The document discusses various methods for measuring disease occurrence and mortality rates in populations. It defines key epidemiological terms like incidence rate, prevalence rate, case fatality rate, crude death rate, age-specific death rates, cause-specific death rates, infant mortality rate, neonatal mortality rate, and maternal mortality ratio. Various factors that influence these rates are also explained. Examples are provided to demonstrate how to calculate different rates.

1. Measuring the Occurrence of
Disease
Dr. Gahayr
Lec-4
1/1/2022

2. To examine the transmission of disease in
human populations, we need to be able to
measure the frequency of disease occurrence
and of deaths from the disease.

3. Measuring the occurrence of Disease
• Morbidity rate- The rates that measure disease
• Mortality rate- The rates that measure death
• Mortality and/or Morbidity data are/is difficulty
to get b/c not recorded by law, pt may not go to
a Dr or Dr may not report the case to the local
health authority.
• So, we have incomplete information on disease

4. What we use morbidity and mortality
rates?
 To identify the characteristics of individual(person)
How have the disease/ health problem and the
environmental conditions that favour condition.
 To show the time changes in occurrence of
condition.
 The rates are used for comparison either from one
place to other or from one time period to another
Therefore, person at risk, time, and place must be
considered in calculation.

5. How do we express the extent of morbidity and
mortality resulting from disease?
 Counts
 Ratio
 a fraction with no specified relationship
 Proportions
 what fraction of the population is affected
 Rates
 how fast things are occurring

6. Morbidity rates
• Incidence rate
• Prevalence rate
• Surveillance rate

7. Incidence
A “motion picture” – describes what is happening
in a population. Incidence is defined as the number
of new cases divided by the population at risk over
time.
Incidence therefore includes three components:
1. New cases
2. Population at risk.
3. Interval of time.
Note that:
• Incidence involves the passage of time.

8. Cont..
A-When the incidence rate is HIGH, it indicates
one of the following:
1- Seasonal increase- when the condition is
occurring in certain seasons of the year-such as
influence, or diarrhoea.
2- Epidemic when the cases increased more
than usual(expected) for the time and place
3-Failure of primary prevention of the condition
B-Incidence rate is Low, it indicates primary
prevention had an effect(Worked).

9. Prevalence
a kind of “still life” picture – is the most basic of
epidemiologic measures.
It is defined as the number of cases (old+new)
divided by the population-at-risk.
Prevalence has three components:
1. Existing cases
2. Population “at risk” to have the condition
3. Point (or sometimes a period) in time to which
the prevalence applies.

10. Factors influencing prevalence rate
Increased by:
 Large duration of disease
 Prolongation of life of patients without cure
 Increased in new case
 In-migration of cases
 Out-migration of healthy people
 In-migration of susceptible people
 Improved diagnostic ( Better reporting)

11. Cont…
Decreased by:
Shorter duration of disease
High case fatality rate from disease
Decrease in new cases
In-migration of healthy people
Out-migration cases
Improved cure rate of cases

12. Prevalence and incidence
incidence
recovery death
Point prevalence is the balance
between inflow to the prevalence
pool (incidence) and outflow (e.g.
recovery and/or mortality).

13. Measures of Mortality rates
• Is one of the principal vital events.
• Is important in public health administration.
• It is complete disappearance of all evidence of
life after live birth (UN,WHO).
• Live birth is complete extraction of the baby
from his mother irrespective of the duration of
pregnancy.

14. Cont..
General mortality rate or crude death rate shows the
overall state of health care. If the mortality rate is
decreased, health care is good and improving

15. Data requirements for mortality
analysis
• Number of deaths
• The size of the population exposed to death.
What are the Basic Mortality Measures?

16. 1) Crude death rate (CDR)
• Midyear population is an approximation of the average
population exposed to risk.
• Typically computed for calendar year so as to eliminate
the effect of seasonal or monthly variations on the
comparability of the rates.
• There is a problem if the mid-point of the time reference
for the numerator does not correspond to the time of the
midpoint population.

17. 1000
year
same
the
during
area
same
in the
population
year
Mid
year
a
during
area
defined
a
in
deaths
of
no
Total
X
CDR 

18. Features of crude death rates
• It is the most commonly used method because
it is the simplest measure of mortality.
• It does not consider the age variation.
• As the result whenever we want to compare
mortalities of two different populations or the
mortality of the same population over time
adjustment for age variation is required.

19. 2) Age and sex specific death rate (ASSDR)
1000
year
&
area
same
the
of
group
sex
and
age
same
the
of
population
year
Mid
year
a
in
area
defined
a
of
group
sex
and
age
specific
a
in
deaths
of
no
Total
X
ASSD 

20. • Example: The average total population of
“sub-village Y” in 1996 was 6000 (3500 female
& 2500 male).
• In the same year 300 people died (100 female
& 200 male).
• Calculate the mortality rate (Crude death rate)
for females?

21. • CDR for females = 100/3500 X 1000 = 29 per
1000 female population.
• That means out of 1000 female population
living in “sub-village Y”, 29 females died in
1996

22. Cont..
Age and Sex- specific death rates are useful to:
Detect a health problem in certain age or sex groups
So, an increase of death rate in any group alerts
public health workers to investigate the reason for
this increase.

