IDSP 7 : www.medicos11.com : IDSP 7 : www.medicos11.com
Basic epidemiology for disease surveillance : Basic epidemiology for disease surveillance IDSP training module for state and district surveillance officers
Module 7
Elements included in the module : Elements included in the module Basic epidemiology relevant to surveillance
Ratios, proportions and rates
Incidence, prevalence and case fatality
Data presentation
Tables
Graphs
Maps
Definition of epidemiology : 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 : 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 : The basic principles of descriptive epidemiology Time
When did the event happen?
Place
Where did the event happen?
Person
Who was affected?
Cases of acute hepatitis by date of onset, Baripada, January-March 2004 : 0 5 10 15 20 25 30 35 40 45 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 Number of cases and deaths Cases Deaths Investigation started Strike Cases of acute hepatitis by date of onset, Baripada, January-March 2004 Time
Attack rate of acute hepatitis by zone of residence, Baripada, Orissa, India, 2004 : Chipat 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 Place
Attack rate of acute hepatitis by age and sex, Baripada, Orissa, India, 2004 : Attack rate of acute hepatitis by age and sex, Baripada, Orissa, India, 2004 Person
Role of the host, the agent and the environment in the occurrence of disease : 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
Uses of epidemiology : 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
1. Examine causation : 1. Examine causation Genetic factors Environmental factors
(Biological, chemical, physical, psychological factors) Good health Ill health Life style
related factors
2. Study natural history : 2. Study natural history Good health Sub-clinical disease Clinical disease Recovery Death
Prevalence of anemia among adolescent girls, Mandla, MP, India 2005 : Prevalence of anemia among adolescent girls, Mandla, MP, India 2005 3. Description of the health status of population
4. Determine the relative importance of causes of illness, disability and death : * Disability-adjusted life years 4. Determine the relative importance of causes of illness, disability and death
5. Evaluation of interventions : 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 : 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
Epidemiological approaches : 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 : 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
Count, divide and compare: The basis of epidemiology : 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
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
A ratio places in relation two quantities that may be unrelated : = 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 : Examples of ratio Number of beds per doctor
85 beds for 1 doctor
Number of participants per facilitator
Sex ratio:
Male / Female
A proportion measures a subset of a total quantity : 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 : 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 : 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
Example of rate : 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 : 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 : Prevalence Number of people with
the disease or condition
at a specified time Total population at risk X Factor P =
Incidence rate : Incidence rate Number of people who get
the disease or condition
in a specified time Total population at risk X Factor I =
Case fatality ratio : 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 : Presenting health information Tables
Graphs
Histograms
Line diagrams
Bar chart
Pie chart
Scatter plot
Map
Tables : 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
Example of one way table: Data tabulated by one variable : Age distribution of a sample of 100 villagers Example of one way table: Data tabulated by one variable
Example of two way table: Data tabulated by two variable : Example of two way table: Data tabulated by two variable Age and sex distribution of a sample of 100 villagers
Graphs : 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 : Malaria in Kurseong block, Darjeeling District, West Bengal, India, 2000-2004 2000 2001 2002 2003 2004 Months Incidence of malaria per 10,000 Incidence of malaria Incidence of Pf malaria Line graph for time series
Histogram to display a frequency distribution : 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
Urinary iodine excretion status, 24 N Parganas, West Bengal, India, 2004 : 0 20 40 60 80 0-19.9 20-49.9 50-99.9 100-300 > 300 Urinary Iodine Excretion levels (µg/L) Percentage 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 : Acute hepatitis by week of onset in 3 villages, Bhimtal block, Uttaranchal, India, July 2005 0 10 20 30 40 50 60 70 80 90 1st week 2nd week 3rd week 4th week 1st week 2nd week 3rd week 4th week 1st week 2nd week 3rd week 4th week 1st week 2nd week 3rd week 4th week 1st week May June July August September Week of onset Number of cases Epidemic curve
Proportions of a total presenting selected characteristics : 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%
Types of unintentional injuries, Tiruchirappalli, Tamil Nadu, India, 2003 : 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
Estimated and projected proportion of deaths due to non-communicable diseases, India, 1990-2010 : 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 Proportion (%) Injuries Communicable diseases Non communicable diseases Cumulated bar chart for the breakdown of many totals in proportions
Comparing proportions across groups : 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 : 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
Prevalence of hypertension by age and sex, Aizawl, Mizoram, India, 2003 : 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 : Cholera cases by residence, Kanchrapara, N-24 Parganas, West Bengal, India, 2004 Spot map
Incidence of acute hepatitis by block, Hyderabad, AP, India, March-June 2005 : 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