This document discusses concepts related to morbidity measurement including incidence, prevalence, and rates. It defines incidence as the number of new cases of a disease in a population over a time period, taking into account those at risk. Prevalence is defined as all current cases, both new and old, existing at a given point in time. Various types of rates are discussed such as cumulative incidence, person-time incidence, point prevalence, and period prevalence. Morbidity measurement provides information about disease burden in a community and is important for health planning and evaluation.

1. B. Ashok
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2. •Any departure ‘subjective or objective’
from a state of physical well being.
•It is known by different names like illness,
disability, sickness.

3.  The extent & nature of the disease load in the
community.
 They provide more comprehensive, accurate
& clinically relevant information on patient
characteristics.
 Starting point for aetiological studies.
 Needed for monitoring and evaluation of
disease control activities.

4.  Incidence and prevalence.
 Notification rates.
 Attendance rates at op depts.
 Admission, readmission & discharge rates
 Duration of stay in hospital.
 Spells of sickness or absence from work or
school.

5.  Three aspects of morbidity are commonly
measured-
Frequency
Duration
Severity
 They are measured in terms of rates or ratios.

6.  Def: Number of NEW cases occurring in a defined population
during a specified period of time.
 An incidence rate includes three important elements:
1. a numerator -- the number of new cases
2. a denominator -- the population at risk
3. time -- the period during which the cases occurred.
no. of new cases of specific disease
during a given time period
INCIDENCE = X 1000
population at risk during that period

7.  NEW CASES : These include those
individuals who developed the disease
during the period of study. Those persons
who already have the disease when study
started are not included.
 POPULTION AT RISK: These include
individuals who are capable of having or
acquiring the disease or condition in
question.
 TIME PERIOD: incidence rate must include
the unit of time used in final expression.

8.  Cumulative Incidence
 Person-time Incidence (Incidence Density)

9.  Cumulative incidence is the proportion of a
fixed population that becomes diseased
during a stated period of time.
No of new cases identified over a
 Cumulative given time interval
Incidence =
rate Estimated population at beginning
of interval

10.  Problem occurs when a population or study group is
under observation for different length of time.
 This may occur for a variety of reasons, including
attrition or dropout, mortality, or development of the
disease under study.
 To allow for varying periods of observation of the
subjects ,one uses a modification of the formula for
the incidence rate in which the denominator becomes
person-time of observation and “incidence density” is
measured.

11. What is the incidence rate from
October 1, 1990 to Sep 30, 1991?
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12. No of new events occurring during a
Person-time specified period of time
Incidence rate =
Total person-time units at risk
 Person-time units:
Combined measure of number of
persons and time they were at risk
 Commonly: Person-Years

13.  Incidence rate of postmenopausal breast cancer
in the IWHS.
 Sample population in Jan 1986 = 41,837
women
 Population at risk =
women who were pre menopausal (n=569),
with breast cancer diagnosis (n=2,293) and
who had breast removed surgically (n=1,870).
41,837 – (569+2,293+1,870) = 37,105
 After 8 yrs of follow-up 1,085 cases were
identified through state health registry.
 Number of new cases during the 8 year period
= 1,085

14.  Cumulative incidence for
8 year for 1 lakh population
: 1085
----------- X 1,00,000 = 2924 (per 8
37,105 years per
1 lakh)
Cumulative incidence per 1 year :
: 2924 / 8 = 365.5 (per year per lakh )

15.  Total number of person time years
= 37,105 X 8 = 2,96,840
Total number of person time years observed in
the study = 2,76,453
Person time incidence rate :
1085
-------------- X 1,00,000 = 392.5 (per
2,76,453 year
per
1lakh)

16. Cumulative incidence :
= 365.5 (per year per lakh)
Person time incidence rate :
= 392.5 (per year per lakh )

17.  All current cases (NEW+ OLD) existing at a
given point or period of time in a given
population.
 Obtained from cross-sectional studies.
 Prevalence is two types:
point prevalence
period prevalence

18.  The point prevalence rate comprises all the cases
of a disease that exist at a point in time.
Number of persons with a specific
disease at a specific point in time
Point Prevalence= X100
Total defined population at
the same time

19.  Period prevalence is all cases whether old, new or
recurrent, arising over a defined period of time.
no. of persons with a specific disease
during a specific period of time.
 Period prevalence = X 100
total defined population during
specific period of time
 Generally the average or mid-interval population
for the specified time period

20. Point and Period Prevalence
1998 1999 2000 2001 2002 2003 2004 2005 2006
1 January

21. I
I
I
I
I
Prevalence
Pot
Increased by: Decreased by:
- longer duration - shorter duration
- prolongation of life - high case-fatality
- increase in new cases - decrease in new cases
- out-migration of healthy - in-migration of healthy
Baseline

22.  It gives information about disease burden
in the community
 To estimate the magnitude of health
problems/ disease in the community.
 For administrative and planning
purposes.
e g; hospital beds, man power need,
rehabilitation facilities.

23.  Assume that population is stable and
unchanged incidence and duration then
Prevalence = Incidence X Duration