Epidemiology is the study of the distribution and determinants of health-related states or events in populations and the application of this study to control health problems. Basic measurements used in epidemiology include rates, ratios, and proportions to describe disease occurrence and burden. Rates measure events over time and include the crude death rate and incidence rate. Proportions compare a part to the whole without time. Ratios compare two rates or quantities. These measurements are essential tools for epidemiologists to investigate disease causation, describe population health, and evaluate interventions.

1. Basic measurements in
epidemiology
Prepared by
Krupa Mathew. M,
Assistant professor

2. 2
Define epidemiology
The study of the distribution and determinants of health-related states or events in
specified population and the application of the study to control of health problems
(J.M. Last 1988)
Uses of epidemiology
 Investigation of causation of disease.
 Study of the natural history and prognosis of diseases.
 Description of the health status of the populations.
 It includes proportion with ill Health, change over time, change with age etc
 Evaluation of the interventions.
 Planning health services, Public policy and programs.

3. Measurements used in epidemiology
• Measurement of mortality
• Measurement of morbidity
• Measurement of disability
• Measurement of natality
• Measurement of disease attributes
• Measurement of health care services
• Measurement of the risk factors
• Measurement of demographic variables

4. Tools of
measurement
4
The epidemiologist usually expresses disease magnitude as a rate, ratio, proportion.
The basic tools of measurement in epidemiology are
• Proportion
• Rate
• Ratio

5. Proportion
5
A part/share or number considered in comparative relation to a whole. Usually expressed as a percentage %.
This is also relation /magnitude between two quantities, and numerator is always part of denominator and
expressed as percentage
• Usually expressed as a percentage %
– Numerator (which is part of denominator)
– Denominator
– Multiplier
– No time factor

6. Proportion – example
6
-Proportion of female students.
-Proportion of anaemic mothers (60% mothers are anaemic)
What proportion of the population is suffering from diabetes?

7. Rate
7
It Measures the occurrence of an event or disease in a given population during a given period (one Year).
(Birth rate, growth rate, accident rate). Usually expressed per 100 or per 1000 population. It has a time
dimension, whereas a PROPORTION does not.
Contains
• Numerator (which is part of denominator)
• Denominator
• Multiplier
• Time period
• Usually expressed per 100 / per 1000 population

8. Rate – example
8
Death rate =
𝑁𝑜.𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 𝑖𝑛 𝑜𝑛𝑒 𝑦𝑒𝑎𝑟
𝑇𝑜𝑡𝑎𝑙 𝑝op𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑡ℎ𝑎𝑡 𝑦𝑒𝑎𝑟
X 1000

9. Ratio
9
The value obtained by dividing one quantity by another - X / Y.
A ratio often compares two rates. Ratio also expresses relation of size between the two quantities.
Numerator is not part of Denominator.
Expressed as X / Y.
Examples:
 Male to female ratio.
 Doctor : Population ratio.
 Male : Female ratio.
 WBC : RBC ratio

10. MEASUREMENT
OF
MORTALITY
10

11. Measurement of mortality
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Disease or injury that initiated the train of morbid
events leading directly to death.
• Crude death rate
• Specific death rate
• Proportional mortality rate
• Case fatality rate
• Survival rate
• Adjusted/standardized rates

12. Crude Death Rate
12
𝐶𝐷𝑅 = 𝑁𝑜.𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 𝑑𝑢𝑟𝑖𝑛𝑔 𝑜𝑛𝑒 𝑦𝑒𝑎𝑟 X 1000
𝑀𝑖𝑑 𝑦𝑒𝑎𝑟 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛
It is defined as “the number of deaths from all causes per 1000 estimated
mid-year population in one year, in a given place”.
The simplest measure of mortality is the crude death rate

