Measures of disease frequency

Measures of
Disease Frequency
Moderators:
Dr Alok Ranjan,
Assistant Professor.
Dr Purushottam Kumar,
Senior Resident
Presenter:
Dr Venkatesh Karthikeyan
JR – 1,Department of CFM
AIIMS Patna

•Basics measurements in epidemiology
•Basics requirements of measurements
•Tools of measurements
•Measures of morbidity
•Measures of disability
•Measures of mortality
Contents

Epidemiology
• The study of the occurrence and distribution
– of health related events, states and
processes – in specified populations, including
the study of determinants influencing such
processes – and application of this knowledge
to control relevant health problems.

Basic measurements in epidemiology
• Epidemiology – key focus is to measure the
morbidity and mortality
• What is required for it?
– Definition of what is to be measured
– Establishment of criteria or standards by which it
can be measured

Measurements in Epidemiology
• Measurement of morbidity
• Measurement of disability
• Measurement of mortality
• Measurement of natality
• Measurement of demographic variables

1
2
3
4
5
Validity
Reliability
Accuracy
Sensitivity
Specificity
Basic requirements of measurements

Variate vs Circumstance
• Variate – Any piece of information referring to
the patient or his disease.
– Discrete (Cancer lung +/-)
– Continuous (Blood pressure)
• Circumstance – Any factor in the environment
that might be suspected of causing a disease
• E.g: Air pollution

Rates Rati
os
Proportio
ns
Tools of Measurement

Measures of morbidity
Morbidity
- Any departure, subjective or objective,
from a state of physiological well being.
Morbidity can be measured in terms of 3 units
*Persons who were ill
*Illness that the person experienced
*Duration of the illness

• Commonly measured aspects of morbidity
– Frequency (measured by incidence and
prevalence)
– Duration
– Severity

Why should we measure it?
• Describe the extent and nature of disease
load in community – assist in establishing
priorities
• Provides data essential for research
• Serve as a starting point for etiological
studies, thus playing a crucial role in disease
prevention
• Monitoring and evaluation of disease control
activities

iNcidence
• Number of New cases occurring in a defined
population during a specified period of time
• Incidence = (Number of new cases of specific
disease during a given time period/population
at risk during that period) x 1000

Incidence
• Example:
– If 500 new cases in a population of 30,000 in a
year is present, then
• Incidence = (500/30000) * 1000
• = 16.7 per 1000 per year
• Incidence rate must include the unit of time
used in the final expression

Incidence – Key features
• Refers only to new cases
• During a give time period
• In a specified population or population at risk
(unless other denominators are chosen)
• Uninfluenced by the duration of the disease
• Usually restricted for describing acute conditions
• It can also refer to new spells or episodes of
disease arising in a given period of time per 1000
population

Incidence – Key features
• Same person having two spells of flu in a year
= contributes to two spells of sickness in that
year.
• Here, incidence rate is
(Number of spells of sickness starting in a
defined period/mean number of persons
exposed to risk in that period) x 1000

Incidence – Special incidence rate
• Attack rate
– Used only when the population is exposed to risk
for a limited period of time, such as during an
epidemic
– It relates the number of cases in the population at
risk and reflects the extent of epidemic
= (Number of new cases of a specified disease
during a specified time interval/ Total population
at risk during the same interval) x 100

Incidence – Special incidence rate
Secondary attack rate
- Number of exposed persons developing the
disease within the range of incubation period
following exposure to the primary case.
SAR = (Number of exposed persons developing
the disease within the range of the incubation
period/Total number of exposed contacts) *
100

Incidence – Secondary attack rate
• The primary case is excluded from both the
numerator and denominator
• SAR helps us to determine whether a disease
of unknown etiology is communicable or not
• SAR helps to evaluate the effectiveness of
control measures such as isolation and
immunization

Incidence rate - Uses
• Useful for taking action to control disease
• For research into etiology and pathogenesis,
distribution of diseases, and efficacy of
preventive and therapeutic measures
• Provides useful insights into the effectiveness
of the health services provided.

Prevalence
• Refers specifically to all current cases (old +
new) existing at a given point in time, or over
a period of time in a given population.
• It is the total number of all individuals who
have a disease at a particular time (or period)
divided by the population at risk of having the
disease at this point in time or midway
through the period.

