This document discusses various measures used to describe disease frequency in populations. It defines key terms like prevalence, incidence, cumulative incidence, mortality rates and risk. Prevalence refers to existing cases at a point in time and is influenced by incidence, duration and cure rates. Incidence only counts new cases and is calculated based on person-time approaches. Mortality rates like crude mortality rate and standardized mortality ratio are also outlined. Examples are provided to demonstrate calculations and differences between measures.

1. Measures of Disease Frequency
Vidya Ramachandran
National Institute Of Epidemiology
(January 2005)

2. Learning - Objectives
At the end of this session scholars must be able to:
1. List, define, and differentiate the various measures
of disease frequency e.g. prevalence, incidence, etc
2. Calculate disease frequency using above measures.
3. List the characteristics, uses and limitations of the
various measures.

3. Measures of Disease Frequency
Rates, Ratios, Proportion
Prevalence, Incidence,
Cumulative Incidence
Mortality, SMR,
Risk

4. Rate, Ratios and Proportion
Three general classes of mathematical
parameters
Often used to relate the number of cases of a
disease or health outcome to the size of the
source population in which they occurred.

5. Ratio
Is obtained by dividing one quantity (x) by
another Quantity (y). These quantities may be
related or may be totally independent.
Usually expressed as x
---- x 10n
y
Example: Number of females per 1000 males
# females
------------ x 1000
# males

6. Proportion
Is a ratio in which the numerator is included in the
denominator.
Example: The number of fetal deaths out of the total
number of births. The answer is often expressed as a
percentage
# of fetal deaths
------------------------------------- x 100
# of live births + fetal deaths

7. Rate
It is a measure of how quickly an event of interest
occurs.
It is calculated by dividing the number of cases by the
corresponding number of people in the population at
risk. It is usually expressed as cases per 10n.
Time, place and population must be specified for each
type of rate
Example: # cases of Typhoid in Chennai, Jan. 2005
--------------------------------------- x 10n
Population at risk in Chennai for Typhoid,
Jan. 2005

8. Population at Risk
Refers to that portion of a population which is
susceptible to a disease. Can be defined on the basis
of demographic or environmental factors.
Example: Population at risk of developing
carcinoma cervix is calculated as follows:
1. Total population minus all males
2. Total female population minus those
below 30 and above 70 years
Therefore population at risk of developing cancer
cervix includes all females aged 30-69 years

9. Prevalence – (P)
Prevalence is the number of existing cases (old and
new) in a defined population at a specified point of
time.
Prevalence rate for a disease is calculated as follows:
# people with disease or conditions at a specified time
P = -------------------------------------------------- x 10n
of people in the population at risk at the specified time
In many studies the total population in the study area
is used as an approximation since data on population
at risk are not available

10. Prevalence – (P) Contd
Point Prevalence:
When the data have been collected at one point in time
P = C / N
- where C = # of observed cases at time ‘t’
N = Population size at time ‘t’
Measures the frequency of disease at a given point in time

11. Prevalence – (P) Contd
Point Prevalence Contd.
Example: Suppose there are 150 persons in a
population and, on a certain day 15 are
ill with flu. What is the estimated
prevalence for this population?
P = 15 / 150 = 10%

12. Prevalence – (P) Contd
Period Prevalence (PP)
When the data are collected over a specified
period of time
PP = C + I / N Where:
C = # of prevalent cases at the beginning of the
time period
I = # of incident cases that develop during the
period
N = size of the population for this same time
period

13. Prevalence – (P) Contd
Period Prevalence (PP) Contd.
Example : Suppose we followed a population of 150
persons for one year, and 25 had a disease of interest
at the start of follow up and another 15 new cases
developed during the year.
Calculate the following :
Period Prevalence for the year
Point prevalence at the start of the period

14. Prevalence – (P) Contd
Period Prevalence (PP) Contd. - Example Contd.
PP = C + I / N = (25 + 15) / 150 = 0.27 or 27 %
P = C / N = 25 /150 = 0.17 or 17 %

15. Prevalence – (P) Contd
Factors Influencing Prevalence include the following:
Severity of the illness
Duration of the illness
Number of new cases

16. Prevalence – (P) Contd
Factors Increasing Prevalence
Long Duration of the illness
Increase in new cases of the illness
Low / poor cure rate of illness
In migration of cases
In migration of Susceptible people
Out migration of healthy people
Improved Case detection / Diagnostic facilities

17. Prevalence – (P) Contd
Factors Decreasing Prevalence
Shorter duration of illness
High case fatality rate associated with the disease
Decrease in new cases
In migration of healthy people
Out migration of cases
Improved cure rate of cases

18. Prevalence – (P) Contd
Uses of Prevalence Data
Assessing the need for health care and in the
planning of such services.
To measure the occurrence of conditions for which
the onset of disease may be gradual
Study of chronic diseases

19. Incidence – (I)
It is the number of new cases arising in a given
period in a specified population.
Incidence Rate is calculated as follows:
# people who get a disease in a specified period
I = ---------------------------------------------------------- x 10n
Sum of the length of time during which each
person in the population is at risk
The numerator refers only to first events of disease.
Time, i.e. day, month, year, should be specified.
Measures the rapidity with which new cases are
occurring in a population.

