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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started

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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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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

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0-19.9 20-49.9 50-99.9 100-300 > 300

Urinary Iodine Excretion levels (µg/L)

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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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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

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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