measures of disease occurrence handouts 1

7/28/2019 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


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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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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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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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



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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




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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


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

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