Prevalence – Point prevalence
• Number of all current cases at one point of
time, in relation to a defined population.
• Point prevalence
= (Number of all current cases of a specified disease
existing at a given point of time/estimated
population at the same point in time) x 100
• In general, “Prevalence rate” denotes point
prevalence.

Prevalence – Period prevalence
• It measures the frequency of all current cases
existing during a defined period of time
expressed in relation to defined population.
• Period prevalence
= (Number of existing cases of a specified disease
during a given period of time interval/Estimated
mid interval population at risk) *100
• It includes cases arising before but extending
into or through to the year as well as those cases
arising during the year

Prevalence – Period prevalence

Uses of Prevalence
• Helps to estimate the magnitude of the
disease problem in the community
• Useful for administrative and planning
purposes

Relationship between prevalence and Incidence
• Prevalence = Incidence x Duration
• Eg;
Incidence = 10 cases/1000 population/year
Mean duration of disease = 5 years
Prevalence = 10 x 5 = 50 per 1000 population
• Similarly,
Incidence = Prevalence / Duration
Duration = Prevalence / Incidence

Prevalence = Photograph Incidence = Videograph

Incidence and Prevalence
• Duration reflects the prognostic factors
• Incidence reflects the causal factors

Quality of Life
• Most diseases have a major impact on the afflicted
individuals above and beyond mortality – may not
be lethal, but may be associated with considerable
physical and emotional suffering resulting from the
associated disability.
• Example : Arthritis
• Hence measuring quality of life and developing
valid indices that are useful for obtaining
comparative data in different patients and different
population is essential

Disability rates
• In recent years, disability rates related to
illness have come into use to supplement
mortality and morbidity indicators.
• They are based on the notion that health
implies full range of daily activities.

Disability rates
• Event type indicators
– Number of days of restricted activity
– Bed disability days
– Work-loss days within a specific period
• Person type indicators
– Limitation of mobility (e.g. Bedridden)
– Limitation of activity (daily activity/major activity)

Health Adjusted Life Expectancy
(HALE)
• It is an indicator used to measure healthy life
expectancy
• Earlier it was called Disability – Adjusted life
expectancy (DALE)
• HALE is based on life expectancy at birth, but
includes adjustment for time spent in poor
health.
• It is the equivalent number of years in full health
that a newborn can expect to live based on
current rates of ill-health and mortality.

Quality – Adjusted life years
• QALY is a measure of disease burden
(including quality and quantity of life lived)
• Used in assessing the value for money of a
medical intervention
• It is based on the number of years of life that
would be added by intervention.

QALY (Contd.)
• Each year in perfect health is assigned a value
of 1.0 down to a value of 0.0 for death, i.e.
• 1 QALY (1 year of life x 1 utility value = 1
QALY) is a year of life lived in perfect health.
• Half a year lived in perfect health is equivalent
to 0.5 QALY (1 year x 0.5 utility value)

Disability free life expectancy (DFLE)
• DFLE is synonymous to active life expectancy
• It is the average number of years an individual
is expected to live free of disability – if
current pattern of mortality and disability
continue to apply

Disability – adjusted life years
• DALY is a measure of overall disease burden
• Expressed as number of years lost due to ill –
health, disability or early death
• DALY = Years of lost life + Years lost to disability
• 1 DALY = One year of healthy life lost

DALY
• Years of lost life is calculated from the number
of deaths at each age multiplied by the
expected remaining years of life according to
a global standard life expectancy.
• Years lost to disability – Number of incident
cases due to injury and illness is multiplied by
the average duration of the disease and a
weighting factor reflecting the severity of the
disease.

Measurement of mortality
• Crude death rate
• Specific death rate
• Case fatality rate
• Proportional mortality rate
• Survival rate
• Adjusted or standardized rates

Measures of mortality
• Mortality data provide starting point for many
epidemiological studies
• Relatively easy to obtain
• Reasonably accurate in many countries
• Major resource for epidemiologist

Limitations of mortality data
• Incomplete reporting of deaths
• Lack of accuracy
– Age
– Cause of death (due to lack of diagnostic
evidence, inexperience of doctor, absence of post
mortem)
• Lack of uniformity
• Choosing single Cause of Death
• Changing coding systems

Why do we need mortality data
• For explaining trends and differentials in
overall mortality
• Indicating priorities for health action
• Allocation of resources
• Designing intervention programs
• Assessment and monitoring of public health
problems and programs
• Important clue for epidemiological research

Crude Death rate
• (Number of deaths from all causes during the
year/Mid year population) x 1000
• Why it is important?
– Shows the composition of population
– Shows the age specific death rates (reflects the
probability of dying)

Crude Death rate
• Disadvantage
– Lack comparability for communities with
populations that differ by age, sex, race, etc.
❑However, CDR should always be examined first
and later the age specific death rate should be
measured – CDR has the ability to portray an
impression in single figure.

