Basic epidemiology for disease surveillance
IDSP training module for state and district surveillance officers
Module 7
Elements included in the module
1. Basic epidemiology relevant to surveillance
2. Ratios, proportions and rates3. Incidence, prevalence and case fatality4. Data presentation
• Tables• Graphs• Maps
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)
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
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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Nu
mb
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ses
and
dea
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Cases
Deaths
Investigation
started
Strike
Cases of acute hepatitis by date of onset, Baripada, January-March 2004
Time
Chip
at
river
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
Attack rate of acute hepatitis by age and sex, Baripada, Orissa, India,
2004Cases 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
Person
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
GenotypeNutritionImmunityBehaviour
SanitationWeatherPollution
Socio-CulturalPolitical
Uses of epidemiology
1. Examine causation2. Study natural history3. Description of the health status of
population4. Determine the relative importance of
causes of illness, disability and death5. Evaluation of interventions6. Identify risk factors
1. Examine causation
Genetic factors
Environmental factors
(Biological, chemical, physical, psychological
factors)
Good health Ill health
Life style related factors
2. Study natural history
Good health
Sub-clinical disease
Clinical disease
Recovery
Death
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
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
5. Evaluation of interventions
Good Health Ill Health
Treatment, Medical care
Health promotion
Preventive measures
Public health services
6. Identify those sections of the population which have the greatest risk from specific causes of ill health
CharacteristicsUnivariate 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
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
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 58City B 35
New AIDS cases
Number Year Population
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
City A: 232/100,000/ year
City B: 250/100,000/ year
3. Compare indicators
Count, divide and compare: The basis of epidemiology
= 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
Examples of ratio
• Number of beds per doctor 85 beds for 1 doctor
• Number of participants per facilitator• Sex ratio:
Male / Female
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
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?
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
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
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
Prevalence
Number of people with the disease or condition at a specified time
Total population at risk X FactorP
=
Incidence rate
Number of people who getthe disease or condition in a specified time
Total population at risk X FactorI =
Case fatality ratio
• Divide Number of deaths Number of cases
• Example: Measles outbreak 3 deaths 145 cases Case fatality ratio: 2.1%
Presenting health information
• Tables• Graphs
Histograms Line diagrams Bar chart Pie chart Scatter plot Map
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
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
Example of two way table: Data tabulated by two variable
Age group (years) Male Female Number
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
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
Malaria in Kurseong block, Darjeeling District, West Bengal, India, 2000-
2004
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2000 2001 2002 2003 2004
Months
Inci
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mal
aria
per
10,
000 Incidence of malaria
Incidence of Pf malaria
Line graph for time series
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
0
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40
60
80
0-19.9 20-49.9 50-99.9 100-300 > 300
Urinary Iodine Excretion levels (µg/L)
Pe
rce
nta
ge
Histogram
Urinary iodine excretion status, 24 N Parganas, West Bengal, India, 2004
Acute hepatitis by week of onset in 3 villages, Bhimtal block, Uttaranchal,
India, July 2005
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Week of onset
Num
ber
of c
ases
Epidemic curve
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%
Road10%
Fall32%
Bites16%
Burns7%
Minor injuries35%
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
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
Pro
port
ion (
%)
Injuries
CommunicablediseasesNon communicablediseases
Cumulated bar chart for the breakdown of many totals in proportions
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
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
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
Cholera cases by residence, Kanchrapara,
N-24 Parganas, West Bengal, India, 2004
Spot map
20-49
50-99
100+
1-19
0
Attack rate per100,000 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