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Indicators of health Epidemiology
Mortality indicator Morbidity indicator
Measures
persons who are ill
(No.of people)
Illness
OrIllnesses
Duration of
illness or illnesses
Morbidity rate
Disease Frequency Disease Duration Disease Severity
By Mac Mohan:
(1960)
The study of the
distribution &
determinantsof
disease frequency
in man.
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Incidence rate Prevalence rate
No. of new cases occurring
in the defined population
during specified period of time.
It refers to No. of current cases
Current cases means blend of old
and new cases
At a given point of time
In a specified population
Distribution of
disease
Time
Place
Person
Determinants of
disease
Identifycause
Testing ofintervention
Cause &effect
DESCRIPTIVE EPIDEMIOLOGY ANALYTIC EPIDEMIOLOGY
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EPIDEMIOLOGICAL METHODS
Epidemiological methods
Observational studies
Descriptive
studies
Analytical studies
Experimental studies
RCT Field trial Community trial
Ecological study Cross-sectional study Case control Cohort study
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DEFINITION OF EPIDEMIOLOGY.
By Mac Mohan: (1960)The study of the distribution & determinantsof disease frequency in man.
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RISK.
Risk, or cumulative incidence, is a measure of theoccurrence of new cases of the disease of interest in the
population.
More precisely, risk is the proportion of unaffectedindividuals who, on average, will contract the diseaseof interest over a specified period of time.
Risk, which has no units, lies between 0 (when no newoccurrences arise) and 1 (when, at the other extreme,the entire population becomes affected during the risk
period).
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CALCULATION OF RISK
Number of new cases A
R =--------------------------------------------- = -------
size of the unaffected population N
R- Estimated risk
A- New casesN- size of the unaffected population under
observation
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EXAMPLE-1
A hypothetical study of six subjects illustrates the calculation of risk.
This study began in 1995 and concluded in 2004.
Individual subjects entered the study at various times, were all free of
the disease of interest at the time of enrollment, and were followed up
for at least 2 years.
Patient Awas enrolled in 1995, was diagnosed with the disease just
prior to 1997, and was followed up until death in 2002. Patient Bwas enrolled in 1997, was followed up until 1999 without
developing the disease, and then discontinued participation in the
study.
Patient Cwas enrolled in 1999, was diagnosed with the disease just
prior to 2002, and survived through the end of the observation period
in 2004.
Patients D, E, and F entered the study in 1997, 1998, and 2002
respectively. Each patient was followed through 2004 without
developing the disease.
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CONT..
Of the six subjects under observation (N = 6),
only two (A = 2) developed the disease.
The 9-year risk of disease, therefore,
Estimated R is 0.3 (33%)
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INCIDENCE.
The incidence rate (IR), like risk, reflectsoccurrence of new cases of the disease ofinterest.
Incidence rate measures the rapidity withwhich newly diagnosed cases of the disease of
interest develop over a period of time.
The unit of measuring incidence isCases/person-time
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CALCULATION OF INCIDENCE RATE
No. of new cases of disease of interest A
IR = ----------------------------------------------- = ---
person-time PT
IR - Incidence Rate
A - No. of new cases of disease of interest
PT - person-time
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FROM EXAMPLE-1
Patient A was enrolled in 1995, was diagnosed with the disease just
prior to 1997.(2YRS)
Patient B was enrolled in 1997, was followed up until 1999 without
developing the disease, and then discontinued participation in the
study. (2YRS)
Patient C was enrolled in 1999, was diagnosed with the disease just
prior to 2002, (3YRS)
Patients D, E, and F entered the study in 1997, 1998, and 2002respectively. Each patient was followed through 2004 without
developing the disease. (7YRS) (6YRS) (2YRS) respectively.
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CONT
With above simplification W.K.T
1. A=2
2. PT= 2+2+3+7+6+2=22
3. Therefore, the IR of disease is 0.09
Cases/person-time
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PREVALENCE.
Prevalence indicates the number of existing casesof the disease of interest in a population.
Specifically, the point prevalence (P) is theproportion of a population that has the disease ofinterest at a particular (given point) time.
Prevalence, like risk, ranges between 0 and 1 andhas no units.
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CALCULATION OF PREVALENCE
No. of existing affected individuals C
P = ----------------------------------------- = -------
No. of Persons in the population N
P- Prevalence
C- No. of existing affected individuals
N- No. of Persons in the population
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FROM EXAMPLE-1
Patient A was diagnosed with the disease -2002
Patient B was enrolled then discontinuedparticipation in the study -
1999
Patient C was enrolled was diagnosed with the disease -2004
Patients D, E, and F entered the study Each patient was followed up
to 2004 without developing the disease.
With above simplification to calculate prevalence( at given point of time
considering 2004) W.K.T
1. No. of existing affected individuals C=1
2. No. of Persons in the population N=4
3. Therefore, the prevalence of disease is 0.25 (25%)
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Characteristics of Risk, Prevalence, and
Incidence Rate.
Characteristic Risk Prevalence Incidence Rate
What is
measured ?
Probability of
disease
Percentage of
population with
disease
Rapidity of
disease
occurrence
Units None None Cases/person-
time
Time of diseasediagnosis Newly diagnosed Existing Newly diagnosed
Synonyms Cumulative
incidence
None Incidence
density
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ANNOTATION-1
Risbud A., Mehendale S., Basu S., Kulkarni S., & al, e. (2002). Prevalence
and incidence of hepatitis B virus infection in STD clinic attendees in Pune,
India. Sexually Transmitted Infections, 78(3), 169-73.
The purpose of the study was to estimate the prevalence
and incidence of hepatitis B virus (HBV) infection among patients
attending three STD clinics in Pune, India, and to identify associated risk
factors. The researcher screened 497 out of 2098 Patients at STD clinics
in Pune during 1996, who returned for at least one follow up visit. The
study showed that out Of 497 participants 3.6%, 26.5%, and 43.2% were
positive for HBsAg, anti-HBs, and anti- HBc respectively. This study
concluded that a high prevalence and incidence of HBV infection, seen in
STD clinic attendees, under scored the need to provide HBV vaccine to
commercial sex workers and their clients in India.
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1. Park. K. Textbook of preventive & social medicine21st edition, M/S Banarasidas Bhanot publishers-Jabalpur, India.p.61-64.
2. Stanhope.M, Lancaster J, public health nursing,8thedition, Elsevierpublication, charlottesvilla,virgina.p257-261.
3. Raymond.S.G Medical epidemiology, 4thedition langepublications. p 5-15
REFERENCES.
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