The document outlines the basic measurements and methodologies in epidemiology used to study health-related states and events in populations. It emphasizes the importance of measuring mortality, morbidity, and disability, as well as their respective indicators such as death rates, incidence, and prevalence rates. Additionally, it describes the epidemiological approach of asking questions and making comparisons to identify disease determinants and inform public health interventions.

1. Basic Measurements in
Epidemiology
Dr. Khem R. Sharma
Associate Prof., SPH&CM

2. Epidemiology
• The study of the occurrence, distribution and determinants of health related
states, events and processes in specified populations, and the application of
this knowledge to control relevant health problems.
• Epidemiologists are concerned with the study of
1) Measurement of frequency of disease & health-related events in the form
of rates and ratios (prevalence rate, incidence rate, health needs, demands,
activities, health care utilization)
2) Disease distribution patterns in the various subgroups of the population by
time, place and person and formulate aetiological hypothesis .
3) Determinants of Disease: to test aetiological hypotheses and identify the
underlying causes (or risk factors) of disease.

3. Epidemiological approach
• The epidemiological approach to problems of health and disease is
based on: i. Asking questions and ii. Making comparisons.
i. Asking questions: "a means of learning or asking questions .... and
getting answers that lead to further questions“ -
What is the event?, its magnitude?, Where, why & when did it
happen? Who are affected? How can it be prevented in the future?
ii. Making comparisons: Basic approach in epidemiology is to make
comparisons & draw inferences on different host/environmental
factors to find the crucial differences between those not/affected.

4. Epidemiological approach
• To identify the diseased from non-diseased, the epidemiologists need
precise definitions of diseases & health events that are:
a) acceptable and applicable for use in large populations;
b) precise and valid,
- Uses of Epidemiology:
1) Investigation of causation of disease.
2) Study of the natural history and prognosis of diseases.
3) Description of the health status of the populations.
4) Evaluation of the interventions.
5) Planning health services, Public policy and programs.

5. Basic Measurements in Epidemiology
I. Measurement of mortality
II. Measurement of morbidity
III. Measurement of disability
IV. Measurement of natality
V. Measurement of disease attributes
VI. Measurement of health care services
VII.Measurement of the risk factors
VIII.Measurement of demographic variables

6. Tools of measurement
• The epidemiologist usually expresses disease magnitude as Rates, Ratios or
Proportions.
1) RATE: Indicates the change in some event that takes place in a population over a
period of time.
• Death rate = Number of deaths in one year/ Mid-year population X 1000
• It comprises of the numerator, denominator, time specification (usually a calendar
year) and a multiplier.
• It can be: a) Crude/unstandardized rates: actual observed (birth rates)
b) Specific rates: actual observed due to specific causes (TB)/groups (<5 yrs).
c) Standardized rates: Obtained by direct/indirect method of standardization or
adjustment (age and sex standardized rates).

7. Tools of measurement…
2) RATIOS: Expresses a relation in size between two random quantities
where the numerator is not a component of the denominator.
Eg: Sex Ratio, Doctor- Population Ratio
3) PROPORTION: Ratio with a relation in magnitude to the whole,
where the numerator is always included in the denominator.
- Usually expressed as a percentage.
Proportion of children with diarrhea (%) =
Number of children with diarrhea at a certain time (usu. year) X 100
Total number of children in the geographical area at the same time.

8. CONCEPT OF NUMERATOR AND
DENOMINATOR
• Numerator: refers to the number of times an event (birth, death) has
occurred in a population, during a specified time-period.
• Denominator: May be related to the Numerator but it has to be
chosen carefully by the epidemiologist and are of different types:
(a) Related to the population: Mid-year population, Population at-risk,
Person-time, Person-distance.
(b) Related to the total events: Infant Mortality Rate (IMR)
-In the case of accidents, the number of accidents "per 1000
vehicles" will be a more useful denominator than the total
population, as many of them may not be using vehicles

9. I. MEASUREMENT OF MORTALITY
• Mortality data are relatively easy to obtain and provide the starting point for
many epidemiological studies.
Death Certificate
- The basis of mortality data
- Two parts: Part I deals with the
immediate cause of death and
the underlying cause.
- Part II deals with any significant
associated disease that contributed
but did not directly lead to death.

10. MORTALITY RATES AND RATIOS
1) Crude death rate
- It measures the rate at which deaths are occurring from various causes in
a given population, during a specified period.
- Crude Death Rate (CDR) = Number of deaths in a year X 1000
Mid-year population
- Although health should not be measured by the number of deaths that
occur in a community, CDR is the only available indicator of health in
many countries
- Usefulness of the crude death rate is restricted because it is influenced by
the age-sex composition of the population

11. MORTALITY RATES AND RATIOS…
2) Specific death rates: When analysis is planned to throw light on
aetiology/cause, it is essential to use specific death rates.
• It can be (a) cause or disease specific (TB), (b) related to specific
groups (age, sex) (c) other variables (income, religion, race, housing).
• Help us to identify groups "at-risk“ requiring preventive actions and
also permits comparisons between different causes within the same
population.
• These rates are the most useful single measures of mortality

12. MORTALITY RATES AND RATIOS…
3) Case fatality Rate (CFR)
- represents the killing power of a disease.
- CFR = Total number of deaths due to a particular disease X 100
Total number of cases due to the same disease
- Used in acute infectious diseases (food poisoning, cholera, measles).