23. 3) Cause specific death rate (CSDR)
1000
year
in that
populaiton
year
Mid
year
a
in
cause
particular
a
to
due
deaths
of
Number
X
CSDR 

24. • Case fatality death rate
• CFR = No of deaths from specific during given time year *100
No. of cases of that disease during same time
• Case fatality rate represents the probability of
death among diagnosed cases
• or the killing power of a disease.

25. Case Fatality Rate
Proportion of cases that resulted in death of
Cholera
 Can be as high as 33%-50% where medical treatment
not available
 In developing countries, CFR can be less than 1%,
when treatment is optimal
 If the CFR is > 10%
 are there problems with case management?
 review treatment routines
 ensure adequate supplies
 increase community's access to care (consider Temporary
Treatment Centers)

26. • Example: In 1996 there were 1000
tuberculosis patients in Aw region.
• Out of the 1000 patients 100 died in the same
year.
• Calculate the case fatality rate of
tuberculosis?
• CFR = 100/1000 x 100 = 10 %
• That means 10% of tuberculosis patients will
die once they develop the disease

27. 4) Infant mortality rate (IMR)
1000
year
and
area
same
in the
births
live
of
No
year
a
during
area
an
in
ocuring
infants
of
deaths
of
No
X
IMR 

28. • Infant mortality rate reflects the health of the
community in which the child is being brought
up.
• Thus, it is high among people who have little
health care, chiefly because infections, such as
– pneumonia,
– diarrhea and malaria, are common among their
infants.
• Malnutrition is also one of the killer of infants in
developing countries.

29. 5) Neonatal mortality rate (NMR)
1000
year
and
area
same
in the
births
live
of
No
year
a
in
area
an
in
age
of
days
28
under
infants
of
deaths
of
No
X
NMR 

30. • Example: In 1996 there were a total of 5000
live births in “Zone B”.
• Two hundred of them died before 28 days
after birth.
• Calculate the Neonatal Mortality Rate (NMR).
• NMR = 200/5000 X 1000 = 40 per 1000 live
births.
• That means out of 1000 live births in 1996, 40
of them died before 28 days after birth.

31. 6) Post-neonatal mortality rate (PNMR)
1000
period
and
area
same
in the
births
live
of
No
year
&
area
an
in
year
1
to
weeks
4
aged
infants
deaths
of
No
X
PNMR 

32. 7) Still birth rate (SBR)
1000
year
and
area
same
in the
occuring
births
still
and
births
live
of
No
year
a
during
area
specified
a
in
occuring
births
still
of
No
X
SBR 

33. 8) Proportionate mortality ratio (Prop. MR)
100
year
and
area
same
in the
deaths
of
no
Total
year
a
during
area
specified
a
in
cause
given
a
from
deaths
of
No
.
Pr X
MR
op 

34. • For example when we say the proportionate
mortality ratio for HIV/AIDS is 30 %,
• this means out of 100 total (of all) deaths 30
of them died from HIV/AIDS.

35. 9) Perinatal mortality rate (PMR) (after 28 days of
pregnancy-7 days old infant)
1000
period
and
area
same
in the
occuring
births
still
and
births
live
of
No
time
of
period
and
area
specified
a
in
occuring
deaths
perinatal
of
No
X
PMR 

36. 10) Maternal mortality ratio (MMR)
000
100
year
and
area
same
in the
births
live
of
No
year
a
in
area
an
in
puerperium
the
and
birth
child
pregnancy,
of
ons
complicati
to
due
deaths
female
of
no
Total
X
MMR 

37. • A maternal death is the death of a woman
while pregnant (or within 42 days of
termination of pregnancy) irrespective of the
duration and the site of the pregnancy, from
any cause related to or aggravated by the
pregnancy or its management but not from
accidental causes

38. Statistical measures of maternal
mortality
• Maternal Mortality Ratio
Number of maternal death during a given time-period per
100 000 live birth during the same time-period.
• Maternal Mortality Rate:
Number of maternal death in a given period per 100 000
women of reproductive age during the same time-period.
38

39. Late maternal death
• The death of a woman from direct or indirect
obstetric causes more than 42 days but less than
one year after the termination of pregnancy.
39

40. Challenges with definitions
• Accurate identification of the causes
• When there is home delivery
• Lack of complete registration
• Even with death certificate, cause may not be indicated
• Even with civil registration, pregnancy status may not
be known
40

41. Home take assignment
• The following information is about village X which was
collected for the year 1999:
– Total average population = 40,000
– Total number of live births = 4000
– Total number of deaths = 400
– Total number of deaths before the age of 28 days =50
– Total number of infant deaths = 200
– Number of women who died from pregnancy related
causes = 160
– New cases of tuberculosis = 100
– All cases of tuberculosis = 300
– –Deaths from tuberculosis = 60

42. • Based on the above information calculate and
interpret the following.
1. The incidence rate of tuberculosis.
2. The period prevalence rate of tuberculosis.
3. The case fatality rate of tuberculosis.
4. The Neonatal mortality rate.
5. The infant mortality rate.
6. The maternal mortality ratio