13. Specific death rate
13
i. Cause specific
e.g. Deaths due to cholera
ii. Age specific
e.g. Infant deaths
𝑁𝑜.𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 due to cholera 𝑑𝑢𝑟𝑖𝑛𝑔 𝑜𝑛𝑒 𝑦𝑒𝑎𝑟 X 1000
𝑀𝑖𝑑 𝑦𝑒𝑎𝑟 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛
𝑁𝑜.𝑜𝑓 infants 𝑑𝑒𝑎𝑡ℎ𝑠 𝑑𝑢𝑟𝑖𝑛𝑔 𝑜𝑛𝑒 𝑦𝑒𝑎𝑟 X 1000
𝑀𝑖𝑑 𝑦𝑒𝑎𝑟 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛
When analysis planned to throw light on aetiology, it is essential to use specific death rates.
The specific death rates may be
(i) cause or disease specific eg. TB, cancer
(ii) related to specific groups eg. Age-specific, sex specific etc.,

14. • Sex specific
e.g. Maternal deaths
• Time specific
– Weekly deaths
𝑁𝑜.𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 among males 𝑑𝑢𝑟𝑖𝑛𝑔 𝑜𝑛𝑒 𝑦𝑒𝑎𝑟
X 1000
𝑀𝑖𝑑 𝑦𝑒𝑎𝑟 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛
𝑁𝑜.𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 in week X 52
X 1000
𝑀𝑖𝑑 𝑦𝑒𝑎𝑟 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛

15. PROPORTIONAL MORTALITY
RATE/RATIO
• It helps to know what proportion of total deaths are due to a
particular cause(eg. cancer) or what proportion of deaths are
occurring in a particular age group (eg. above the age of 50yrs)
• It expresses the number of deaths due to a particular cause (or in
a specific age group) per 100(or 1000) total deaths.

16. Proportional mortality rate
a. Proportional mortality rate from a specific disease
𝑁𝑜.𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 𝑑𝑢𝑒 𝑡𝑜 𝑎 𝑝𝑎𝑟𝑡𝑖𝑐𝑢𝑙𝑎𝑟 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛 in a year
b. Under five proportionate mortality rate
𝑇𝑜𝑡𝑎𝑙 𝑛𝑜.𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 from all causes in that year
X 100
𝑁𝑜.𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 due to cholera 𝑑𝑢𝑟𝑖𝑛𝑔 𝑜𝑛𝑒 𝑦𝑒𝑎𝑟 X 1000
Total no. of deaths during the same period

17. Case fatality rate
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CFR = 𝑁𝑜.𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 𝑑𝑢𝑒 𝑡𝑜 a particular disease
X100
𝑇𝑜𝑡𝑎𝑙 𝑛𝑜.𝑜𝑓 𝑐𝑎𝑠𝑒𝑠 due to same disease

18. Survival rate
18
It is the proportion of survivors in a group, studied and followed over a period.
It is a method of describing prognosis in certain disease conditions.
• Used in research studies
• Like cancer therapy or survival
=
𝑇𝑜𝑡𝑎𝑙 𝑛𝑜.𝑜𝑓 𝑝𝑎𝑡𝑖𝑒𝑛𝑡𝑠
𝑃𝑎𝑡𝑖𝑒𝑛𝑡𝑠 𝑎𝑙𝑖𝑣𝑒 𝑎𝑡 𝑡ℎ𝑒 𝑒𝑛𝑑 𝑜𝑓5 𝑦𝑒𝑎𝑟 X 100

19. ADJUSTED OR STANDARDIZED RATES
• To compare the death rates of two populations with different age –
composition, the crude death rate is not the right yardstick.
• Hence we are using the adjusted or standardized rates
• The rate is either “age adjustment” or “age standardization” which
removes the confounding effect of different age structures and yields a
single standardized or adjusted rate, by which the mortality experience
can be compared directly.
• Standardization is carried out by one of the two methods – direct or
indirect standardization. Both the methods begin by choosing a
standard population not the age-structures of the populations

20. Standardized mortality ratio
20
SMR =
Observed deaths
Expected deaths
X 100

21. Direct standardization
21

22. Indirect standardization
22

23. 37
Total population
Total deaths – all causes
Cholera deaths
CDR
Specific DR
PMR

