Frequency measures of health is an important aspect in the planing of the type of services required in a specific population. This is due to the fact that they are able to indicate the type and level of health problems being faced In that population during a specified period of time.

1. LUAPULA SCHOOL OF HEALTH
SCIENCES
COURSE :PUBLIC HEALTH CARE
CODE: PHC210
TOPIC:EPIDEMIOLOGICAL CALCULATIONS
BY: CHIPILI JOYCE. BSc Env. Health. Cet IT. Lec. Lushes.
(chipilijoyce84@gmail.com)

2. Introduction
• Scope of measurement in epidemiology is broad and
includes measurements of:
Mortality, morbidity and disability.
Presence, absence or distribution of characteristics of
disease
Medical needs, health care facilities, utilization of
services.
Demographic variables

3. • Requirement for this measurement:
To define what is to be measured
Establish criteria or standards by which it can be
measured.
• Definition must be
Precise and valid (help ID cases from non-cases).
Acceptable and applicable for use in large populations.
• Disease magnitude expressed as:
Rate
Ratio
Proportion

4. Frequency measures
• compare one part of the distribution to another part of the
distribution, or to the entire distribution.
• Common frequency measures are
ratios ,
proportions,
and rate s .
• All three frequency measures have the same basic form:
numerator
𝑛𝑢𝑚𝑒𝑟𝑎𝑡𝑜𝑟
𝑑𝑒𝑛𝑜𝑚𝑖𝑛𝑎𝑡𝑜𝑟
× 10𝑛

5. Common frequency measures cont.
1. A ratio
This is the relative magnitude of two quantities or a
comparison of any two values.
It is calculated by dividing one interval- or ratio-scale
variable by the other.
The numerator and denominator need not be related.
Therefore, one could compare apples with oranges or apples
with number of physician visits.
• Ratio=
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑟 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑒𝑣𝑒𝑛𝑡𝑠,𝑖𝑡𝑒𝑚𝑠,𝑝𝑒𝑟𝑠𝑜𝑛𝑠,𝑒𝑡𝑐.𝑖𝑛 𝑜𝑛𝑒 𝑔𝑟𝑜𝑢𝑝
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑟 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑒𝑣𝑒𝑛𝑡𝑠,𝑖𝑡𝑒𝑚𝑠,𝑝𝑒𝑟𝑠𝑜𝑛𝑠,𝑒𝑡𝑐.𝑖𝑛 𝑎𝑛𝑜𝑡ℎ𝑒𝑟 𝑔𝑟𝑜𝑢𝑝

6. Ratio cont.
• After the numerator is divided by the denominator, the result
is often expressed as the result “to one” or written as the
result “:1.”
• In certain ratios, the numerator and denominator are different
categories of the same variable, such as males and females, or
persons 20–29 years and 30–39 years of age.
• In other ratios, the numerator and denominator are completely
different
• variables, such as the number of hospitals in a city and the
size of the population living in that city.

7. Ratios cont.
• Properties and uses of ratios
used as both descriptive measures and as analytic tools
the numerators and denominators of a ratio can be related or
unrelated
Usually, the values of both the numerator and denominator of
a ratio are divided by the value of one or the other so that
either the numerator or the denominator equals 1.0.

8. Types of common frequency measures cont
2. A proportion
This is the comparison of a part to the whole.
It is a type of ratio in which the numerator is included in
the denominator.
For example it can be used to describe what fraction of
clinic patients tested positive for HIV, or what percentage
of the population is younger than 25 years of age.
It proportion may be expressed as a decimal, a fraction, or
a percentage.

9. Proportion cont.
• Method for calculating a proportion
P =
𝑁𝑢𝑚𝑏𝑒𝑟
𝑜𝑓 𝑝𝑒𝑟𝑠𝑜𝑛𝑠 𝑜𝑟 𝑒𝑣𝑒𝑛𝑡𝑠 𝑤𝑖𝑡ℎ 𝑎 𝑝𝑎𝑟𝑡𝑖𝑐𝑢𝑙𝑎𝑟 𝑐ℎ𝑎𝑟𝑎𝑐𝑡𝑒𝑟𝑖𝑠𝑡𝑖𝑐
𝑇𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟
𝑜𝑓 𝑝𝑒𝑟𝑠𝑜𝑛𝑠 𝑜𝑟 𝑒𝑣𝑒𝑛𝑡𝑠,𝑜𝑓 𝑤ℎ𝑖𝑐ℎ 𝑡ℎ𝑒 𝑛𝑢𝑚𝑒𝑟𝑎𝑡𝑜𝑟 𝑖𝑠 𝑎 𝑠𝑢𝑏𝑠𝑒𝑡
× 10𝑛
For a proportion, 10𝑛
is usually 100 (or n=2) and is often
expressed as a percentage.

10. Proportion cont.
• Properties and uses of proportions
Proportions are common descriptive measures used in all
fields.
They are also used to describe the amount of disease that
can be attributed to a particular exposure.
The numerator is always a subset of the denominator.
can be expressed as a fraction, a decimal, or a percentage.
can easily be converted to ratios

11. Types of common frequency measures cont.
3. rate
In epidemiology, a rate is a measure of the frequency with
which an event occurs in a defined population over a specified
period of time.
Because rates put disease frequency in the perspective of the
size of the population, they are particularly useful for
comparing disease frequency in
different locations,
at different times,
among different groups of persons with potentially different
sized populations; that is, a rate is a measure of risk.

12. RATE CON’T
• A rate measures occurrence of a particular event
(disease/death) in a particular population during a given time
period.
• A rate comprises:
Numerator (number of cases).
Denominator (population at risk or mid year
population).
Time specification (calendar year).
Multiplier (1000) – included to avoid fractions.