20. Basic Measures of Disease Frequency`
Number of New cases in a population
during a given period of time
Cumulative Incidence = ------------------------------------------------------
Population free of disease
at start of period
Risk
This assumes that the entire population at risk at the start of the study has
been followed-up for the time period of observation
Number of new cases in a population
during a given period
Incidence Density = ---------------------------------------------------------
Total Person-time of observation
True Rate & Average

21. Hypothetical example for Risk & Incidence Density
Hypothetical example for Risk & Incidence Density
Onset
Time (years)
   
1 2 3 4 End
Diag
Lost
-------------
Diag
--------------
Diag Died
1980 1981 1982 1983 1984 1985
x
x
x
x
x
x
Disease-free observation period x Marks Diagnosis of disease
-------------Time observed after diagnosis

22. One year Risk = 1/5 = 20%
Incidence Density =
= 0.33 cases per PY
= 33 cases per 100 PY
years
Person
cases
new

9
3

23. Computation of Person-Time (PT)
Fixed Cohort Dynamic Cohort
No new members added to the New members get added to the
population during follow-up. Population during follow-up.
PT=Sum of Disease-free (i) PT=No. of Individuals x Length
periods in all persons in of follow-up adjusting for
the cohort. Migration in and out.
If, however, No. of new entrants
Person-years approach assumes into cohort equals number who
the risk to remain constant leave, the cohort is in a steady state.
through the period of In such a cohort
observation. (ii) PT = Total Population x Length
of follow-up.
If, however, the individual follow-up periods vary substantially in both
Fixed & Dynamic cohorts we have to use Acturial (Life-Table) method for
computing risk (This will be dealt with later in the course.)

24. Incidence – (I) Contd.
Cumulative Incidence (CI)
Is a simpler measure of disease occurrence. It
measures the denominator only at the beginning of a
study
Cumulative Incidence rate is calculated as follows:
# people who get disease during a specified period
C I = -------------------------------------------------------------- x 10n
# people free of disease in the population at risk in
the beginning of the period
Is often expressed as cases per 1000 population
CI = I / N

25. Incidence – (I) Contd.
Cumulative Incidence (CI) Contd.
C I = I / N
I= # of new cases during follow-up
N = # of disease free subjects at start of follow up
Example: N = 1000 men aged 45 years ; I = 50
developed prostate cancer. Follow-up =10 years. No
one lost to follow up. No one withdrew from study.
CI = I /N = 50/1000 = 0.05 = 5 %
10 year risk = CI = I/N = 5%

26. Risk
It is the probability that an individual with certain
characteristics like age, race, sex, smoking status
will experience a health status change over a
specified follow – up period.
The concept assumes that the individual does not :
Have disease at the start of follow up
Die from other causes during follow up

27. Prevalence - Incidence
Prevalence depends on both the incidence rate and
the duration of the disease from onset to termination.
P = I x D
Change in prevalence from one time period to another
may be the result of changes in incidence rates,
changes in the duration of disease, or, both.
The relationship between prevalence and incidence
varies according to the disease. There may be high
prevalence and low incidence e.g. Diabetes Mellitus,
or low prevalence and high incidence - common cold

28. Mortality
Mortality data however crude provide information
on trends in a population’s health status.
The Crude mortality rate (CMR) is calculated thus:
# deaths in a specified period
CMR = ------------------------------------------------ x 10n
Average total population during
that period
CMR does not take into account factors such as
age, sex, race, socio economic status, etc.

29. Mortality Contd.
Standardized Mortality Ratio (SMR)
SMR is a ratio measure. It is calculated by dividing
the observed number of cases of disease in the
cohort by the expected number ( O / E ). It is usually
expressed as a percentage.
The expected number is calculated by applying age
specific standard disease rates from a reference
population ( e.g. national rates) to the age
distribution of the cohort

30. Some Examples of Incidence and Prevalence Measures
Rate Type Numerator Denominator______
Morbidity Incidence New cases of non- Mid-year population
fatal disease
Mortality Incidence No. of deaths from Mid-year population
all causes
Case-fatality Incidence No. of deaths from a No. of cases of that
disease disease
Attack Incidence No. of cases of a Total population at
disease risk for a limited
period of time
Disease at Prevalence No. of cases of a No. of persons
autopsy disease autopsied
Birth defect Prevalence No. of babies with No. of live births
a given abnormality
(Source: Epidemiology in Medicine; Charles H. Hennekins & Julie E. Buring)