology
In descriptive epidemiology, data is organised and summarised according to time, place and person. These characteristics are also referred to as epidemiologic variables
Reasons for this organisation
Investigator becomes familiar with the data and with the extent of the public health problem

8. Descriptive Epidemiology
Descriptive epidemiology is the first step in epidemiological study
Careful observation of available information have led to successful preventive measures.
Describing health or disease occurrence according to place, person and time can lead to meaningful hypothesis formulations

9. Descriptive Epidemiology
It provides a detailed description of the health of population
Analysis of data identifies the population that are at greatest risk of acquiring a particular disease
Information provides important clues to the cause of the disease and the clues can be turned into testable hypotheses

10. Descriptive vs. Analytic Epidemiology
Descriptive Analytic
Used when little is Used when insight
known about a about various aspects disease of disease is available
Rely on preexisting Rely on development of
data new data
Who, where, when Why, How
Illustrates potential Evaluates the causality
associations of associations

11. Descriptive Epidemiology
What (Case definition)
Who? (Persons)
Where) (Place)
When? (Time)
How many (Measures)

12. Descriptive Epidemiology
Study of the occurrence and distribution of disease
Terms
Time : When is the disease occurring?
Place: Where is the diseases occurring?
Person: Who is getting the disease?

13. Variables of Descriptive Epidemiology
PERSON
PLACE
TIME
Think of this as the standard dimensions used to track the occurrence of a disease.

14. Descriptive Epidemiology
TIME (When)
Secular
Periodic
Seasonal
Epidemic

16. Time Con’t
Study of disease occurrence by time is a basic aspect of epidemiologic analysis.
Occurrence is usually expressed on a monthly or annual basis, daily in epidemic
Some diseases occur periodically or cycles
E.g. Malaria is one of the of diseases with seasonal periodicity, high peaks occur in relation to the rainy season when stagnant water are convenient for the breeding of mosquitoes

17. Time Disease rates change over time
Some changes occur regularly and can be predicted
Examining events that precede a disease rate increase or decrease, the causes can be identified and appropriate actions to control or prevent further occurrence of disease can be taken

18. Time Con’t Secular Trend Long term trend of disease occurrence
Occurs slowly over long periods of time.
Steady trend towards increase or decrease over a long period of time.
Commonly use these trends to predict the future incidence of disease
Used to evaluate programmes or policy decisions or to suggest what caused an increase or decrease in disease occurrence, particularly if it indicates when related event took place

19. Secular Trend

20. Secular Trend

21. Epidemiologic basis of public health
Time Con’t
Seasonal Variations
A cyclic variation in disease frequency by time of year & season.
Seasonal fluctuations
Environmental factors
Occupational activities
Recreational activities
Seasonal variations can be used to suggest possible etiology
Seasonal variations can be seen for some diseases or conditions falling within calendar year
Detels - PH Sept 2009

22. Seasonal Variation
Seasonal variation can be seen for some diseases or conditions falling within a calendar year

23. Seasonal Variation . Migratory Birds?
Seasonal variation can be used to suggest possible etiology

24. Epidemiologic basis of public health
Time Con’t
Point Epidemics
An increase in incidence above the expected in a defined geographic area within a defined time period
Short-term changes occur over limited time frame. Hours, days, wks, months
To show the time course of a disease outbreak or epidemic, we use a specialized graph called an epidemic curve
Used for short term exposures or diseases with short incubation period and /or illness duration
Detels - PH Sept 2009

25. Point Epidemics

26. Epidemic Curve
Number of Cases
of Disease
Time

27. Cyclic Trends
Cyclic trends may be either long-term or short term events.
Some are “seasonal” while others are cyclic due to other factors:
Immigration
School year

28. Cyclic Trends

29. Endemic, Epidemic and Pandemic
Endemic: The habitual presence (or usual occurrence) of a disease within a given geographic area
Epidemic: The occurrence of an infectious disease clearly in excess of normal expectance and generated from a common or propagated source
Pandemic: A worldwide epidemic affecting an exceptionally high proportion of the global population

30. Time clustering Time Place Cluster/disease cluster
Epidemiologic basis of public health
Time clustering Time Place Cluster/disease cluster
A group of cases occur close together & have a well aligned distribution pattern in terms of time and place
Time clustering data can sometimes be used to trace the beginning to the introduction of specific casual agent
Cluster analysis-used for rare or special disease events.
Thalidomide and birth defects: was first marketed in Europe in the 1950s as a sleeping pill and to treat morning sickness in pregnant women
Detels - PH Sept 2009

31. Place (Where)
An area defined by natural boundaries may have a high or low incidence of certain diseases because it is characterized by some particular environmental or climatic conditions, e.g temperature, humidity, rainfall, altitude, mineral content of soil, or water supply.
Place can assist to infer factors that increase the risk factors are present in persons (host factors) living there or in the environment or both

32. Place (Where?

33. Place (Where)
Health event may be described by place to gain insight into geographical extent of the problem. e.g. place of
Residence, Birth
Country, Continent, district, urban, rural
Frequency of disease is different in different places. These differences can occur because of the natural boundaries (e.g. forests, rivers, deserts).

34. Five Criteria of Place
 Rate observed in all ethnic groups in the area
 Rate NOT observed in persons of similar groups inhabiting other areas
Healthy persons entering area get ill at same frequency
People who leave do NOT show similar levels
Similar levels of infestation in other species (if zoonotic disease)

35. Place
Place gives an idea of where the agent that causes disease normally lives, multiplies, what may carry it and how it spreads
Where are the rates higher? Lower?
Geographic location of source
Geographic location of reservior

36. Epidemiologic basis of public health
Spot maps show location of individual cases
Detels - PH Sept 2009

37. Characteristics Relating to Place
International
Variation within countries
Urban-rural
Local

38. Person WHO is getting the disease?
Many variables are involved and studied, but factors such as sex, age & race often have a major effect.

39. Person Age Hobbies Gender Hereditary Marital status Ethnicity/Race
Occupation Travel
Immunization status Personal Habits
Underlying disease Stress
Medication Family unit
Nutritional status School
Socioeconomic factors Genetics
Crowding Lifestyle
Behaviour Religion/Culture

40. Age (Who?)

41. Sex (Who?)

42. Age (Who?)

43. Summary of Descriptive epidemiology :
Patterns of Disease Occurrence
Distribution of disease in populations
By “person” : age, race / ethnicity, gender, occupation, education, marital status, genetic marker, sexual preference
By “place” : residence (urban vs. rural), worksite, social event
By “time” : week, month, year: sporadic, seasonal, trends

44. Procedure in conducting a Descriptive Study
Defining the population
Defining the disease under study
Describing the disease by
Time
Place
Persons
Measurements of the disease
Comparing with known indices
Formulation of an aetiological hypothesis