BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
LESSON 08
MEASURES OF DISEASE FREQUENCY AND MORBIDITY
LEARNING OUTCOMES:
•
Define important terms used in epidemiologic data measurements.
•
Compare different measures of morbidity, including incidence rates, cumulative incidence, attack rates,
prevalence rates, and person-time at risk.
•
Illustrate why incidence data are necessary for measuring risk.
•
Discuss the interrelationship between incidence and prevalence.
•
Identify limitations in defining the numerators and denominators of incidence and prevalence
measurements.
DEFINITION OF TERMS
___________________
▣ Number of diseases, injury, death, and etc.
▣ Compare one part of distribution to another part of the distribution or to entire population
MEASURES OF DISEASE FREQUENCY
•
are used to describe how common an illness (or other health event) is with reference to the size of the
population (the population at risk) and a measure of time
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
LESSON 08
___________________
▣ Number of cases of a particular health event
▣ Examples:
□
Number of college students who smoke
□
Vehicular fatalities in Davao City during a 24-hour period
___________________
▣ Division of one quantity by another
▣ Includes any expression with a numerator and a denominator (general term)
▣ Examples:
□
Community X has:
■
600,000 males: 400,000 females
■
600:400 or 3:2
□
Vehicular fatalities 18:1 male to female
□
675 persons/square mile
▣ Formula:
□
___________________
▣ A ratio in which the numerator is contained in the denominator
▣ Tells us what fraction of the population is affected
▣ Expressed as a percentage, decimal, or fraction
▣ Examples:
□
□
Community X: 600,000 males and 400,000 females
■
Proportion of males: 600,000/1,000,000 = 60%
■
Proportion of females: 400,000/1,000,000 = 40%
0.13 (13%) of Davao City adults are current smokers
▣ Formula:
□
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
LESSON 08
___________________
▣ A ratio representing change over time
▣ Describes how quickly disease occurs in a population
▣ Can be expressed as a percentage or per unit time
▣ Examples:
□
Velocity of a car (km/h)
□
Incidence rate of dengue in IGACOS
□
20 deaths in a population of 1,000 in 2018
■
the death rate is 2%
■
2% per year or 2% in 2018
MEASURES OF MORBIDITY
▣ ___________________: Refers to the presence of disease in a population
▣ Two categories of measurement to describe frequency of disease in a specified population
□
___________________ (new cases)
□
___________________ (existing cases – old and new)
NUMERATORS
●
In Mathematics: top number in a fraction
●
In Epidemiology:
○
count of persons with a specific trait or condition
○
count of persons using a program or intervention
○
count of positive or negative events
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
●
LESSON 08
Sources of numerator data:
○
surveys, registries, telephone book
○
researcher’s own counts or observations
○
in the context of a formal study
DENOMINATORS
●
In Mathematics: bottom number in a fraction
●
In Epidemiology:
●
○
total population in an area over a defined period of time
○
it is the reference population in which health events are drawn and measured
Sources of denominator data:
○
census
○
population statistics
A. INCIDENCE (INCIDENT CASES)
▣ Frequency of occurrences of disease, injury, illness, or death
▣ Number of ______________ diagnosed cases
▣ Can be measured using:
□
Counts = incident cases / incidence
□
Element of time = incidence rate
□
Proportion = cumulative incidence
INCIDENCE RATE
▣ Rate is a measure of disease risk
□
Transition from a non-diseased state to a diseased state
▣ Numerator:
□
Reflects the number of new cases during the time period
▣ Two types of denominators:
□
___________________ (defined time period) – closed population
□
___________________ (not observed full time) – open/dynamic population
INCIDENCE RATE FOR CLOSED POPULATION
▣ Problem:
□
In Community X with a population of 600,000 males and 400,000 females, 70 new cases of uterine
cancer were diagnosed in a year. What is the incidence rate of uterine cancer in women?
▣ Formula:
□
▣ Answer:
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
LESSON 08
INCIDENCE RATE FOR DYNAMIC POPULATION
▣ Also known as ___________________
▣ Formula:
□
PERSON TIME AT RISK
If the population was observed the entire study period
If the population was observed in different length of time
INCIDENCE RATE FOR DYNAMIC POPULATION
•
A follow-up study was conducted to
determine which sexual behaviors were
associated with the greatest risk of becoming
HIV+. The study was conducted in a group of
female prostitutes.
•
The subjects were tested prior to the
beginning of the study, and five HIV+ women
were excluded. The remaining ten women were
followed for six years beginning in January
1989.
•
Each woman was contacted and retested at the beginning of January each year. The table below
summarizes the findings these ten subjects.
•
A circled plus sign (+) indicates when a subject was found to be HIV+; a question mark (?) indicates when a
subject became lost to follow-up. The dashed lines indicate continued follow-up.