Specific death rates
• Why it is needed?
– Helps us to identify at-risk groups : for preventive
action
– Permit comparison between different causes
within the same population

Specific death rates
• Cause or disease specific
– E.g: (Number of deaths from tuberculosis during a
calendar year/Mid year population) x 1000
• Related to specific groups
– E.g: (Number of deaths among males during a
calendar year/Mid year population of males) x
1000

Case fatality Rate
CFR = (Total number of deaths due to a
particular disease/Total number of cases due
to the same disease) x 100
• Denotes the Killing power of the disease

Case fatality Rate
• It is a proportion
• Typically used in acute infectious diseases like
cholera
• Usefulness in chronic disease is limited
(period from onset to death is long and
variable)

Proportional mortality rate
PMR = (Number of deaths from the specific
disease in a year/Total deaths from all causes
in that year) x 100
Under 5 PMR = (Number of deaths under 5
years of age in the given year/Total number
of deaths during the same period) x 100

Proportional mortality rate
• Does not indicate the risk of members of the
population contracting/dying from the
disease.
• Used when population data is not available
• Limited value in making comparisons between
population groups or different time periods

Survival rate
• Proportion of survivors in a group studied and
followed over a period
• Survival rate = (Total number of patients alive
after 5 years/Total number of patients
diagnosed or treated) x 100

Survival rate
• It is a method of describing prognosis
• Can be used as a yardstick for assessment of
standards of therapy
• Specially used in cancer studies

Adjusted or standardized rates
• Standardization removes the confounding
effect of different age structures
• Yields a single standardized or adjusted rate
• by which the mortality experience can be
compared directly
• Adjustment can be made not only for age, but
also sex, race, parity, etc.

Measures of mortality - Standardized rates
Direct standardization
(446/53500) x 1000
(60/4000)x1000

• First step – Choose a Standard population
• It is one for which the numbers in each age
and sex group are known
• It can be created by combining two
populations

• Second step – Apply to the standard population, the age
specific rates of the population whose crude death rate is to
be adjusted or standardized 🡪
• As a result, for each age group, an “expected” number of
deaths in the standard population is obtained 🡪
• These are added together from all age groups to give the total
expected deaths
• The final operation is to divide the “expected” total number
of deaths by the total of the standard population, which
yields the standardized or age – adjusted rate.

---------------

• “Standardizing” for age distribution has
reduced the crude death rate from 8.3 to 6.56
• Standardized rates have been calculated so
that they can be compared between
themselves – they have no intrinsic meaning
other than this purpose.

Indirect age standardization
• Standardized Mortality Ratio
• Life table
• Regression techniques
• Multivariate analysis

Measures of mortality - Standardized rates - Indirect age standardization
Standardized Mortality Ratio
• It is a ratio of the total number of deaths that
occur in the study group - to the number of
deaths that would have been expected to
occur - if that study group had experienced
the death rates of a standard population.
• SMR = (Observed deaths/Expected deaths) x 100

• It gives a measure of the likely excess risk of
mortality due to occupation
• If SMR is >100, then the particular occupation
would appear to carry greater mortality risk
than that of the whole population – and the
vice versa

Why SMR?
• SMR has the advantage over the direct
method of age adjustment in that it permits
adjustment for age and other factors, where
age specific rates are not available or
unstable because of small numbers.
• It is possible to use SMR if the event of
interest is occurrence of disease , rather than
death.

Interpreting observed changes in mortality

Summary
• Basics measurements in epidemiology
• Basics requirements of measurements
• Tools of measurements
• Measures of morbidity
• Measures of disability
• Measures of mortality
References:
• Park’s textbook of Preventive and Social Medicine – 26th
edition
• Gordis Epidemiology – Sixth edition

Measures of disease frequency

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Measures of disease frequency

Similar to Measures of disease frequency (20)

More from Dr Venkatesh Karthikeyan

More from Dr Venkatesh Karthikeyan (20)

Recently uploaded

Recently uploaded (20)

Measures of disease frequency