Basic epidemiologic concept

Dr. Mehrdad Askarian, MD,
MPH
Professor of Community Medicine
Shiraz Medical School

Basic Epidemiologic Concepts
Greek language
Epi: upon
Demos: population
Logy: study of
Epidemiology is defined as:

• Epidemiology is defined as:
– The study of something that affects a
population.
– Study of factors that determine the
occurrence and distribution of disease in
population.
– One of the ways in which diseases are
studied.

• So, there are four levels at which the scientific
study of disease can be approached:
– The submolecular or molecular level
– The tissue or organ level
– The level of individual patients
– The level of population

Epidemiologic Measurements
• Frequency
– The frequency of a disease, injury or death
can be measured in different ways, and it can
be related to different denominators,
depending on the purpose of the research
and the availability of data.
• The concepts of incidence and prevalence are of
fundamental importance to all epidemiology.

• Incidence (incident cases)
– Frequency (number) of new occurrences of
disease, injury, or death-that is the number of
transitions from well to ill, from uninjured to
injured, or from alive to dead-in the study
population during the time period being
examined.

• Prevalence (Prevalent cases)
– Sometimes called point prevalence which is
the number of persons in a defined population
who have a specified disease or condition at
a point in time, usually the time a survey is
done.

• Differences between point prevalence and
period prevalence
– Prevalence usually is meant point prevalence
– Period prevalence is the number of persons
who had the disease at any time during the
specified time interval.
• Period prevalence is the sum of the point
prevalence at the beginning of the interval plus the
incidence during the interval.

Epidemiologic Measurements (concepts)
Jan 1 Dec 31
1
2
3
4
5
6
7
8
A given disease in a defined population in
which there is no emigration or immigration,
Line starts with illness and finishes with
death or cure
t1 t2

• Incident cases during the year:
• The point prevalence at t1 :
• The period prevalence between t1 and t2 :

• Incident cases during the year :

• Incident cases during the year : 3, 4, 5, 7

Jan 1 Dec 31
1
2
3
4
5
6
7
8
Incident cases during the year : 3, 4, 5, 7
t1 t2

• The point prevalence at t1 : 1, 2, 6, 8

Jan 1 Dec 31
1
2
3
4
5
6
7
8
The point prevalence at t1 : 1, 2, 6, 8
t1 t2

• The point prevalence at t2 : 1, 3, 5

Jan 1 Dec 31
1
2
3
4
5
6
7
8
The point prevalence at t2 : 1, 3, 5
t1 t2

Epidemiologic Measurements (concepts(
• The period prevalence between t1 and t2 :

Jan 1 Dec 31
1
2
3
4
5
6
7
8
A given disease in a defined population in
which there is no emigration or immigration,
Line starts with illness and finishes with
death or cure
t1 t2

• The period prevalence between t1 and t2 : 1 to 8

Jan 1 Dec 31
1
2
3
4
5
6
7
8
The period prevalence between t1 and t2 : 1 to 8
t1 t2

• Relationship Between Incid. & Prev.
– Prevalence is the result of:

• Relationship Between Incidence &
Prevalence
– Prevalence is the result of:
• Periodic (annual( number of new cases
• Immigration & emigration of persons with a disease
• Duration of illness

P=I*D

Risk
• Epidemiologically risk is defined as:
– The proportion of persons who are unaffected
at the beginning of a study period but who
undergo the risk event during the study period
– The risk event may be death, disease, injury,
& the persons at risk for the event are called a
cohort.
– Cohort is clearly defined group of persons
studied over time.

Limitations of the Concept of Risk
• Who is truly at risk? (susceptible
population( who is Ab negative?
• Risk of death from an infectious disease
looks simple but is actually complex;
WHY?

Total populationTotal population
Number of susceptible
Number of exposed
Number of infected
Number of ill
Number of dead

(# of dead / Total population( =
{(# of dead / # of ill( *
(# of ill / # of infected( *
(# of infected / # of exposed( *
(# of exposed / # of susceptible( *
(# of susceptible / Total population(}
This is why the calculation is
difficult

Rates
• The frequency of events that occur in a
defined time period, divided by the average
population at risk
– The midperiod population is often used as the
denominator of a rate
Rate = (Numerator / Denominator( * Constant multiplier

• Crude death rate =
(Number of deaths “known place and time”/
Midperiod population ”known place and
time”( * 1000

Criteria of valid use of the term
“Rate”
• All the events counted in the numerator
must have happened to persons in the
denominator
• All of the persons counted in the
denominator must have been at risk for
the events in the numerator.

