Basic epidemiology for disease surveillance.

Basic epidemiology for disease surveillance.

1. Basic epidemiology for disease
surveillance
IDSP training module for state and
district surveillance officers
Module 7

2. Elements included in the module
1.Basic epidemiology relevant to surveillance
2.Ratios, proportions and rates
3.Incidence, prevalence and case fatality
4.Data presentation
•Tables
•Graphs
•Maps

3. Definition of epidemiology
Epidemiology is the study of the distribution
and determinants of health-related events or
states in population groups and the application
of this study to the control of health problems
(Last JM ed. Dictionary of Epidemiology, Oxford University Press, 1995)

4. Comparing the job of a clinician
and the job of an epidemiologist
The clinician
•Deals with patients
•Takes a history
•Conducts a physical
•Makes a diagnosis
•Proposes a treatment
•Follows up the patient
The epidemiologist
•Deals with populations
•Frames the question
•Investigates
•Draws conclusions
•Gives recommendations
•Evaluates programmes

5. The basic principles of
descriptive epidemiology
•Time
When did the event happen?
•Place
Where did the event happen?
•Person
Who was affected?

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Cases
Deaths
Investigation
started
Strike
Cases of acute hepatitis by date of
onset, Baripada, January-March 2004
Time

7. C
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Attack rate of acute hepatitis by zone of
residence, Baripada, Orissa, India, 2004
0 - 0.9 / 1000
1 - 9.9 / 1000
10 -19.9 / 1000
20+ / 1000
Attack rate
Underground water supply
Pump from river bed
Place

8. Attack rate of acute hepatitis by age and
sex, Baripada, Orissa, India, 2004
CasesPopulationAttack rate
per 1000
Age 0-4 1 1012 0.1
5-9 11 21802 2
10-14 37 74004 5
15-44 416 51358 81
45+ 73 56153 13
Sex Male 341 102683 3.3
Female 197 101646 1.9
Person

9. Role of the host, the agent and the
environment in the occurrence of disease
VECTOR
AGENT
HOST
ENVIRONMENT
Biologic,
Chemical,
Physical (injury, trauma)
Social
Psychological
Genotype
Nutrition
Immunity
Behaviour
Sanitation
Weather
Pollution
Socio-Cultural
Political

10. Uses of epidemiology
1.Examine causation
2.Study natural history
3.Description of the health status of
population
4.Determine the relative importance of
causes of illness, disability and death
5.Evaluation of interventions
6.Identify risk factors

11. 1. Examine causation
Genetic
factors
Environmental factors
(Biological, chemical,
physical, psychological
factors)
Good health Ill health
Life style
related factors

12. 2. Study natural history
Good health
Sub-clinical
disease
Clinical
disease
Recovery
Death

13. Prevalence of anemia among adolescent girls,
Mandla, MP, India 2005
Age in years
Hemoglobin <12 g%
TotalNumber (%)
12-13 71 93.4 76
14-15 88 93.6 94
16-17 71 97.3 73
18-19 27 77.1 31
Total 257 93.8 274
3. Description of the health status of
population

14. Disease DALYs*
(000)
Mortality
(000)
Included in IDSP
Tuberculosis 7577 421 Yes
Measles 6471 190 Yes
Malaria 577 20 Yes
* Disability-adjusted life years
4. Determine the relative importance of
causes of illness, disability and death

15. 5. Evaluation of interventions
Good Health Ill Health
Treatment,
Medical care
Health promotion
Preventive measures
Public health services

16. 6. Identify those sections of the
population which have the greatest risk
from specific causes of ill health
Characteristics
Univariate
odds ratio
(95% CI)
Adjusted odds
ratio
(95% CI)
Hookworm infestation 12 (5-29) 10 (4-24)
Consumption of IFA < 90 days 4.1 (2-8) 2.7 (1-7)
Education below middle school * 4 (3-7) 2.3 (1-4)
Number of pregnancy > 2 3.6 (2-6) 1.9 (1-4)
* Middle school = Seventh class in Orissa
Factors associated with
anemia among pregnant women, Orissa, 2004

17. Epidemiological approaches
•Descriptive epidemiology:
What is the problem?
Who is involved?
Where does the problem occurs?
When does the problem occurs?
•Analytical epidemiology:
Attempts to analyze the causes or determinants of disease
•Intervention or experimental epidemiology:
Clinical or community trials to answer questions about
effectiveness of control measures

18. Count, divide and compare:
The basis of epidemiology
1. Count the number of new AIDS cases in two cities
No. of new of AIDS cases
City A58
City B35

19. New AIDS cases
Number YearPopulation
City A 58 2004 25,000
City B 35 2004-5 7,000
2. Divide the number of cases by the population
City A: 58/25,000/ 1 year
City B: 35/7,000/ 2 years
Count, divide and compare:
The basis of epidemiology

20. City A: 232/100,000/ year
City B: 250/100,000/ year
3. Compare indicators
Count, divide and compare:
The basis of epidemiology

21. = 5 / 2 = 2.5/1
A ratio places in relation two quantities
that may be unrelated
•The quotient of two numbers
•Numerator NOT necessarily INCLUDED in the
denominator
•Allows to compare quantities of different
nature

22. Examples of ratio
•Number of beds per doctor
 85 beds for 1 doctor
•Number of participants per facilitator
•Sex ratio:
Male / Female

23. 2 / 4 = 0.5=50%
A proportion measures a
subset of a total quantity
•The quotient of two numbers
•Numerator NECESSARILY INCLUDED
in the denominator
•Quantities have to be of the same nature
•Proportion always ranges between 0 and 1
•Percentage = proportion x 100

24. Example of proportion
•Tuberculosis cases in a district:
400 male cases
200 female cases
•Question
What is the proportion of male cases among all
cases?
What is the proportion of female cases among all
cases?

