It explains about measurements of mortality and morbidity in epidemiology

1. Dr. Jayaramachandran S
Associate Professor
Dept. of Community Medicine
04.11.2019

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Define Epidemiology
• The study of the distribution and determinants of health-related
states or events
• in specified populations
• and the application of this study to the control of health problems

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Specific Learning Objectives
• At the end of the session, the students shall be able to
List the basic measurements in epidemiology
Select an appropriate tools of measurement
Measure morbidity & mortality
Perform standardization of rates

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Basic measurements in
epidemiology

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Basic measurements in epidemiology
• Measurements of mortality
• Measurements of morbidity
• Measurements of disability
• Measurements of natality
• Measurements of the presence, absence or distribution of the
characteristics or attributes of the disease

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Basic measurements in epidemiology
• Measurements of the presence, absence or distribution of the
environmental and other factors suspected of causing the disease.
• Measurements of the medical needs, health care facilities, utilization
of health services and other factors suspected of causing the disease.
• Measurements of demographic variables.

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Tools of measurement

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Tools of measurement
1. Rates
2. Ratios
3. Proportions

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1. Rate
• A rate measures the occurrence of some particular event (
development of disease or the occurrence of death) in a population
during given time period.
• CDR = No. of deaths in one year ×1000
Mid year population
• Consists : Numerator, denominator, time factor and multiplier
• e.g.. CBR, CDR.

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Various categories of rates
a) Crude rates: These are the actual observed rates such as the birth
and death rates. (unstandardized rates)
b) Specific rates: These are the actual observed rates due to specific
causes (e.g.. tuberculosis); or occurring in specific groups (e.g., age-
sex groups) or during specific time periods (e.g., annual, monthly or
weekly rates).
c) Standardized rates: These are obtained by direct or indirect method
of standardization or adjustment, e.g. Age and Sex standardized
rates.

11. 2. Ratio
• It expresses a relation between two random quantities.
• 𝑥 ∶ 𝑦 or 𝑥
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𝑦
• E.g. Sex ratio, child-woman ratio, doctor- population ratio.

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3. Proportion
• A proportion is a ratio which indicates the relation in magnitude of a
part of the whole.
• The numerator is always included in denominator.
• The proportion is usually expressed in percentage.

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Measurements of mortality

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Crude death rate
• CDR is the number of deaths (from all causes) per 1000 estimated
mid year population in one year, in a given place.
• CDR = No. of deaths in one year ×1000
Mid year population
• Limitation of CDR is exposed when we compare age-specific death
rates b/w two populations.
• Major disadvantage of CDR is that they lack comparability for
communities populations that differ by age, sex, race, etc.

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Specific death rate (SDR)
• SDR = Death due to speci0ic cause ×1000
Mid year population
• Useful when planning to find out etiology :
• Cause or disease specific e.g. TB, Cancer, etc.
• Related to specific groups- age specific, sex specific, etc.

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Specific death rate
• Advantages: It helps us to identify particular groups or groups at risk
for preventive action. They permit comparisons b/w different causes
with in same population
• Disadvantages: mainly used in developed countries where they have
civil registration system.

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Case fatality rate (CFR)
•CFR = Total no. of deaths due to particular disease ×100
Total no. of cases due to the same disease
• It represents the Ratio of death to cases
• Virulence of organism & killing power of a disease
• Useful in acute infectious diseases e.g. cholera, measles
• Time interval is not specified

19. Proportional mortality rate (ratio)
• No of deaths due to a particular cause or in a specific age group per
100 or 1000 deaths”.
a) Proportional mortality of a specific disease
=
No. of deaths from the specific disease in a year
×100
Total deaths from all causes in that year
b) Under 5 proportionate mortality rate
= No. of deaths under 5 yrs of age in a given year ×100
Total no. of deaths during the same period
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Proportional mortality rate (ratio)
• Proportional mortality rates are usually used for broad disease group
and for specific disease of major public health importance e.g.,
cancer, CHD
• It is used when population data is not available
• Depends upon only two variables, which differ. So, it cannot be used
for comparison b/w population groups or different time periods.

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Survival rate (SR)
•SR = Total no. of patients alive over a period ×100
Total no. of patients diagnosed or treated
• It is the population of survivors in a group studied and followed over a
period. e.g. for 5 years period
• Helps in describing prognosis in certain disease
• Can be used as a yard stick for the assessment of standards of
therapy.