Before comparisons of rates can be
made, the following 3 items must
be true
• Definition or diagnosis of all numerators
should be the same
• The constant multipliers should be the
same
• The same time interval

Incidence rate
• Number of incident cases over a defined
study period, divided by the population at
risk at the midpoint of the study period.
• Per 1000, 10 000, or 100 000

Prevalence rate
• Actually a proportion and not a rate,
however the term is common in use and
is:
– The proportion of persons with a defined
disease or condition at the time of study

Incidence density
• Frequency (density( of new events per
person-time and is especially useful
when the event of interest can occur in a
person more than once during the
study period

Use of crude rates vs. specific
rates
• Categories of rates
– Crude rates
– Specific rates
– Standardized rates

rates
– Those that apply to an entire population,
without reference to any characteristics of the
individuals in it (crude rates(.

rates
– When a population is divided into more
homogenous subgroups based on a particular
characteristics of interest (age, sex, race, risk
factors or comorbidity). (specific rates)

rates
– crude rates that have been modified to
control for the effects of age or other
characteristics and thereby allow for valid
comparison of rates. (standardized rates).

rates
– crude rates that have been modified to control
for the effects of age or other characteristics
and thereby allow for valid comparison of
rates. (standardized rates).

Crude death rate and life
expectancy for three countries
Country Crude Death
Rate (per 1000)
Life
Expectancy at
birth (years)
A 7.4
B 8.8
C 10.8

Crude death rate and life
expectancy for three countries
Country Crude Death
Rate (per 1000)
Life
Expectancy at
birth (years)
A 7.4 63.4
B 8.8 71.3
C 10.8 74.2

Why do we use crude rates?
• Three major reasons:
– Numerator not known for subgroups
– Size of subgroups is not known
– Too small number of persons at risk to
provide stable estimates of the specific rates

Standardization
• Direct
• Indirect

Direct standardization
• Most commonly used method to remove
the biasing effect of the differing age
structure of different populations.

Calculation of the crude death rates
Age
Group
Population A
P. size ASDR Expected
number of
death
Young 1000 * 0.001 = 1
Middle 5000 * 0.010 = 50
Older 4000 * 0.100 = 400
Total 10000 451
Crude Death Rate= 451/10000 = 4.51%

Age
Group
Population B
number of
death
Young 4000 * 0.002 = 8
Middle 5000 * 0.020 = 100
Older 1000 * 0.200 = 200
Total 10000 308

Direct Standardization
Age
Group
Population A
number of
death
Young 5000 * 0.001 = 5
Middle 10000 * 0.010 = 100
Older 5000 * 0.100 = 500
Total 20000 605

Age
Group
Population B
number of
death
Young 5000 * 0.002 = 10
Middle 10000 * 0.020 = 200
Older 5000 * 0.200 = 1000
Total 20000 1210
Crude Death Rate= 1210/20000 = 6.05%

Indirect standardization
• Useful if
– ASDRs is not available
– Small population
uses standard rates and applies them to the
known age groups in the population to be
standardized

Age
Group
Males in the standard population
Prop of
stand
Pop.
ASDR Observed
death rate
Young 0.4 * 0.0001 = 0.00004
Middle 0.3 * 0.0010 = 0.00030
Older 0.3 * 0.0100 = 0.00300
Total 1 0.00334
observed Death Rate= 0.00334, or 334/10000

Age
Group
Males in the company
Number
of
workers
ASDR Observed
death rate
Young 2000 * ? = ?
Middle 3000 * ? = ?
Older 5000 * ? = ?
Total 10000 48
observed Death Rate=48/10000

Age
Group
Males in the standard population
Prop of
stand
Pop.
ASDR Observed
death rate
Young 0.4 * 0.0001 = 0.00004
Middle 0.3 * 0.0010 = 0.00030
Older 0.3 * 0.0100 = 0.00300
Total 0.1 0.00334
Expected Death Rate= 0.00334, or 334/10000

Age
Group
Males in the company
Number
of
workers
ASDR Observed
death rate
Young 2000 * 0.0001 = 0.2
Middle 3000 * 0.0010 = 3.0
Older 5000 * 0.0100 = 50.0
Total 10000 53.2
Expected Death Rate=53.2/10000

Calculation of standardized
mortality ratio (SMR)
• SMR=
(Observed death rate for males in company)/
(Observed death rate for males in community)
*100
=(0.00480/0.00532) *100 = (0.9) (100)= %90
Company protects the death

SMR
• If SMR:
• >1: Increased risk in this group
• <1: Decreased risk in this group

