MEASUREMENTS IN
EPIDEMIOLOGY
PRACHETH R.
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Tools of measurement
Measurement of mortality
Mortality rates and ratios
Standardization
Measurement of morbidityIncidence, Prevalence
• Uses of epidemiology
• If you can measure that of which you speak,
and can express it by a number, you know
something of your subject, but if you cannot
measure it, your knowledge is meager and
unsatisfactory.
—William Thomson, Lord Kelvin, engineer,
mathematician, and physicist (1824–1907)[1]
MEASUREMENTS IN EPIDEMIOLOGY
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Measurement of mortality
Measurement of morbidity
Measurement of disability
Measurement of natality
Measurement of the presence, absence or
distribution of characteristic or attributes of
the disease
6) Measurement of medical needs,health care
facilities,utilization of health services and
other health – related events.
7) Measurement of presence, absence or
distribution of environmental and other
factors suspected of causing disease
8)Measurement of demographic variables
CHARACTERISTICS
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Valid
Reliable
Accurate
Sensitive
Specific
• VARIATE:
o Discrete: Ca lung, rash in measles
o Continuous: Serum cholesterol, B.P, height
• CIRCUMSTANCE:
air pollution, polluted water
TOOLS OF MEASUREMENT
1.Counts:
• Simplest ,most frequently used
• Number of persons in the group studied –
particular disease/characteristic
• Doesn’t take into account source population
from which counts are derived
2. Rates
• Measures occurrence of some particular event
in a population during a given time period.
• Frequency of events occur in a defined time
period,divided by average number of people
at risk during the period being studied.
• Change in some event that take place in a
population over a period of time
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Eg: Death rate
Numerator
Denominator
Time specification
Multiplier
Expressed per 1,000 or some other round
figure.
• Population at risk during the same time period
no of cases or events occurring during a given time
period
x 1,000
• Persons in the denominator must reflect the
population from which the cases in the numerator
arose.
• Counts in the numerator and denominator should
cover the same time period.
• Persons in denominator must be “at risk” for the
event.
• Ratio:
 Relation between 2 random quantities
Numerator- not a component of denominator
x:y or x/y
x independent of y
• Proportion:
Ratio which indicates relation in magnitude of
whole
x is included in y.
Eg: No of ppl with TB at a certain point of time
Total no of ppl in village at same time
x 100
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Sex of children attending an immunization
clinic :
1.male/female
2. female/all
Uses of rates, ratios , proportions
• Characterise population by age, sex, race ,
exposure
• Describe:
 Morbidity ( disease)
 Mortality (death)
 Natality (birth)
NUMERATOR AND DENOMINATOR
• Numerator:
No. of times an event occurred in a specified
time period.
Is a component of denominator in calculating
rate,but not in ratio
• Denominator:
Related to population
Related to total events
• Related to population:
1. Mid- year population
2. Population at risk
3. Person –time
4. Person-distance
5. Sub-groups of population
• Related to total events
MEASUREMENT OF MORTALITY
• Easy to obtain
• Many countries have routine systems to collect
mortality data
• Starting point for epidemiological studies
• Measure of disease severity
• Determine whether treatment for a disease has
become effective over time
• Surrogates for incidence rate- severe and lethal
disease
INTERNATIONAL DEATH CERTIFICATE
• Is the basis of mortality data.
• To ascertain frequency of disease in a
population
• National and international comparibility
Part 1:
• Immediate cause
• Underlying cause
Part 2:
• Any significant associated diseases
Limitations of mortality data
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Incomplete reporting of deaths
Lack of accuracy
Lack of uniformity
Choosing a single cause of death
Changing
Diseases with low fatality
Uses of mortality data
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Explain trends in overall mortality
Indicate priorities for action
Allocation of resources
Designing intervention programmes
Assessment and monitoring public health
problems and programmes
• Clues for epidemiological research
MORTALITY RATES AND RATIOS
• CRUDE DEATH RATE:
Simplest measure
No. of deaths per 1000 estimated mid year
population in 1 year in a given place
No.of deaths during a year
X 1000
Mid –year poplulation
Summarizes the effect of :
• Population composition
• Age specific death rates
Disadvantage:
• Lack comparibility for communities with
populations that differ by age, sex, race. Etc.
• CAUSE SPECIFIC MORTALITY RATE:
Analysis planned to know aetiology
May be:
Cause/ disease specific
Related to specific group
Identify particular groups at risk for preventive
action
 Comparisions between different causes
within same population
Satisfactory civil registartion system
High proportion of deaths certified
CASE FATALITY RATE :
• Killing power of a disease, Severity of disease
• Ratio of deaths to cases
• Total no. of deaths
due to a particular disease X 100
Total no. of cases due to
same disease
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Time interval not specified
Acute infectious diseases
Measure benefits of new therapy
Vary in different epidemics
Difference between C.F.R and mortality rate
 Proportional mortality rate:
• Proportion of deaths due to particluar disease
or occur in a particular age group
• Number of deaths due to a particular cause
/sp. age group per 100 (or 1000) total deaths.