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Adjusted or standardized rates

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Adjusted or standardized rates
• It’s major advantage is able to compare death rates of two
populations with different age- composition.
• This is because rates are only comparable if the populations upon
which they are based are comparable.
• It removes the confounding effect of different age structures and
yields a single standardized or adjusted rate by which we can
compare mortality directly

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Two methods of standardization
• Direct standardization
• Indirect standardization

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Direct standardization
• First a “ standard population” is selected (nos. in each group are
known)
• Secondly age specific rates are applied to standard population whose
crude death rate is intended to be adjusted or standardized

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Indirect standardization
• Standardized Mortality Ratio (SMR) (expressed as percentage)
• SMR = Observed deaths ×100
Expected deaths
• Applications of SMR: SMR compares the mortality in a study group
(e.g.. an occupational group) with the mortality that the occupational
group would have had if they had experienced national mortality
rates.

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Indirect standardization
• If SMR is greater than 100, then the occupation would appear to carry
a greater mortality risk than that of the whole population.
• If SMR is less than 100, then the occupation risks of mortality would
seem to be proportionately less than that for the whole population.
• The SMR is better than direct standardization because it permits
adjustment for age and other factors.

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Measurements of morbidity

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Measurements of morbidity
• Morbidity is defined as “any departure, subjective or objective, from
a state of physiological well-being”.
• Morbidity rates/ratios measured by
a) Frequency of disease: incidence and prevalence
b) Duration of illness: days, months and years
c) Severity of illness: case fatality rates

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Importance of morbidity data
• To know extent and nature of the disease load in community and
help in forming priorities.
• Provide more comprehensive and accurate and clinically relevant
information on patient characteristics – essential for basic research.
• Starting point of etiological studies and plays crucial role in disease
population.
• Needed for monitoring and evaluation of disease control activities.

31. Incidence
• Incidence
=
No. of new cases of specific disease during a given time period
×1000
Population at risk during that period
• New spells or episodes of disease in a given population in the given
duration
• Usually restricted to acute conditions
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32. Special incidence rates
• Attack rate (case rate), Secondary attack rate, Hospital admission rate
• Attack rate: An attack rate is an incidence rate (usually expressed as a
per cent), used only when the population is exposed to risk for a
limited period of time such as during an epidemic.
• It reflects the extent of the epidemic.
=
Number of new cases of a specified disease during a specified time interva
×100
Total population at risk during the same interval
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Secondary attack rate
• SAR =
No of exposed persons developing disease with in the range of the incubation ×100
Total no exposed/ “susceptible” contact
• Primary case is excluded from both numerator and denominator.
• Limitation:
• Useful only for disease with short incubation period
• Not possible to identify susceptible cases

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Uses of incidence rate
• To control disease, and
• For research into aetiology and pathogenesis, distribution of diseases,
and efficacy of preventive and therapeutic measures.

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Cumulative incidence
• CI =
No. of people who get a disease during a specified period ×1000
No. of people free of the disease in the population at risk at the beginning of the period
• Cumulative incidence is a simpler measure of the occurrence of a
disease or health status.
• Unlike incidence, it measures the denominator only at the beginning
of a study.
• In a statistical sense, the cumulative incidence is the probability that
individuals in the population get the disease during the specified
period.

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Cumulative incidence
• The period can be of any length but is usually several years, or even
the whole lifetime.
• The cumulative incidence rate therefore is similar to the “risk of
death” concept used in life-table calculations.
• The simplicity of cumulative incidence rates makes them useful when
communicating health information to the general public.

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Prevalence
• The term "disease prevalence" refers specifically to all current cases
(old and new) existing at a given point in time, or over a period of
time in a given population.
• Prevalence is a ratio but expressed as a rate
• Two types
• Point prevalence
• Period prevalence

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Prevalence
• Point prevalence =
No of all current cases (old + new) at one point of time in a defined population ×100
Estimated population at the same point in time
• Period prevalence =
No. of existing cases (old+ new) during a given period of time interval ×100
Estimated mid − interval population at risk

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Relationship between prevalence & incidence
• Prevalence = Incidence x Duration.
• (if population is stable and incidence and duration are unchanging)
• Longer the duration: greater is the prevalence rate e.g. TB
• Acute disease: short duration and rapid recovery. So prevalence is less
than incidence.

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Uses of prevalence
• Estimating magnitude of health/ disease problems in the community
• Identify potential risk populations
• Useful for administrative and planning purposes

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Summary
• List the basic measurements in epidemiology
• Select an appropriate tools of measurement
• Measure morbidity & mortality
• Perform standardization of rates

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Any Questions???