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
LESSON 08
2. CUMULATIVE INCIDENCE
▣ Also called ___________________ or simply “risk”
▣ Measures probability that healthy people will develop disease during a specified time
▣ The proportion of initially susceptible individuals in a population who develop new cases of a disease in a
specified time period
▣
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
LESSON 08
EXAMPLE #2: CUMULATIVE INCIDENCE
▣ Problem:
o
What is the cumulative incidence (risk) for Schistosomiasis in Agusan del Sur in 2015 if 22 new
cases were reported among a population of 700,653?
▪
•
•
▪
Important – assume everybody in the area is at risk
▣ Criteria needed:
o
Knowledge of health status of the study population (diseased or not)
o
Time of onset (acute vs. chronic)
o
Specification of numerator
o
Specification of population “at risk”
▪
Denominator should NOT include people with disease or who are not susceptible
▪
Number of persons at risk varies over time. If so, consider using population size at mid-point
of the time period
o
Period of observation: Usually 1 year
3. ATTACK RATES
▣ AKA cumulative incidence / incidence proportion
▣ Usually applied to outbreak settings
▣ Types:
□
___________________
■
□
Total number of new cases / total population
___________________
■
Number of persons who ate a specific food item and became ill / total number of persons
who ate the food
□
___________________
■
Number of persons exposed to the risk factor for developing a disease within the
incubation period following exposure to the primary case
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
LESSON 08
B. PREVALENCE (PREVALENT CASES)
▣ Definition of Prevalence:
□
Not a measure of risk
□
Usually expressed as a percentage (multiplier = 100)
□
Used to express burden of disease (new + old cases)
□
Number of persons in a defined population who have a specified disease or condition at a given
point in time
▣ Types of prevalence:
□
___________________
□
___________________
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
LESSON 08
▣ Types of prevalence:
□
Point prevalence
▣ Proportion of persons with a particular disease or attribute on a particular date
□
Period prevalence
▣ Proportion of persons with a
particular disease or attribute at
any time during the interval
▣ Answers the question how
many people have had the
disease at any point during a
certain time period?
B. PREVALENCE RATE
▣ Number of affected persons present in the population at a specific time divided by the number of persons in
the population at that time (___________________)
▣ Expressed as number of existing cases per X population
▣ Visit every household in the community, using interviews or physical examinations to get a count of those
affected
▣
IMPORTANCE OF PREVALENCE
KEY POINTS:
▣ Used by health planners for determining workload
▣ For monitoring control programs for chronic conditions
▣ If incidence data not available, can use prevalence to estimate importance of disease
EXAMPLE #4
▣ Suppose we followed a population of 150 persons for 1 year, and 25 had the disease of interest at the start
of follow-up and another 15 new cases developed during the year.
o
What is the period prevalence for the year?
o
What is the point prevalence at the start of the period?
o
What is the cumulative incidence for the one-year period?
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
LESSON 08
RELATIONSHIP BETWEEN INCIDENCE AND PREVALENCE
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
MEASUREMENT
LESSON 08
CASES
Incidence
Point prevalence (t1)
Point prevalence (t2)
Period prevalence
RESEARCH QUESTIONS
Got sick at the start of the study
period?
Got sick in the middle of the
study but recovered before the
study ended?
Got sick in the middle of the
study period and prevailed until
end of study?
Using chest x-rays, 2,000 persons were screened for tuberculosis:
▣ Can we therefore conclude that the risk of tuberculosis is higher in Hitown people than in Lotown people?
▣ ___________________ of acquiring tuberculosis in Lowtown
▣ Higher prevalence of tuberculosis in Hitown due to longer survival
▣ Differences in ___________________ factors affect incidence & prevalence
▣ High prevalence does not automatically mean ___________________
Consider a hypothetical clinical trial that was
conducted to determine whether taking low-dose
aspirin reduced the frequency of heart attacks in
middle-aged and elderly men. The time line below
summarizes events 12 subjects labeled 1-12, all of
whom were allocated to the placebo-treated group
What is the incidence density and cumulative
incidence of getting a heart attack?
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
LESSON 08
___________________
▣ Lists the clinical criteria by which public health
professionals determine whether a person’s illness is
included as a case in an outbreak investigation.
▣ Helps decide who to count in your numerator.
▣ Helps to differentiate who might have similar but not
exactly the same symptoms.
PROBLEMS WITH NUMERATORS
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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BE-LEC 100: DISEASE FREQUENCY AND MEASURES OF MORBIDITY | FINALS TOPIC 1
LESSON 08
PROBLEMS WITH DENOMINATORS
SPC-SMLS | Prepared by: JC Louise P. Bandala, RMT
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