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8/2/2019 [CRP II] (2012.03.15) Basic Epidemiologic Measure and Vital Statistics
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BASIC EPIDEMIOLOGIC
MEASUREBony Wiem Lestari
Departement of Epidemiology and
Biostatistics
2012
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Learning objectives:
To understand:
• Basic concept of measuring disease occurence
• Define and distinguish between cumulative
incidence, incidence rate, and prevalence• Morbidity statistics
• Mortality statistics
• Demographic studies
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What is Epidemiology?
The study of the
distribution and
determinants of diseasein populations.
(J. Last, Dictionary of Epidemiology, 1983)
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What Can Epidemiology Do?
• Determine the impact of disease in groups of people.
• Detect changes in disease occurrence in groupsof people. (measuring disease occurence)
• Measure relationships between exposure anddisease. (measuring association)
•
Evaluate the efficacy of health interventions andtreatments.
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Demographic Studies -
Purpose
• The means by which the epidemiologist
can assess the health status of a
population from the perspective of morbidity and mortality,
• Inexpensive, and the first test of an
etiologic hypothesis,
• Can be conducted by using readily
available vital and health statistics
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Measuring Disease Occurence
I. Morbidity
II. Mortality
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Measures of Disease Frequency
The most basic measure of disease
frequency is a simple count of affected individuals.
However, counting is not enough!
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Measures of Disease Frequency
Why is a simple count not enough?
• 3 cases of cancer per year from a city of 1,000 people is very different than 3 cases
per year from a city of 100,000 people
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Measures of Disease Frequency
So, in epidemiology we must know:
• the size of the population from which theaffected individuals come, and
• the time period the information was collected.
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Measures of Disease Frequency
Type of population
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Measures of Disease Frequency
1. Absolute value
2. Relative value : the value of each sub categorycompared to the whole population.
- Ratio -----> a + b -------> a/b- Proportion ---> a + b -------> a /(a+b)
- Rate -------> the same as proportion but thenumerator and denominator are of the samecertain time, e.g: incidence
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Measures of Disease Frequency
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Measures of Disease Frequency
Rate
• basic measure in epidemiology
• the frequency with which an event occursin a group of people
• used to compare the occurrence of disease in different groups
• tells us how fast the disease is occuring ina population
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Measures of Disease Frequency
Rate = Number of events in a specified time period
Average population during the time period
***the measure of time is a critical part of a rate!
Such as, the number of newly diagnosed cases of breast cancer per 100,000 women during 1999.
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Measures of Disease Frequency
Common Rates
• mortality (death) rate is the number of deaths ina defined group of people during a specified
time period.
• birth rate is the number of live births in a definedgroup of people over a specified time period.
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Measures of Disease Frequency
Incidence Rate
• a type of rate
• the number of new cases that
develop in a group of individuals
during a specific time period
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• Some medical statistic in Epidemiology :
• Morbidity statistic
• Mortality statistic• Measurement of Risk *)
• Screening *)
• Standardization *)
• Statistical activity in Epidemiology :
• to count the frequency
• to compare : Rate, Ratio, Proportion, Relative Risk,
cause and effect association.• to predict by analyzing and conclusion
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STATISTICAL MEASURE IN
EPIDEMIOLOGY1.
INCIDENCE RATE
2. INCIDENCE PROPORTION
3. ATTACK RATE
4. PREVALENCE PROPORTION
5. PROPORTIONAL DISTRIBUTION
6. MORTALITY RATE
7. RISK (Measure of association/ cause-effect)
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• The Incidence rate, attack rate, prevalencerate are used for measuring the occurrence
of the disease ( Morbidity Statistic ) or acertain condition.
• If the incidence rate of certain disease or conditions in a group of people is higher
than the other group for the same disease or condition, the first group is stated as “higher risk than the other group”. In wide populatione.g. the age group, there are group which are
the “ High Risk Group “. Because they aremost sensitive to get disease or other condition than the other group.
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INCIDENCE RATE
• The number of new events (cases) divided bythe amount of person time in the basepopulation. It can be measured in closed or open populations. It is often restricted to
include a maximum of one event per person.
• = number of new events during a period of time
number of persons at risk during this time period
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Incidence Rate
X
X
X
X
Time
Risk =# Events
# of people initially at risk
Incidence Rate =# Events
∑ individual time periods at risk
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Incidence Rate
• The incidence rate is an instaneous
measure. Although it is measured over a
period of time, the measurement time has
no connection with the time units used toexpress rate, and whatever its value, it
refers to only an instant in time.
• E.g: vehicle speed
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Incidence Proportion
• The average risk for a population
• The proportion of a population that
develops a disease during a specifiedtime.
• It is measured only in closed populations
• = cumulative incidence
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Incidence Proportion
and Incidence Rate
• For small incidence proportion,
Incidence Proportion ≈ I ∆T
•
For larger incidence proportion, if I isconstant over time,
Incidence Proportion = 1 – e -I ∆T
•
When I varies over time,Incidence Proportion = 1 – e ∑-Ii∆Ti
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ATTACK RATE• Similar as Incidence rate
• It is used in a short period of an event –an outbreak -, the population being
affected is limited closely, the durationof epidemic is short period.
• The formula is the same as the
incidence rate.
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Prevalence proportion
• The proportion of a population that has diseaseat given instant.
Period Prevalence
• The total number of persons known to have hadthe disease or attribute at any time duringspecified period .
