[CRP II] (2012.03.15) Basic Epidemiologic Measure and Vital Statistics

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



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



What is Epidemiology?

The study of the

distribution and

determinants of diseasein populations.

(J. Last, Dictionary of Epidemiology, 1983)



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.



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



Measuring Disease Occurence

I. Morbidity

II. Mortality



Measures of Disease Frequency

The most basic measure of disease

frequency is a simple count of affected individuals.

However, counting is not enough!




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




So, in epidemiology we must know:

• the size of the population from which theaffected individuals come, and

• the time period the information was collected.




Type of population




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







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




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.




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.




Incidence Rate

• a type of rate

• the number of new cases that

develop in a group of individuals

during a specific time period



• 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



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)



• 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.



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



Incidence Rate

X

X

X

X

Time

Risk =# Events

# of people initially at risk

Incidence Rate =# Events

∑ individual time periods at risk



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



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



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



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.



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



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.



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 )



THE INCIDENCE

CHARACTERISTICS

4. The change in incidence rate meansthe change of etiological balance

5. The Incidence Rate obtainedaccurately by cohort study



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



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.









RISK

• The probability that an individual will

develop disease in a specified time



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



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



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




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




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)



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



Morbidity Statistics:

• Time - Incubation period; Time &

Space clusters

• Place -

• Person - age, gender, ethnicity and

social status can influence morbidity.



VITAL EVENTS:

• Births

• Marriages• Divorces

• Deaths

Mortality Statistics:



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



• Autopsy records

• Financial records (insurance, pension)

• Hospital records

• Occupational records

• Death Certificates

Mortality Data Sources



• 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



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



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



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.



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.



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



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



YPLL be ore age 65 yearsamong

children younger than 20.



Documents

[CRP II] (2012.03.15) Basic Epidemiologic Measure and Vital Statistics