Measures of morbidity

1. 15
Measures of Morbidity
In a given population morbidity for a given time may be
measured in terms of new cases (incidence) and in the terms
of both new and old cases (Prevalence).
[Q: What are the differences between "incidence" and
"prevalence". Mention the type of studies needed for the
estimation of rates of incidence and prevalence. (BSMMU,
MD Radiology, July, 2010)]
Incidence rate
Incidence rate is defined as the number of NEW cases
occurring in a defined population during a specified period of
time.
Total number of new cases
during a definedperiod
Incidence rate = ×1000
number of population
exposed to risk in same period
Problem: population of Dhaka 5000000 in 1995 and the
number of motor accident in that year was 45. Calculate the
Incidence rate of motor accident
Ans.
45×1000
Incidence rate = = 0.009/thousand
5000000 /year
Note; Incidence rate must include the unit of time used in the
final expression. If you wrote 0.009 per 1000, this would be
inadequate. The correct expression is 0.009 per 1000 per year.
It will be seen from the above definition that incidence rate
refers

2. Biostatistics-145
 only to new cases
 During a given period (usually one year )
 In a specified population or “population at risk” , unless
other denominators are chosen
It can also refer new spells or episodes of disease arising in a given period of
time, per 1000 population. For example, a person may suffer from common
cold more than once a year. If he had suffered twice, he would contribute 2
spells of sickness in that year. The formula in this case would be
Number of spells of sickness
starting in a defined period
Incidence rate = ×1000
Mean number of persons
exposed to risk in that period
Incidence measures the rate at which new cases are occurring in a
population. It is not influenced by the duration of the disease. The use of
incidence is generally restricted to acute conditions.
Prevalence
The term “diseases 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.
A broader definition of prevalence is a follows. “ the total
number of all individuals who have an attribute or disease at
a particular time ( or during a particular period) divided by
the population at risk of having the attribute or disease at this
point in time or midway through the period”.
It is of two kinds:
1. Period prevalence: It is used to measure the frequency of
an illness in existence during a defined period (day, week.
month. year. etc). It includes all the cases, in the defined
period—old and new cases occurring during the same period.

3. Biostatistics-146
Total number of new and old
cases found during a specified period
period prevalence = ×1000
no.ofexposed to risk
at the same period
2. Point prevalence This term is used to measure the number
of cases of illness, new and old, existing at a particular
point of time, such as at 2 PM on Monday, the 15th July.
1983.
Total number of new and old cases
found at a particular point of time
pointprevalence = ×1000
Population exposed to risk at
the same point of time
Relationship between prevalence and incidence
Prevalence depends upon 2 factors, the incidence and
duration of illness. Prevalence can be expressed as;
P= I x D = incidence x mean duration
The above equation (P= Ix D) shows that the longer the
duration of the disease, the greater its prevalence. For
example, tuberculosis has a high prevalence rate relative to
incidence. This is because new cases of tuberculosis keep
cropping up throughout the year, while the old ones may
persist for months or years.
Incidence can be obtained from “longitudinal” studies, and
prevalence from “cross- sectional” studies.
Uses of prevalence
(a) Prevalence helps to estimate the magnitude of health
disease problems in the community, and identify
potential high- risk populations
(b) Prevalence rates are especially useful for administrative
and planning purposes, e.g ., hospital beds, manpower
needs, rehabilitation facilities, etc.

4. Biostatistics-147
Some special rates
Number of live births
Birth rate = Mid- year population
Number of live births
Fertility rate =
Number of women aged15- 44 years
Number of infant (<1year) deaths
Infant mortality rate = Number of live births
Number of intrauterine
deaths after 28 weeks
Stillbirth rate =
Totalbirths
Number of stillbirths
+
deaths in1st week of life
Perinatal mortality rate =
Total births
NB: These rates are usually related to one year
[Name types of studies that determine prevalence and
incidence of a disease. [BSMMU, Radiology, January,
2012]