Measures of morbidity Unit-8, lecture epidemiology

Health care

1. Measures of Morbidity
Acknowledgment : Dr Mufiza Farid

2. Learning Objectives
By the end of the session students should be able to
Define , calculate and interpret rates, ratios and
proportions.
Define incidence and prevalence and discuss the
relationship between incidence and prevalence.
REFERENCES:
Leon Gordis. Chapter 3: Measuring the occurrence of
disease Epidemiology, W. B.Saunders Company .1996.P 30-
57

3. Measures of health status convey information about
the occurrence of disease. They include:
•Counts
•Proportions
•Ratios
•Rates
Quantitative Measures of Health Status

4. To investigate distributions and determinants of disease, To investigate distributions and determinants of disease,
it is necessary to know:it is necessary to know:
The size of the source population from which affected
individuals were derived
The time period during which the data were collected.

5. •Simplest measure in epidemiology
•Refers to the number of cases of a disease or
other health phenomenon being studied e.g.
Cases of influenza in a squatter settlement of
Karachi in January, 2002
Number of dengue positive patients admitted in
AKUH in July 2006
•Useful for allocation of health resources
•Limited usefulness for epidemiologic purposes without
knowing size of the source population
Counts

6. Relationship between two numbers
Example: Occurrence of Major Depression
Female cases = 240 240
---------------------------- =----2:1 female to
male
Male cases = 120 120
Ratios

7. Persons included in the numerator are always
included in the denominator:
A
Proportion: --------
A + B
In epidemiology, proportion tells us the fraction
of the population that is affected
Proportions

8. Proportions - Example
A B Total (A + B)
# persons with
hypertension
# persons without
hypertension
Total study
population
1,400 9,650 11,050
P = A / (A + B) = (1,400 / 11,050) = 0.127

9. Rate
Measures the probability of occurrence of a particular
event
X = number of times an event has occurred (cases)
Y = number of persons exposed to the risk of the event
K = some round number (10
a
where a=2,3,4,5,6) or
base depending upon the relative magnitude of X and
Y
Condition
Time, place and population must be specified
X
------- x K
Y

10. INCIDENCE:
Incidence quantifies the number of new events or
cases of disease that develop in a population of
individuals at risk during a specified time period .
IR = number of new cases of disease during a specified
period of time
x10
a
Population at risk

11. Incidence
Incidence quantifies the “development” of disease
Measure of risk (i.e transition from non-diseased to
diseased state)
Denominator represent the number of people at
risk for developing the disease
Any individual who is included in denominator
must have a potential to become a part of the group
that is counted in the numerator
e.g. Uterine cancer

12. Types of Incidence
Cumulative incidence (CI)
Incidence rate (IR) / Incidence density

13. Cumulative incidence (CI)
is the proportion of people who become diseased
during a specified period of time. It provides an
estimate of the probability, or risk, that an individual
will develop a disease during a specified period of'
time

14. Cumulative Incidence (CI)
No. of new cases of disease during a given period
CI = --------------------------------------------------------------------------
Total population at risk during the given period
Example: During a 1-year period, 10 out of 100 “at risk”
persons develop the disease of interest.
10
CI = -----= 0.10 or10.0%
100

15. Incidence Density (Rate)
It measures the rate (speed) at which new
events occur in a population.
Slide 15
of 61
10xI
α
nobservatio of time-person Total
timeof periodgiven a during disease of cases new ofNumber

Measures probability to become ill by unit of time.

16. Incidence Density
Observation Period
Jan
1989
July
1989
Jan
1990
July
1990
Jan
1991
July
1991
Years
contributed
A
1.5
B
2.0
C X 1.0
D X 1.5
E
2.5
2 cases
8.5 person years
x100= 24 per 100 person years
Initiation of observation
loss-to-follow-up
X Disease of interest detected
ID =

17. Figure 3.22. Breast cancer incidence rates in while women and distribution of cases by age. (data
from Cultler SJ, Young Jr: Third National Cancer Survey: Incidence data. Natl Cancer Mongor 41,
1975.)

18. PREVALENCE
 Prevalence quantifies the proportion of
individuals in a population who have the
disease at a specific instant and provides
an estimate of the probability that an
individual will be ill at a point in time

19. POINT PREVALENCE
Prevalence can be thought of as the status of the
disease in a population at a point in time and as
such is also referred to as point prevalence.

This "point" can refer to a specific point in calendar
time or to a fixed point in the course of events that
varies in real time from person to person, such as
the onset of menopause or puberty or the third
postoperative day.

20. PERIOD PREVALENCE
It represents the proportion of cases that exist within
a population at any point during a specified period of
time
The numerator thus includes cases that were
present at the start of the period plus new cases that
developed during this time

21. “Point” Prevalence
No. of cases of a disease present in a
population at a specified time
P =--------------------------------------------------------------------
No. of person in the population at that
specific time

22. “Point” Prevalence
Example:
On June 30, 1999, neighborhood A has:
•population of 1,600
•29 current cases of hepatitis B
P = 29 / 1600 = 0.018 or 1.8%

23. “Period” Prevalence
Number of existing cases
Pp = ----------------------------------------
Total population
During a certain time period
(i.e. May 1 - July 31, 1999)
Includes existing cases on May 1, and those
newly diagnosed until July 31.

24. “Period” Prevalence
Example:
Between June 30 and August 30, 1999, neighborhood
A has:
•average population of 1,600
•29 existing cases of hepatitis B on June 30
•6 incident (new) cases of hepatitis B between July
1 and August 30
Pp = (29 + 6) / 1600 = 0.022 or 2.2%

25. Examples of Point and Period Prevalence in Interview Studies
of Asthma
Interview Question
Do you currently have
asthma
Have you had asthma
during last (n) years?
Type of Measure
Point Prevalence
Period Prevalence

26. Uses of Incidence & Prevalence
Prevalence: Snap shot of disease or
health event
Help health care providers plan to deliver
services
Indicate groups of people who should be
targeted for control measures
Reflects determinants of survival

27. Uses of Incidence & Prevalence
Incidence: Measure of choice to:
--- Estimate risk of disease development
--- Study etiological factors
--- Evaluate primary prevention programs

28. Relation between Incidence &
Prevalence
Longer duration of the
disease.
Prolongation of life of
patients without cure.
Increase in new cases
(increase in incidence).
In-migration of cases.
Out-migration of healthy
people.
In-migration of susceptible
people.
Improved diagnostic
facilities (better reporting).
Shorter duration of
the disease.
High case-fatality rate
from disease.
Decrease in new cases
(decrease in
incidence).
Improved cure rate of
cases.

29. PROBLEMS WITH INCIDENCE & PREVALENCE
MEASURES
Problems with Numerators:
•Defining who has the disease

30. PROBLEMS WITH INCIDENCE & PREVALENCE MEASURES
Problems with Denominators:
•Classification of population subgroups may be
ambiguous (i.e race/ethnicity)
•Difficult to identify and remove from the
denominator persons not “at risk” of developing
the disease.
e.g those who had hysterectomy are no more at risk