13. MORTALITY RATES AND RATIOS…
4) Proportional mortality rate (Ratio)
- Useful to know what proportion of total deaths are due to a particular
cause (cancer) or what proportion of deaths are occurring in a
particular age group (above the age of 50 years)
- Proportional mortality from a specific disease =
Number of deaths from the specific disease in a year X 1000
Total deaths from all causes in that year
- Valuable indicator that highlights the importance of specific disease/
disease groups as a cause of death.

14. MORTALITY RATES AND RATIOS…
5) Survival rate: Proportion of survivors in a group (patients) studied
and followed over a period (5 year) and is a method of describing
prognosis in certain disease conditions like cancer.
6) Adjusted or standardized rates: to compare the death rates of two
populations with different age-composition (CDR is not useful)
- removes the confounding effect of different age structures and yields
a single standardized/adjusted rate, by which the mortality
experience can be compared directly.
a) Direct Standardization b) Indirect Standardization

15. a) Direct standardization
• A standard population is defined for which the numbers in each age
and sex group are known.
• Feasible only if specific rates in subgroups of the observed population
are available, along with the number of individuals in each subgroup.

16. Direct standardization..

17. b) Indirect standardization
i. Standardized mortality ratio (SMR)
- simplest and most useful form of indirect standardization
- compares the mortality in a study group (e.g. , an occupational group) with the
mortality that the occupational group would have had if they had experienced
national mortality rates .
- the more stable rates of the larger population are applied to the smaller study
group.
- SMR = Observed Deaths X 100
Expected Deaths
- Value > 100 indicates the occupation would appear to carry a greater mortality risk
than that of the whole population and values < 100, vice versa.

18. Indirect standardization…

19. Indirect standardization…
ii. Other standardization techniques
a) Life table is an age-adjusted summary of current all-causes
mortality
b) Regression techniques
c) Multivariate analysis

20. II. MEASUREMENT OF MORBIDITY
• Mortality indicators do not reveal the burden of ill-health in a community
(mental illness, rheumatoid arthritis).
• Morbidity indicators supplement mortality data and can be measured in
terms of 3 units:
1) Persons who were ill (Frequency) - Incidence/Prevalence Rates
2) The periods/spells of illness experienced (Severity) – CFR
3) The duration (days, weeks) of illness (Duration) - Disability Rate
(Average number of days of disability per person).
• Do not measure the subclinical or in-apparent diseased states (submerged
part of iceberg).

21. INCIDENCE
• Incidence measures the rate at which new cases are occurring in a
population and its use is generally restricted to acute conditions.
• Incidence = Number of new cases of specific disease during a given time period X 100
Population at-risk during that period
Special incidence rates
1) Attack rate: The rate at which acute disease is spreading and is used
during epidemics and expressed in percentages.
2) Secondary attack rate: Percentage of exposed persons developing the
disease within the range of the incubation period following exposure
to a primary case.

22. PREVALENCE
• All current cases (old and new) of a disease existing at a given point or
over a period of time in a given population
1) Point prevalence: the number of all current cases (old and new) of a
disease at one point of time, in relation to a defined population.
2) Period prevalence: the number of all current cases (old and new) of
disease existing during a defined period of time (annual), expressed
in relation to a defined population.

23. INCIDENCE Vs PREVALENCE
• Probability of developing a
disease
• Refers only to new cases but can
also be new spells/episodes of
disease (common cold)
• Not influenced by duration of
disease
• Preferred measure when
studying cause and effect usually
in acute conditions
• Probability of already having a
disease
• Refers to new and old cases
• Influenced by duration of
disease
• Preferred measure when
estimating the population
based burden of a chronic
disease/attribute (smoking)

24. INCIDENCE Vs PREVALENCE

25. Relationship between Incidence and Prevalence
• Given the assumption that the population is stable, and incidence and
duration (D) are unchanging, the relationship between incidence (I) and
prevalence (P) can be expressed as :
Prevalence = Incidence x Mean Duration of Disease
• Prevalence has been compared with a photograph - an instantaneous
record; and incidence with film, - a continuous record.
• Incidence rates should be used in formulation and
testing of aetiological hypotheses
• Prevalence rates may have to be used when they are
not available.

26. III. MEASUREMENT OF DISABILITY
• Death rates have not changed markedly in recent years, despite
massive health expenditures
• Disability rates related to illness and injury have come into use to
supplement mortality and morbidity indicators & are of two categories
a) Event-type indicators
• Number of days of restricted activity
• Bed disability days
• Work-loss days

27. III. MEASUREMENT OF DISABILITY…
b) Person-type indicators
• Limitation of mobility: (confined to bed, confined to the house,
special aid in getting around)
• Limitation of activity: (limitation to perform the basic activities of
daily living like eating, washing, dressing)
Sullivan's index = life expectancy of the nation - the probable duration
of bed disability and inability to perform major activities
• It is an expectation of life free from disability

28. III. MEASUREMENT OF DISABILITY…
HALE (Health Adjusted Life Expectancy)
• The equivalent number of years in full health that a newborn can be
expected to live based on the current rates of ill health and mortality
DALY (Disability Adjusted Life Years)
• Expresses the years of life lost to premature death and years lived with
disability adjusted for the severity of disability
• One DALY is "one lost year of healthy life“
• Simplest and the most commonly used measure to find the burden of
illness and the effectiveness of the interventions in a defined population

29. Best Wishes !!!