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Occurrence of Disease Morbidity
Dr. Swe Mar Myint Lwin
University of Public Health
• Morbidity has been defined as “any departure ,subjective or
objective from a state of physiological well-being”.
• The term is used equivalent to such terms as sickness ,illness
Tools of Measurements
Basic tools of measurements in epidemiology are-
• Ratio is one number divided by another number
• Numerator is not a component of denominator
• For example-sex ratio, doctor-population ratio , child-women
X KX:Y (or)
a + b X 100
• Proportion is also one number divided by another, but the
entities represented by these numbers are related to one
• The numerator is always a subset of the denominator.
• It has no dimension because the unit of the numerator cancels
out the unit of the denominator
• Proportion is usually expressed as a percentage.
The no. of children with scabies at a certain time X 100
The total no. of children in the village at the same time 5
• A rate measures the occurrence of some particular event
(development of disease or occurrence of death) in a population
during a given time period.
• Eg: Number of death in one year
Death rate = X 1000
Mid- year population
Categories of rates
• Crude rates- the actual observed rates such as the birth and
death rates. Crude rates are also known as unstandardized rates.
• Specific rates-These are the actual observed rates due to specific
causes (e.g ;TB) or occurring in specific groups (eg; age or sex
group), or during specific time periods (annual ,monthly ,weekly
• Standardized rates- These are obtained by direct or indirect
method of standardization or adjustment eg; age or sex
The concept of numerator and denominator
Numerator - a component of denominator in rate, but not in ratio.
Denominator - Related to total population
(i) Mid-year population
(ii) Population at risk
(iii) Person-times (eg; person-years,person-months )
(iv) Person-distance (passenger-miles)
(v) Subgroup of the population (age, gender, etc .)
- Related to total events
i.-in IMR; CFR
ii. Per 100 vehicles or per million vehicles-miles in case of
What are they—ratio, proportion, or rate?
• The number of people who ate tuna salad divided by the
number of people who did not eat tuna salad
• The number of sick people who ate tuna salad divided by
the number of people who ate tuna salad
• The number of people diagnosed with influenza on a cruise
ship divided by the number of person-weeks of follow-up of
people on the cruise ship
Population: A population is defined as a group of people with a
common characteristic such as place of residence, age, or the
occurrence of an event.
• There are two main types of populations, fixed and dynamic (or
• The membership of a fixed population is defined by a life event and
is permanent, whereas the membership of a dynamic population is
defined by a changeable characteristic and is transient.
• Three factors should always be considered when
measuring how commonly a disease occurs in a
(1) the number of affected individuals or cases,
(2) the size of the population from which the cases
(3) the amount of time that this population is
-Same population size and time period should be used for the
MEASURES OF MORBIDITY
a. Period Prevalence
b. Point Prevalence
a. Cumulative Incidence (Risk)
b. Incidence density or Incidence Rate
• Prevalence is defined as the number of affected persons present
in the population at a specific time divided by the number of
persons in the population at that time.
• Of 10,000 female residents in town A on 1st January 2006,
1,000 have hypertension.
• The prevalence of hypertension among women in town A on
this date is calculated as:
• 1,000/10,000 = 0.1 or 10%
• For example, if we are interested in knowing the
prevalence of arthritis in a certain community on a
certain date, we might visit every household in that
community and, using interviews or physical
examinations, determine how many people have
arthritis on that day.
• This number becomes the numerator for prevalence.
• The denominator is the population in the community
on that date.
A. Point prevalence
• Point prevalence refers to the proportion of the
population that is diseased at a single point of time.
Number of existing cases of disease At a point in time
Number in total population
• Returning to point prevalence, practically speaking, it is
virtually impossible to survey an entire city on a single day.
• Therefore, although conceptually we are thinking in terms
of a single point in time, in reality, the survey would take
• When we see the word prevalence used without any
modifier, it generally refers to point prevalence.
• Although you should always specify the point in time to
which point prevalence refers, prevalence is a proportion so
it is dimensionless (i.e., it has no units).
B. Period prevalence
• Period prevalence refers to the proportion of the population
that is diseased during a specified duration of time (usually a
month , a year, or 5 year period).
Number of existing cases of disease During a period of time
Number in total population
• Prevalence = a photograph (instantaneous
• Incidence = a film (continuous record)
• The incidence rate of a disease is defined as the number
of new cases of a disease that occur during a specified
period of time in a population at risk for developing the
• If we are calculating incidence of uterine cancer, the denominator
must include only women, because men would not have the
potential to become part of the group that is counted by the
numerator, that is, men are not at risk for developing uterine
• Another important issue regarding the denominator is the issue of
• Incidence measures can use two types of denominators: people at
risk who are observed throughout a defined time period or, when all
people are not observed for the full time period, person-time (or
units of time when each person is observed).
