measures of disease occurrence handouts 1
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Dr. H. Mohammed
Measures of Disease Occurrence
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Objectives To describe measures of disease
occurrence and frequency of healthevents
To calculate measures of disease
frequency
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Types of Measures Epidemiology is concerned with the
presence of health problems or theoccurrence of new health events in apopulation
The most common types of measuresare:
Counts
Ratios
Proportions
Rates
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Types of Measures
Count
=Number of animals who have aparticular disease
5,000 cows in St. Kitts have mastitis
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Types of Measures
Ratio
=A fraction in which the numerator isnot part of the denominator
e.g. Fetal death ratio:Fetal deaths/live births
By definition, fetal deaths are not includedamong live births
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Types of Measures
Proportion
=A fraction in which the numerator is part ofthe denominator
e.g. Fetal death rate:
Fetal deaths/all births
All births include both live births and fetaldeaths
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Proportions
Synonyms include:
Risk Percentage (if expressed as a value per
100)
Hint: Most fractions in epidemiology are
proportions
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Types of MeasuresRate
An expression of the frequency with which
an event occurs in a defined population There is a relationship between the numerator
and the denominator and a specified timeperiod
e.g. 0.36 per 1,000 pigs reported with swine flu in 1997-98
A proportion in which change over time isconsidered
However, is often used interchangeably withproportion (without reference to time)
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Measures of DiseaseFrequency
When measuring it is important inepidemiology to define:
What is being measured? Person or animals included
Place or location of the studypopulation
Time period of the study
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Measures of DiseaseFrequency
When measuring it is important inepidemiology to define:
What Vaccination status of equines Animals Equines
Place Nevis Time JulNov 2009
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Measures of DiseaseFrequencyThis helps to:
Define the population at risk
Clarify relationships between the numerator and
denominator for the calculation of rates andratios
Identify potential associations between outcomeand the factors being studied Do stroke rates differ by geographical location?
Does the prevalence rate of AIDS differ by geographical
location?
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Measures of DiseaseFrequencyKey Measures
Prevalence
Point Prevalence
Period Prevalence
Incidence
Cumulative Incidence
Incidence Rate or Incidence Density
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PrevalenceI. Point Prevalence
Number of cases of disease present inthe population at a particular time
Total population at that specified time
Is expressed as:
Percentage
No. diseased/1,000 population
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PrevalenceI. Point Prevalence
Is a snap shot of the disease frequencyat a point in time
Is the most common measure ofprevalence
Is often determined by cross-sectionalsurveys
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PrevalenceII. Period Prevalence
Number of cases of disease present inthe population over a period of time
Total population over that period of
time
Is expressed as:
Percentage
No. diseased/1,000 population
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PrevalenceExample
Blood samples are taken from a herd of173 dairy cows to assess the frequencyofNeospora caninum infection
15 animals test positive
Prevalence =15/173 = 0.09 (9%)
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Measures of DiseaseFrequencyKey Measures
Prevalence
Point Prevalence
Period Prevalence
Incidence
Cumulative Incidence
Incidence Rate or Incidence Density
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Incidence
Is the number of new cases/eventsin a population over a specificperiod of time
Example:The incidence of AIDS in the
Australian population was 178 in2001 i.e. 178 new cases of AIDS were
diagnosed in Australia in 2001
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Incidence
There are 2 main ways incidence is
reported:I. Cumulative Incidence
II. Incidence rate or incidence density
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Incidence
I. Cumulative Incidence (CI)
The proportion of individuals in a
population who became diseased duringthe specified period of time
CI = No. new cases of disease orevents during time period
Total population at risk atthe beginning of the time
period
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Cumulative Incidence
Assumptions:
Entire population at risk has beenfollowed from the beginning of thestudy till the end
All participants are at risk of the
outcome of interest
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Cumulative Incidence
Example 1:
The daily incidence of chickenpox infirst grade children at Cayon primaryschool during the 1998 epidemic was:
10 new cases per 100 children
If there were 200 children in the firstgrade, how many new cases wouldthere be each day?
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Cumulative Incidence
Example 2:
818 women had in vitro fertilization in Puerto
Rico in the year 2000
80 developed a clinical pregnancy within one
month of follow-up after the first embryotransfer procedure. The CI of pregnancy was?
CI= 80/818 9.8 cases per hundred women on the program
A k R
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Attack Rate
Is a type of cumulative incidence
Is applied to a narrowly defined populationobserved for a limited period of time, such asduring an epidemic
Attack rate =No. new cases of illness during a
specified time period
Total population at risk during thatspecified period
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Attack Rate - Example
The Ministry of Health was called in to investigate morethan 20 reports of people being ill with gastroenteritis aftereating at a large restaurant in Frigate Bay during the firstweek of April 2009
An investigation was conducted by interviewing all patronswho ate at the restaurant during that week
They found 2000 patrons ate at the restaurant that weekand 400 became sick.
What was the attack rate?