13. • general formula for death rate
•Death rate=
𝒐𝒇 𝒅𝒆𝒂𝒕𝒉𝒔 𝒅𝒖𝒓𝒊𝒏𝒈 𝒂 𝒔𝒑𝒆𝒄𝒊𝒇𝒊𝒆𝒅 𝒑𝒆𝒓𝒊𝒐𝒅
𝒆𝒈.𝟐𝟎𝟐𝟎
𝒎𝒊𝒅 𝒚𝒆𝒂𝒓 𝒑𝒐𝒑𝒖𝒍𝒂𝒕𝒊𝒐𝒏
X 100

14. Examples
• During the first 9 months of national surveillance for Malaria,
Mansa DHO received 1068 case reports which specified the
sex; 893 cases were females and 175 were males. During the
same period 50 deaths were reported.
a. Calculate the ratio of female to male.
b. Calculate proportion of malaria cases that are male
a. Calculate the death rate in this period if the population of
Mansa is 57500

15. 15
Types of rates
1. Crude rates: Apply to the total population in a given area
2. Specific rates: Apply to specific subgroups in the
population (age, sex etc) or specific diseases
3. Standardized rates: used to permit comparisons of rates in
population which differ in structure (e.g age structure)
Two methods of standardization: Direct, indirect

16. Measuring health and disease
• Definitions of health states used by epidemiologists tend to
be simple, for example, “disease present” or “disease absent”
• Case definitions: Whatever the definitions used in
epidemiology, it is essential that they be clearly stated, and
easy to use and measure in a standard manner in a wide
variety of circumstances by different people.
• Several measures of disease frequency are based on the
concepts of prevalence and incidence.

17. Population at risk
• An important factor in calculating measures of disease
frequency is the correct estimate of the numbers of people
under study.
• Ideally these numbers should only include people who are
potentially susceptible to the diseases being studied.
• The people who are susceptible to a given disease are called
the population at risk, and can be defined by demographic,
geographic or environmental factors.

19. FERTILITY RATES
this is defined as the total number of live births per 1000 women aged 15-44 years

20. Types of Fertility rates
1. Crude Birth rate
the total number of live births per 1,000 population divided by
the length of the period in years.
2. General fertility rate
the number of live births in a geographic area in a year per 1,000
women of child bearing age which is usually defined as 15-44
3. Age specific rate
the number of births to women of a specified age or age group
per 1,000 women in that age group
(e.g 15-19, 20-24, 25-29, 30-34, 35-39, 40-45)

22. MEASURES OF MOBIDITY

23. Measures of morbidity cont
• Morbidity is defined as any departure, subjective or objective,
from a state of physiological or psychological well-being.
• In practice, morbidity encompasses disease, injury, and
disability.
• In addition, it can also be used to describe the periods of illness
that these persons experienced, or the duration of these
illnesses.
• Measures of morbidity frequency characterize the number of
persons in a population who become ill (incidence) or are ill at
a given time (prevalence)

24. Measures of morbidity cont.
1. Prevalence
• The frequency of existing cases in a defined population at a
given point in time
• Prevalence is often expressed as cases per 100 (percentage),
or per 1000 population.

26. Measures of morbidity cont
2. Incidence
• The rate of occurrence of new cases arising in a given period
in a specified population
• Incidence takes into account the variable time periods during
which individuals are disease-free and thus “at risk” of
developing the disease.

27. •Incidence rates are the most common way of
measuring and comparing the frequency of disease in
populations.
•Appropriate denominator can be the following:
Average size of the population over the time period
Size of the population (either total or at risk) at the
middle of the time period
Size of the population at the start of the time period.

28. The deference between Incidence vs Prevalence

29. Mortality Frequency
Measures

30. Mortality Rates
• A mortality rate is a measure of the frequency of occurrence
of death in a defined population during a specified interval.
• The formula for the mortality of a defined population, over
specified period of time, is:

31. Types of Death Rates
1. Crude Death Rate: The crude mortality rate is the mortality rate
from all causes of death for a population.
2. Cause-specific Death Rate: is the mortality rate from a
specified cause for a population.
The numerator is the number of deaths attributed to a specific
cause.
The denominator remains the size of the population at the
midpoint of the time period.

32. Types of death rates cont
3. Proportional Mortality: is a mortality rate limited to a
particular age group.
The numerator is the number of deaths in that age group
the denominator is the number of persons in that age
group in the population. E.g. neonatal, postneonatal, and
infant mortality rates.

34. Exercise
part A: For each of the fractions shown below, indicate whether it is a
ratio, a proportion, a rate, or none of the three.
1.
number of women in State A who died from heart disease in 2004
number of women in State A who died in 2004
2.
number of women in State A who died from heart disease in 2004
estimated number of women living in State A on July 1, 2004
3.
number of women in State A who died from heart disease in 2004
number of women in State A who died from cancer in 2004
4.
number of women in State A who died from lung cancer in 2004
number of women in State A who died from cancer (all types) in 2004
5.
number of women in State A who died from lung cancer in 2004
estimated revenue (in dollars) in State A from cigarette sales in 200

35. PART B: In 2001, a total of 15,555 homicide deaths occurred
among males and 4,753 homicide deaths occurred among females.
The estimated 2001 midyear populations for males and females
were 139,813,000 and 144,984,000, respectively. (use per 100,000)
a. Calculate the homicide-related death rates for males and for
females.
b. What type(s) of mortality rates did you calculate in
Question 1?
c. Calculate the ratio of homicide-mortality rates for males
compared to females.
d. Interpret the rate you calculated in Question 3 as if you
were presenting information to a policymaker.