People at Risk Who Are Observed throughout
a Defined Time Period
• The choice of time period is arbitrary: (can calculate incidence
in 1 week, incidence in 1 month, incidence rate in 1 year,
incidence rate in 5 years, and so on).
• The important point is that whatever time period is used in
the calculation must be clearly specified, and all individuals
included in the calculation must have been observed (at risk)
for the entire period.
A. Cumulative incidence or Risk
• The incidence calculated using a period of time during which all
of the individuals in the population are considered to be at risk
for the outcome is also called cumulative incidence, which is a
measure of risk.
• It is important to note that the denominator is the total number of
people who were free of disease at the start of the time period.
• This is defined as the ‘population at risk’.
• Assume that the population at risk is followed over a specified time
period, and that all those who are included at the beginning of the time
period are counted at the end of the period.
• This is called a closed population or a cohort.
• For example, if a group of 100 people were studied for a year, and 25
had caught a cold at some point during the year, you would say that the
risk of catching a cold was 0.25 or 25% in that year in that group.
When All People Are Not Observed for the Full Time
period, Person-Time, or Units of Time When Each Person
• When different individuals are observed for different
lengths of time, calculate an incidence rate (also called
an incidence density), in which the denominator consists
of the sum of the units of time that each individual was
at risk and was observed.
• This is called person-time and is often expressed in terms
of person-months or person-years of observation.
• Let us consider person-years (py):
• One person at risk who is observed for one year = one
• One person at risk observed for 5 years = 5 person-years
• But 5 people at risk, each of whom is observed for only 1
year, also = 5 person-years.
• If people at risk are observed for different periods of time, the
incidence rate is:
• Although it is most accurate to determine person-time on an
individual basis, as was done in this study, this method is very
time consuming and labor-intensive.
• In much larger studies, where it might be difficult to measure the
exact person-time for every individual in the study, the mid-
period population can be used as an estimate for the total
person-time at risk.
• In this situation, the average population at the mid-point of the
calendar period of interest is multiplied by the number of years
over which the study took place.
• Figure: Hypothetical Disease Occurrence
• Point prevalence rate at t1 = 4 / 10
• Point prevalence rate at t2 = 3 / 10
• Incidence cases between t1 and t2 = (4, 6, 7, 8)
• Period prevalence = (Point prevalence t1 + Incidence cases) /10 =
(4 + 4)/10
• Cumulative Incidence Rate for 2004 = (4) / (10-4) = 4/6
Relationship Between Prevalence and Incidence
• This equation assumes that the population is in steady state
(that is, inflow equals outflow) and that the incidence rate
and duration do not change over time.
P = I x D
• P is prevalence (the proportion of the total population with the
• I is incidence rate
• D is the average duration (or length of time) that an individual
has the disease.
Special Incidence rates
• Attack rate or case rate
• Secondary attack rate
• Hospital admission rate
Attack Rate or case rate
Used only when limited exposure to a risk (during an epidemics)
Reflects the extent of the epidemic.
Attack Rate= No. of New cases of a specific d/s
during a specified time x100
Total population at risk
during the same interval
Secondary attack rate
It is defined as the number of exposed persons developing the
disease within the range of the incubation period following
exposure to a primary case.
Uses of prevalence
1. Helps to estimate the magnitude of health or disease problems
in the community
2. Helps to identify potential high risk population.
3. Useful projecting demands for medical services
4. Useful for administrative and planning purposes
e.g. Hospital beds, Manpower needs, Rehabilitation facilities etc.
Uses of incidence rate
Incidence rate is useful for taking action
1) to control disease &
2) For research into
• Etiology and pathogenesis
• Distribution of diseases
• Effectiveness of preventive and therapeutic measures.
REVIEW QUESTIONS FOR CHAPTER 3
Q. A prevalence survey conducted from January 1 through
December 31, 2012, identified 1,000 cases of schizophrenia in a
city of 2 million persons. The incidence rate of schizophrenia in
this population is 5/100,000 persons each year. What percent of
the 1,000 cases were newly diagnosed in 2012?
• Q. Population of the city of Atlantis on March 30, 2012 =
• No. of new active cases of TB occurring between January 1 and
June 30, 2012 = 26
• No. of active TB cases according to the city register on June 30,
2012 = 264
6. The incidence rate of active cases of TB for the 6-month period
a. 7 per 100,000 population
b. 14 per 100,000 population
c. 26 per 100,000 population
d. 28 per 100,000 population
e. 130 per 100,000 population
7. The prevalence rate of active TB as of June 30, 2012,
a. 14 per 100,000 population
b. 130 per 100,000 population
c. 144 per 100,000 population
d. 264 per 100,000 population
e. None of the above