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

2. Elements included in the module
Basic epidemiology relevant to surveillance
Ratios, proportions and rates
Incidence, prevalence and case fatality
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?

6. Time
Cases of acute hepatitis by date of onset, Baripada, January-March 2004
Investigation 45
started
Cases 40
Deaths 35 30 25
Strike
Number of cases and deaths 20 15 10 5
1/1/04
1/3/04
1/5/04
1/7/04
1/9/04
1/11/04
1/13/04
1/15/04
1/17/04
1/19/04
1/21/04
1/23/04
1/25/04
1/27/04
1/29/04
1/31/04
2/2/04
2/4/04
2/6/04
2/8/04
2/10/04
2/12/04
2/14/04
2/16/04
2/18/04
2/20/04
2/22/04
2/24/04
2/26/04
2/28/04
3/1/04
3/3/04
3/5/04
3/7/04

7. Place
Attack rate of acute hepatitis by zone of residence, Baripada, Orissa, India, 2004
Attack rate
Underground water supply
Pump from river bed
/ 1000
/ 1000
/ 1000
20+ / 1000
Chipat river

8. Person
Attack rate of acute hepatitis by age and sex, Baripada, Orissa, India, 2004
Cases
Population
Attack 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

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

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

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

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

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

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

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

16. Factors associated with anemia among pregnant women, Orissa, 2004
6. Identify those sections of the population which have the greatest risk from specific causes of ill health
Factors associated with anemia among pregnant women, Orissa, 2004
Characteristics
Univariate odds ratio
(95% CI)
Adjusted odds ratio
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
This slide summarizes the factors that were associated with anemia in univariate and multivariate analysis.
Factors that were associated with anemia included partial consumption of IFA, a number of pregnancy exceeding two, an education level below middle school and hook worm infestation.

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 A 58
City B 35

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

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

21. 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
= 5 / 2 = 2.5/1

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

23. 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
2 / 4 = 0.5=50%

24. Example of proportion Tuberculosis cases in a district: Question
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
Observed in 2004 2
----- = 0.02 / year
100

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
Population at risk

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

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

30. Case fatality ratio Divide Example: Measles outbreak 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. Example of one way table: Data tabulated by one variable
Age distribution of a sample of 100 villagers
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 and sex distribution of a sample of 100 villagers
Age group (years)
Male
Female
Number
0-4 10 9 19
5-14 12 13 25
15-44 20 40
45+ 7 16
Total 49 51
100

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

36. Line graph for time series
Malaria in Kurseong block, Darjeeling District, West Bengal, India, 5 10 15 20 25 30 35 40 45
January
February
March
April
May
June
July
August
September
October
November
December
Incidence of malaria
Incidence of Pf malaria
Incidence of malaria per 10,000
2000
2001
2002
2003
2004
Months

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. Histogram
Urinary iodine excretion status, 24 N Parganas, West Bengal, India, 2004 80 60 40
Percentage 20
0-19.9
> 300
Urinary Iodine Excretion levels (µg/L)

39. Epidemic curve
Acute hepatitis by week of onset in 3 villages, Bhimtal block, Uttaranchal, India, July 2005 90 80 70 60 50
Number of cases 40 30 20
This graph represents the number of cases by week of onset between the month of May and September 2005 in the the tree villages.
The first case in the area developed an illness during the first week of May. The outbreak peaked during the fourth week of July – when the investigated began – and the number of cases decreased during the early part of the month of August. The shape of the curve suggested a common source outbreak.
You can see on this graph that before the outbreak, there were a number of initial cases that could be considered as index cases. 10
3rd week
1st week
1st week
2nd week
3rd week
4th week
1st week
2nd week
4th week
1st week
3rd week
2nd week
4th week
1st week
2nd week
3rd week
4th week
May
June
July
August
September
Week of onset

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. Incidence: 9.6 per 100 person-month (95% C.I. 8-11
Pie chart for the breakdown of a total in proportions
Types of unintentional injuries, Tiruchirappalli, Tamil Nadu, India, 2003
Incidence: 9.6 per 100 person-month (95% C.I. 8-11
Road
10%
Minor injuries
35%
In this rural population of predominantly agricultural workers, despite the current low agricultural activities, the leading contributor to disability was due to agricultural and other occupational injuries. These findings were consistent with other studies.32, 34 A high rate of morbidity due agricultural injuries has also been reported by community-based surveys in India.7,10,11, 16, 57
Fall
32%
Bites
16%
Burns 7%

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

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

45. Vertical bar chart
Prevalence of hypertension by age and sex, Aizawl, Mizoram, India, 2003 70 60 50 40
Male % 30
Female 20
Overall prevalence was 34.5% with a 95% Confidence interval of 32.3 and 36.7.
As we can see from the graph below, prevalence was higher among men in all the age groups as compared to women. We observed that the prevalence of hypertension is increasing with increasing age. This was consistent for both the sexes. The trend was significant.
We will now present the proportion aware of disease status among the hypertensive status. 10
30-39
40-49
50-59
60-69
70 +
Age group (years)

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

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