Epidemiologic Methods
• Primary concern is to study disease
occurrence in people, these studies can
be classified as:
– Observational
– Experimental

Observational studies
• Descriptive
• Analytical
– Ecological or correlational with population as
unit of study
– Cross-sectional or prevalence, with
individuals as unit of study
– Case-control or case-reference, with
individuals as unit of study
– Cohort or follow-up, individuals as unit of
study

Experimental, Interventional studies
• Randomized controlled trials or clinical
trials, with patients as unit of study
• Field trials community intervention studies,
with healthy people as unit of study
• Community trials, with communities as
unit of study

Descriptive epidemiology
• Descriptive studies are usually the first
phase of an epidemiologic investigation.
These are concerned with:
– Observing the distribution of disease
– Heath related characteristics
– Identifying the characteristics with which the
disease in question seems to be associated
(hypothesis).

Descriptive studies
• Basically ask the questions:
– When is the disease occurring?
• Time distribution
– Where is it occurring?
• Place distribution
– Who is getting the disease?
• Person distribution

Procedures in descriptive studies
• Defining the population to be studied
– Total numbers (the study groups)
– Composition
• Age
• Sex
• Occupation
• Cultural characters etc…
• Defining the disease under study
• Describing the disease by:
– Time
– Place
– person
• Measurement of disease
• Comparing with known indices
• Formulation of an etiological hypothesis

Defined population
• The whole population in the area
• A representative sample taken from it
or
• A selected group
– Age
– Sex
– Occupational
– Hospital patients
– School children
– A group of people can be fairly accurately counted.

Defined population
• The defined population should:
– Large enough so that age, sex … rates are
meaningful.
– Stable community (no migration into & out)
– Community participation
– This population should not be overtly different
from other communities in the region
– A close health facility to provide easy access
for patients requiring medical services

Defining the disease under study
• Clinicians:
– May not need a precise definition of disease for
immediate patient care
• Epidemiologist:
– Needs a definition that is both precise and valid to
enable him to identify those who have the disease
from those who do not.
• The diagnostic method for use in epidemiological studies
must be
– Acceptable to the population
– Applicable to their use in large population

Describing the disease
• Primary objective of descriptive
epidemiology is to describe the diseases
by:
– Occurrence
– Distribution
by
• Time
• Place
• person

Characteristics frequently
examined in descriptive studies
• Time
– Year
– Season
– Month
– Week
– Day
– Hour of onset
– Duration

• Place
– Climatic zones
– Country region
– Urban/rural
– Local community
– Towns
– Cities
– Institutions

• Person
– Age
– Sex
– Marital status
– Occupation
– Social status
– Education
– Birth order
– Family size
– Height
– Weight
– Blood pressure
– Blood cholesterol
– Personal habits

Time distribution
• Pattern may be described by the time of
occurrence
– Week
– Month
– Year
– The day of week
– Hour of onset

Time distribution
• Time distribution shows raises questions:
– Seasonal occurrence
– Periodic increase or decrease
– Following a consistent time trend
All of the above mentioned issues may yield
important clues about the source and
etiology of the disease, suggesting
potential preventive measures.

Kinds of epidemiologic time trends
• Short-term fluctuations
• Periodic fluctuations
• Long term or secular trends

Short-term fluctuations
• Epidemic

Periodic fluctuations
• Seasonal trend
– Communicable diseases
• Measles
• meningitis
• Cyclic trend
– Every 2-3 years, less or more
– Herd immunity

Long-term or secular trends
• Changes in the occurrence of disease
over a long period of time generally
several years or decades
• In other words there is a consistent
tendency to change in a particular
direction
– Different chronic diseases

Place distribution
• International variations
• National variations
• Rural – Urban differences
• Local distributions
• Migration studies

Place distribution
• International variations
– cancers
• National variations
– Endemic goiter
• Rural – Urban differences
– accidents
• Local distributions
– Clustering of cases
• Migration studies
– Cancer of stomach vs. colon

Person distribution
• Age
– Progressive increment of disease with age
– Bimodality
• The study material is not homogenous
• Two distinct sets of causal factors might be
operative
• Small number of observations

Person distribution
• Sex
– Basic biological differences
– Cultural and behavioral differences

Person distribution
• Ethnicity
• Marital status
– Married persons lower mortality rate
• Health status of the person
• More secure and protected
• Occupation
• Social class
• Behavior
• Stress
• Migration

Measurement of disease
• Cross sectional
• Longitudinal

Thank you
for your kind
attention!

Basic epidemiologic concept

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