• Deaths due to particular cause X 100
Deaths from all causes
1.PMR from a specific disease:
2. Under- 5 PMR
3. PMR for aged 60 above
Uses:
• Broad disease group, specific disease of public
health importance
• Importance of specific disease /disease group as
cause of death
Drawbacks:
• Limited in making comparisons between
population groups or different time periods
• Does not tell the risk of dying from a disease
 Survival rate:
• Proportion of survivors studied and followed
over a period.
• Describes prognosis in certain disease
conditions
• Assessment of standards of therapy
• Total no. of pts alive after 5 years
X 100
Total no. of pts diagnosed or treated
Adjusted/ standardized rates:
• Removes confounding effect of different age
structures
• Compare mortality experience of different
populations
• Age, sex, race, parity
DIRECT STANDARDIZATION
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Standard population selected
Apply age specific rates
Expected number of deaths
Total expected deaths
Divide total expected deaths/ total of
standard population =
STANDARDIZED/ AGE ADJUSTED RATE
• Useful for making comparisons
• Do not reflect true mortality risk of a real
population
• Feasible if actual specific rates in subgroups of
observed population, number of persons in
age group available
INDIRECT STANDARDIZATION
 Standardized mortality ratio:
• Ratio of total number of deaths that would
have been expected to occur if the study
group had experienced death rates of a
standard population.
• Number of deaths for each age specifc
stratum not available
• Excess risk of mortality due to an occupation
• Observed deaths X 100
Expected deaths
• > 100: greater mortality risk
• < 100: lesser mortality risk
• Advantage over direct method:
 Other techniques:
• Index death rate and a standardizing factor
for each population of interest
• Life table
• Regression techniques
• Multivariate analysis
MEASUREMENT OF MORBIDITY
WHO Expert Committee on Health Statistics 6th
report:
• persons who were ill
• Illnesses (periods/ spells of illness) that these
persons experienced
• Duration of illnesses
 Frequency
 Duration
 Severity
Value of morbidty data:
• Extent and nature of disease load in
community, help to establish priorities
• More comprehensive, accurate, clinically
relevant information on patient characteristics
• Starting point for aetiological studies, major
role in disease prevention
• Monitoring and evaluation of disease control
activities
INCIDENCE
• No. of new cases occurring in a defined
population during a specified period of time
• No.of new case of specific
disease during a given time period
X 100
Population at risk during that period
• Unit of time must be included
• Only to new cases
• During a given period (usually 1 year)
• In a specified population or “population at
risk”
• New spells/ episodes of disease arising in a
given period of time per 1000 population
• Incidence rate (spells)=
No. of spells of sickness
starting in a defined period
X 1000
Mean no of persons exposed
to risk in that period
 Measures rate at which cases are occurring in a
population
 Not influenced by duration of disease
 Used in acute conditions
• People at risk who are observed throughout a
defined time period
• People are not observed for full time period:
Denominator consists of sum of units of time
that each individual was at risk and observedPERSON TIME
Special incidence rates
 Attack rate:
• Population is exposed to risk for a limited period
of time- epidemics
• Number of cases in population at risk
• Reflects extent of epidemic.
• No. of new cases of a specified
disease during a specified time interval X 100
Total population at risk during same
interval
 Secondary attack rate:
• No. of exposed persons developing the disease
within range of incubation period, following
exposure to the primary case
Uses of Incidence:
• Health status indicator To control disease
 Research into aetiology, pathogenesis,
distribution of diseases, efficacy of preventive
and therapeutic measures
• Increase- failure of current control
programmes
• Change- change in aetiology of disease
PREVALENCE
• All current cases (old +new), at a given point
of time or over a period of time in a given
population
• Total no. of individuals who have an attribute
or disease at a particular time divided by
population at risk of having the
attribute/disease at this point of time or
midway through period.
• Ratio
• No. of cases of a disease present in
a population at a specified time
X 1000
No. of persons in the populatn at that
specified time
Types:
 Point prevalence:
• No. of all current cases (old+new) of a disease
at a given point of time
• 1 day, several days or few weeks
 Period prevalence
• How many ppl have had the disease during a
certain time period
• Number of all current cases ( old+ new)
existing during a defined time period in
relation to a defined population
• Do you currently have asthma ?- Point
• Have you had asthma during the last (n) yearsPeriod
Uses of prevalence
• Estimate magnitude of health/ disease
problems in community, identify potential
high-risk populations
• Measure of burden of disease in a community
• Administrative, planning purposes
Drawbacks of prevalence
• Not an ideal measure for studying disease
aetiology and causation
• To measure risk, incidence used
Uses of Epidemiology
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Study rise and fall of a disease in a population
Community diagnosis
Planning and evaluation
Evaluation of an individuals risk and chances
Syndrome identification
Completing the natural history of a disease
• Searching for causes and risk factors
• Measure disease frequency in terms of
magnitude problem (i.e morbidity, mortality)
• Descriptive epidemiology- formulate an
aetiological hypothesis
REFERENCES
THANK YOU