Attack rate = 400/2000
= 20 ill per 100 patrons
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Incidence
II. Incidence Rate (IR)
The incidence rate or incidence density is the
number of new cases in a population dividedby the total time units each individual in thepopulation at risk was observed
IR = No. new cases of disease or eventsduring time period
Sum of the length of time duringwhich each individual in the
population is at risk
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Incidence Rate - Example
In investigating the incidence ofduodenal ulcer following the use of
a specific drug in 14 subjects:
4 subjects started the study in Jan 1990,
all of whom finished the study in Dec 1999 10 subjects joined the study in Dec 1995
and finished the study in Nov 1996
After the period of observation: 5 peopledeveloped duodenal ulcer while taking thedrug
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Incidence Rate - Example
What is total length of time thatpersons in the population were at
risk (denominator)?
4 X 10 years = 40 years
10 x 1 year = 10 years Total = 50 person-years New cases = 5 What is the incidence rate of duodenal ulcers
after taking the drug? Incidence rate = 5 cases / 50 person-years
= 10 cases per 100 person-years
JanJuly 1996JanJulyJan 1995
Follow-up Period
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Developmentof diseaseX
Follow-up
period
6.0
Total person years at risk
Zero Time
0.5XE
2.0D
1.0XC
1.5B
1.0A
Subjects
Duration of individual follow-up
Jan
1997
July 1996Jan
1996
July
1995
Jan 1995
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Incidence RateCan be presented in many different
ways:
10 cases/1000 person-years
1 case/100 person-years
0.1 cases/10 person-years 0.01 cases/1 person-year
These are all equivalent!
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Incidence
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Mortality Rate
The incidence of death
Example: All-cause mortality ratefor the Australian population 2001=
No. deaths during 2001
Total population in Australia atmidyear 2001
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Mortality Rate
Example:
If there were 50,000 deaths in one year and
20,000,000 people living in Australia, whatwould be the mortality rate for Australia in thatyear?
Mortality rate = 50,000 deaths/20 million
= 2.5 deaths per 1000 pop
= 250 deaths per 100,000 pop
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Cause-Specific Mortality Rate
Mortality rate due to a specific disease orevent
Example:
Annual Mortality Rate for Lung Cancer per1,000 population =
No. new deaths from lung cancer per year
No. of persons in the population at mid yearx 1000
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Mortality Rate - Example We can place restrictions on more than one
characteristic simultaneously e.g. age and cause ofdeath:
No. new deaths from leukemia per year in
children under 10 years
No. of children under 10 years in the population at
mid-yearX 1,000
Note: Time must be specified in a mortality rate
Mortality can be calculated over 1 year, 5 years, orlonger
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Case Fatality RateNo ofindividuals dying during a
specified period of time after diseaseonset or diagnosis
Total number of individuals with thedisease during that specified period
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Example Assume a population of 100,000 people of whom 20
are sick with disease X in 1999. In that same year,
18 die from the disease.
What is the mortality rate?
What is the case-fatality rate?
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A quick recap
An incident case of disease
A new case
A prevalent caseAn existing case
Relationship between
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An incident case becomes a prevalentcase and remains a prevalent case until
recovery or death
Where a population is in a steadystate, prevalence depends on incidenceand duration of disease
Thus prevalence of a disease mayincrease when incidence remains stablebut survival of cases improves
Relationship betweenIncidence and Prevalence
Relationship between
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Relationship between
Incidence and Prevalence In a steady-state population, prevalence
depends on:
Incidence of illness
Duration of illness
The relationship can be can be expressed asfollows:
Point prevalence =Incidence density x Duration of
condition
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Factors influencing prevalence
rate
Prevalence
Increased by:-longer duration
of disease
-prolongation of
life without cure
-
increase in newcases
-in-migration of
cases
-out migration of
healthy people
-improved
diagnosis
Decreased by:-short duration of
disease
-high case-
fatality rate from
disease-decrease in new
cases
-in-migration of
healthy people
-out-migration of
cases
-improved cure
rate
Incidence and Prevalence
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Incidence is generally used for acutelyacquired diseases, prevalence is used for morepermanent states, conditions or attributes of
ill-health
Incidence is more important when thinking ofthe etiology of the disorder, prevalence whenthinking of societal burden of the disorderincluding the costs and resources consumed as
a result of the disorder
Incidence always requires a duration,
prevalence may or may not
Incidence and Prevalence
Incidence and Prevalence
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Incidence generally requires an initial disease-free interval before counting starts, becauseincidence is measured only in those at-risk of
disease
If one wishes to look at a change in disease(e.g. studies of causality, acute conditions orevents, outbreak investigation) useincidence. For example:
Looking at the change in the incidence of cancer isimportant to know whether current prevention,screening and treatment activities are working
Incidence and Prevalence
Diabetes Prevalence, US (CDC)
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Diabetes Prevalence, US (CDC)
Trends in Diabetes Prevalence, US (CDC)
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, ( )
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Recap Prevalence
Incidence
Mortality rate
Case-fatality rate
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References Epidemiology (2nd Ed), L. Gordis
Essentials of Infectious Disease
Epidemiology, M. Magnus
Dr. N. Paneth