24. MEASUREMENT
OF
MORBIDITY
24

25. Measurements of morbidity
Morbidity is 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, disability etc.
WHO Expert committee on Health statistics that morbidity could be measured in
terms of 3 aspects
(i) persons who were ill
(ii) the illness that these persons experienced
(iii)the duration of these illnesses

26. 26
• Incidence
– Occurrence of new cases
• Prevalence
– Existence of new and old cases
• Incidence – how many people with the disease are newly
diagnosed each year (like video)
• Prevalence - how many people in a population currently have
the disease (like snapshot)

27. Incidence
27
No. of 𝐍𝐄𝐖 cases of a disease
Total population at risk during the same
time period
in a particular time period
X 1000
The incidence rate is defined as ‘the number of NEW cases occurring in a defined
population during a specified period of time’.

28. Special incidence
rates
28
Attack rate:
The rate at which acute disease is spreading. It is used during
epidemics & expressed in %.
Secondary attack rate:
% of exposed persons developing disease after primary case
exposure.

29. Uses of incidence
rate
29
• For taking action to control disease
• More suited for acute or infectious conditions
• For research

30. Example of use of
incidence
30

31. Points to remember about
incidence
31
• Refers only to new cases
• Not influenced by duration of disease
• Refers to a particular time period
• Denominator is people at risk

32. Prevalence
• The term disease prevalence refers to specifically to all current
cases(old and new) existing at a given point in time, over a period of
time in a given population.
• The total number of all individuals who have an attribute or disease at
a particular time divided by the population at risk of having the
attribute or disease at this point in time or midway through the period

33. Prevalence
33
=
No.of 𝐎𝐋𝐃 𝐚𝐧𝐝 𝐍𝐄𝐖 cases of a disease
Total population at risk during
the same time period
in a particular time point/period
X 1000

34. Types of prevalence
34
Point prevalence
Number all current cases at any given point of time. 4% TB cases on 1st April. It is defined as number of all
current cases (old and new) of a disease at one point of time, in relation to a defined population. The point in a
point prevalence, may for all practical purposes consists of a day, several days or even a few weeks depending
upon the time it takes to examine the population sample.
No of all cases (old & new) of a specified
disease existing at a given point in time X 100
Estimated population at the same point in time
Period prevalence
Number all current cases at a given period of time. A less commonly used measure of prevalence is period
prevalence. It measures the frequency of all current cases (old and new) existing during a defined period of time.
e.g (Annual Prevalence) expressed in relation to defined population. It includes cases arises before but extending
into or through to the year as well as those cases arising during the year.
No of all cases (old & new) of a specified
disease during a given period of time interval X 100
Estimated mid interval population at risk

35. Prevalence increases
35
• Longer duration of disease
• Prolongation of life with treatment
• Increase in incidence
• Immigration of new cases
• Better reporting of cases
• Emigration of healthy people

36. Prevalence decreases
36
• Shorter duration of diseases
• Improved cure rate
• Decrease in incidence
• Emigration of new cases
• Under reporting of cases
• Immigration of healthy people

37. Uses of prevalence
37
• Magnitude of disease problems
• Identify potential high-risk populations
• Administrative and planning purposes, e.g., hospital beds,
manpower needs, rehabilitation facilities

38. Points to remember about prevalence
38
• Refers to new and old cases
• Influenced by duration of disease
• Refers to a particular time period
• Denominator is people at risk

39. Relation between incidence & prevalence
39

40. Relation between incidence & prevalence
40
• Prevalence = Incidence X duration
• Incidence = 10 cases/1000 population/year
• Mean duration of disease = 5 years
• Prevalence = 10 x 5 = 50 per 1000 population

41. USES OF MORBIDITY RATES
 Give important clues for epidemiological research.
 Assessment of public health problems and programs.
 Designing intervention programs.
 Indicating priorities for health action and resource allocation.
 Explaining trends in overall mortality.