25. A rate measures the speed of occurrence
of health events
•The quotient of two numbers
•Defined duration of observation
•Numerator
 Number of EVENTS observed for a given time
•Denominator (includes time)
Population at risk in which the events occur
2
----- = 0.02 / year
100
Observed in 2004

26. Example of rate
•Mortality rate of tetanus in country X in 1995
Tetanus deaths: 17
Population in 1995: 58 million
Mortality rate = 0.029/100,000/year
•Rate may be expressed in any power of 10
100, 1,000, 10,00, 100,000

27. Measures of disease frequency
•Prevalence
Number of cases of a disease in a defined
population at specified point of time
•Incidence
Number of new cases, episodes or events occurring
over a defined period of time

28. Prevalence
Number of people with
the disease or condition
at a specified time
Total population at risk
X FactorP =

29. Incidence rate
Number of people who get
the disease or condition
in a specified time
Total population at risk
X FactorI =

30. Case fatality ratio
•Divide
Number of deaths
Number of cases
•Example: Measles outbreak
3 deaths
145 cases
Case fatality ratio: 2.1%

31. Presenting health information
•Tables
•Graphs
Histograms
Line diagrams
Bar chart
Pie chart
Scatter plot
Map

32. Tables
•Data presented in columns and rows by one
or more classification variable
•Title- Concise, self explanatory explaining
clearly all information being presented
•Rows and columns should be clearly labeled
•Categories should be clearly shown

33. Age distribution of a sample of
100 villagers
Example of one way table:
Data tabulated by one variable
Age group (years) Number
0-4 19
5-14 25
15-44 40
45+ 16
Total 100

34. Example of two way table:
Data tabulated by two variable
Age group (years)Male FemaleNumber
0-4 10 9 19
5-14 12 13 25
15-44 20 20 40
45+ 7 9 16
Total 49 51 100
Age and sex distribution of a sample of 100 villagers

35. Graphs
•Charts based on length
•Bar charts (horizontal, vertical, grouped, stacked)
•Charts based on proportion
•Pie chart
•Geographic co-ordinate charts (maps)
•Spot map
•Area map

36. Malaria in Kurseong block, Darjeeling
District, West Bengal, India, 2000-2004
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Incidence of malaria
Incidence of Pf malaria
Line graph for time series

37. Histogram to display a frequency
distribution
•Graphic representation of the frequency distribution
of a continuous variable
•Rectangles drawn in such a way that their bases lie
on a linear scale representing different intervals
•Areas are proportional to the frequencies of the
values within each of the intervals
•No spaces between columns
•No scale breaks
•Equal class intervals
•Epidemic curve is an example of histogram with time
on the x axis

38. 0
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80
0-19.9 20-49.9 50-99.9 100-300 > 300
Urinary Iodine Excretion levels (µg/L)
P
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Histogram
Urinary iodine excretion status, 24 N
Parganas, West Bengal, India, 2004

39. Acute hepatitis by week of onset in 3
villages, Bhimtal block, Uttaranchal, India,
July 2005
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80
90
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May June July AugustSeptember
Week of onset
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Epidemic curve

40. Proportions of a total presenting
selected characteristics
•Breakdown of a total in proportions:
Pie chart
•Breakdown of more than one total into
proportion:
Juxtaposed bar charts cumulated to 100%

41. Road
10%
Fall
32%
Bites
16%
Burns
7%
Minor injuries
35%
Types of unintentional injuries,
Tiruchirappalli, Tamil Nadu, India, 2003
Incidence:
9.6 per 100 person-month
(95% C.I. 8-11
Pie chart for the breakdown of a total in proportions

42. Estimated and projected proportion of
deaths due to non-communicable
diseases, India, 1990-2010
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
1990 2000 2010
Year
P
r
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(
%
)
Injuries
Communicable
diseases
Non communicable
diseases
Cumulated bar chart for the breakdown
of many totals in proportions

43. Comparing proportions across groups
•No logical order: Horizontal bar chart
Sort according to decreasing proportions
•Logical order: Vertical bar chart
Not a continuous variable: Do not display axis
Continuous variable: Display axis

44. Causes of non vaccination as reported by the
mothers, Bubaneshwar, Orissa, India, 2003
0% 20% 40% 60% 80% 100%
Lack of money
Lack of facility
Lack of time
Lack of motivation
Irregularity by health staff
Child sick
Lack of awareness
India FETP
Horizontal bar chart

45. 0
10
20
30
40
50
60
70
30-39 40-49 50-59 60-69 70 +
Age group (years)
%
Male
Female
Prevalence of hypertension by age and
sex, Aizawl, Mizoram, India, 2003
Vertical bar chart

46. Cholera cases by residence, Kanchrapara,
N-24 Parganas, West Bengal, India, 2004
Spot map

47. 20-49
50-99
100+
1-19
0
Attack rate per
100,000
population
Pipeline crossing
open sewage drain
Open drain
Incidence of acute hepatitis by block,
Hyderabad, AP, India, March-June 2005
Hypothesis generated:
Blocks with hepatitis are those
supplied by pipelines crossing
open sewage drains
Incidence by area