• Annual Prevalence
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PROPORTIONALDISTRIBUTION
• The percentage of a disease or event in a definedpopulation to all diseases or events happened inthose population.
Example :• Percentage of death of heart disease compared to all
death happened in the population.
• Percentage of sickness cause by certain diseasecompare to all disease happened in the population.
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THE INCIDENCECHARACTERISTICS
1. Indicated for the new events or casesoccurred in the population
2. A fundamental tool in etiological study
of disease3. Act as direct indicator of the risk of
getting sick ( the probability of getting
sick )
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THE INCIDENCE
CHARACTERISTICS
4. The change in incidence rate meansthe change of etiological balance
5. The Incidence Rate obtainedaccurately by cohort study
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THE CHARACTERISTIC OF
PREVALENCE1. The subjects are all cases / events in
population
2. Depend on the incidence and duration of thedisease
3. The low Prevalence Rate because of lowincidence, short duration of disease or fastcured of the disease
4. Therapy to prevent the death but can notomit the disease might increasing theprevalence of the disease
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THE CHARACTERISTIC OFPREVALENCE
5. In chronic disease the prevalence = I x D
6. It is useful to describe the chronic disease
7. It is an important and useful measure of the
burden of disease in a community8. Determined by a single survey (cross
sectional study)
9. It is valuable for planning health services.
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RISK
• The probability that an individual will
develop disease in a specified time
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Distinguishing Characteristics of Incidenceand Prevalence
Measure Type of number
Units Range Type of cases Major uses
CumulativeIncidence
Proportion None 0 to 1 New Research oncauses,
incidence,prevention,
andtreatment of
disease
Incidencerate
True rate 1/time 0 to infinity New Research oncauses,
prevention,and
treatment of disease
Prevalence Proportion None 0 to 1 Existing Resourceplanning
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Morbidity Statistics
• Disease control programs
• Tax-financed public assistance programs
• School & employment records
• Insurance data
• Special research programs
• Morbidity surveys on population samples for
illness in general and for specific diseases
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Some sources of Morbidity
Statistics1. Disease reporting : communicables disease,
cancer registries
2. Data accumulated as a by-product of insurance
and prepaid medical care plans
a. Group health and accident insurance
b. Prepaid medical care plans
c. State disability insurance plans
d. Life insurance companies
e. Hospital insurance plans
f. Railroad Retirement Board
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Some sources of Morbidity
Statistics3. Tax-financed public assistance and medical
care plans
a. Public assistance, aid to the blind, aid to the
disabled
b. State or federal medical care plansc. Armed forces
d. Veterans Administration
4. Hospitals and clinics
5. Absenteeism records: industry and schools
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Some sources of Morbidity
Statistics6. Pre-employment and periodic physical
examinations in industry and schools
7. Case-finding programs
8. Records of military personnel9. Morbidity surveys on population samples (e.g.,
Susenas, Riskesdas)
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Morbidity Statistics:Surveillance systems
• Focused on identification of infected
individuals, with the goal of isolation to
minimize disease transmission
• Ongoing collection of data by a data center,analysis, dissemination and implementation
of a response based upon analyses
• Sentinel Surveillance
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Morbidity Statistics:
• Time - Incubation period; Time &
Space clusters
• Place -
• Person - age, gender, ethnicity and
social status can influence morbidity.
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VITAL EVENTS:
• Births
• Marriages• Divorces
• Deaths
Mortality Statistics:
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Mortality Studies
• Distribution of mortality in populations
• Time: Trend in mortality rates - secular
trends - trends over time
• Place: Migrant studies - helps to
establish an environmental
contribution to disease
• Person: Age, gender, race & ethnicity,social class, birth cohort
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• Autopsy records
• Financial records (insurance, pension)
• Hospital records
• Occupational records
• Death Certificates
Mortality Data Sources
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• Autopsy data and hospital records may bemore accurate about details, but may not
represent the general population
•
Useful for investigating diseases with a high-case fatality
• Problem of selection bias - impossible to
correlate an autopsy series with any well-
defined population at risk, or to estimate thefrequency of disease
Mortality statistics - Autopsies
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MORTALITY STATISTIC
The are 3 elements :
1. Group of population who are at risk of death(denominator)
2. Designed period of time
3. Number of death in group of population in
designed period of time (numerator)
MORTALITY RATE :The rapidity with which persons within a given
population die from a particular disease
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CRUDE DEATH RATE (CDR)• Number of population dying every year or
number of death in the community per 1000
population divided by population taken atmidyear.
A S ifi D h R
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Age Specific Death Rate
• Number of death of specific groupevery year per 1000 of specific groupdivided by number of specific group.
• E.g. : Neonatal Mortality Rate, PerinatalMortality Rate, Infant Mortality Rate,Maternal mortality Rate, etc.
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CAUSE SPECIFIC DEATH RATE
• Death rate for any specific disease,such as heart disease, may be statedfor the entire population or for any age,
race, or sex subgroup.
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CASE FATALITY RATE ( in % )
• The number of death due to the diseasein a specified period of time divided bythe number of cases of the disease in
the same period of time.• The k = 100
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Other Demographic Measures:
•
Survival AnalysisUsed to make demographic predictions and toanalyze data in clinical trials
• Years of Potential Life Lost (YPPL)
o Recognizes that death occurring in the same personat a younger age clearly involves a greater loss of future productive years than were it to occur at anolder age.
o Measures the impact of premature mortality on apopulation
YPLL be ore age 65 years
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YPLL be ore age 65 yearsamong
children younger than 20.
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