EPB PHC 6000 EPIDEMIOLOGY FALL, 1997

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Unit 2 Measures of Disease Occurrence
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• Unit 2 Learning Objectives
• Understand counts, ratios, proportions, and
  rates.
• Define, calculate, and interpret incidence.
• Understand the use of person-time denominators.
• Distinguish between cumulative incidence and
  incidence rate.
• Define, calculate, and interpret prevalence.
• Distinguish between point and period prevalence.

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• Unit 2 Learning Objectives (cont.)
• Understand special types of incidence and
  prevalence measures.
• Understand the interrelationship between
  incidence, prevalence, and duration of disease.
• Differentiate the use of incidence and prevalence
  measures.
• Become familiar with methods used in survival
  analysis.

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Assigned Readings Textbook (Gordis) Chapter
3, pages 32-58 (Measuring occurrence of morbidity
and mortality Chapter 6, pages 96-112 (Person
years and survival analysis)
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Quantitative Measures of Health Status
Measures of health status convey information
about the occurrence of disease. They
include Counts Proportions Ratios Rat
es
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Counts
Simplest/most frequently performed measure in
epidemiology Refers to the number of cases of
a disease or other health phenomenon being
studied i.e. cases of influenza in Allegheny
county in January, 2002 i.e. Number of persons
involuntarily referred for psychiatric crisis
intervention Useful for allocation of health
resources Limited usefulness for epidemiologic
purposes without knowing size of the source
population
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Counts Limited Interpretation
New Cases Reporting Location of Disease
Period Population City A 20 1998
100 City B 100 1998
1000 Annual Rate of Occurrence City A 20 / 100
1 / 5 City B 100 / 1000 1 / 10
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Proportions

• Persons included in the numerator are always
  included in the denominator
• A
• Proportion --------
• A B
• Indicates the magnitude of a part, related to the
  total.
• In epidemiology, tells us the fraction of the
  population that is affected.

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Proportions - Example
P A / (A B) (1,400 / 11,050) 0.127
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Proportions

• Numerical value of a proportion 0 to 1.0
• Linked to probability theory (i.e. risk of
  developing disease)
• For ease of usage, can multiply a proportion by
  100 to get a percentage
• Example p 0.127 12.7

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Ratios

• Like a proportion, is a fraction, BUT without a
  specified relationship between the numerator and
  denominator
• Example Occurrence of Major Depression
• Female cases 240 240
• ------------------------ ---- 21 female
  to male
• Male cases 120 120

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Rates

• A ratio in which TIME forms part of the
  denominator
• Epidemiologic rates contain the following
  elements
• disease frequency (in the numerator)
• unit size of population
• time period during which an event occurs

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Rates Example

• Calculate crude annual death rate in the US
• Annual death count
• Crude death rate ----------------------- x
  1,000
• Reference population
• (during midpoint of year)
• Death count in U.S. during 1990 2,148,463
• U.S. population on June 30, 1990 248,709,873
• 2,148,463
• Crude death rate -------------- x 1,000 8.64
  per 1,000
• 248,709,873

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Discussion Question 1
What does a crude annual death rate of 8.64 per
1,000 mean?
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Discussion Question 1

• It means that over the course of a year
• About 9 persons in 1,000 died.
• About 864 persons per 100,000 died.
• The risk of dying was about 0.9 (see below)
• 2,148,463
• Crude death rate -------------- 0.0086 x 100
  0.86
• 248,709,873

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Incidence

• The development of new cases of a disease that
  occur during a specified period of time in
  previously disease-free or condition-free (at
  risk) individuals.

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Incidence

• Incidence quantifies the development of
• disease --- Most fundamental measure of
  disease frequency and leads to the development of
  the concept of risk (i.e transition from
  non-diseased to diseased state)
• - Cumulative incidence (CI)
• (Incidence proportion)
• - Incidence rate (IR)
• (Incidence density)

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Cumulative Incidence (CI)

• PROPORTION of individuals who become diseased
  during a specified period of time
• (e.g. all new cases during 1998)
• Range 0 to 1.0

Also referred to as incidence proportion.
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Cumulative Incidence (CI)
No. of new cases of disease during a given
period CI --------------------------------------
------------------------ Total population at
risk during the given period Example During a
1-year period, 10 out of 100
at risk persons develop the disease of
interest. 10 CI ----- 0.10
or 10.0 100
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Cumulative Incidence (CI)

• To accurately calculate cumulative incidence, we
  need to follow the entire population for the
  specified time interval. Often times, this does
  not fully occur.
• Cumulative incidence provides an estimate of the
  probability (risk) that an individual will
  develop a disease during a specified period of
  time.

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Cumulative Incidence (CI)

• Keep in mind that over any appreciable period of
  time, it is usually technically impossible to
  measure risk.
• This is because if a population is followed over
  a period of time, some people in the population
  will die from causes other than the outcome under
  study
• The phenomenon of being removed from a study
  through death from other causes is referred to as
  competing risks.

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Incidence Rate (IR)
No. new cases of disease during a given
period IR -------------------------------------
---------------------- Total person-time of
observation Range 0 to Infinity
Since the number of cases is divided by a measure
of time of observation, rather than people, this
helps address the problem of competing risks.
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Incidence Rate (IR)
What is person time?

• When we observe a group of individuals for a
  period of time in order to ascertain the
  DEVELOPMENT of an event.
• - The actual time each individual
• is observed will most likely
• vary.

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Discussion Question 2
In a 2-year study of the development of disease
X, why might the actual time each individual is
observed vary?
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Discussion Question 2

• Because
• Subjects may be recruited at different times
• Subjects may emigrate
• Subjects may choose to leave study
• Subjects may die
• Subjects may get the disease we are studying

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Person-Time

• Each subject contributes a specific person-time
  of observation (days, months, years) to the
  denominator

Person Follow-up Time on Study Person
Yrs. 1 lt-----------------------------------
--gt 2 2 lt---------------------------------
-----D 2 3 lt-----------------WD
1 4 lt-----------------------------------------
--------------gt 3 5 lt-------------------
------------------gt 2 1995 1996 1997 1998
Jan. Jan. Jan. Jan.
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Person-Time
Person Follow-up Time on Study Person Yrs.
1 lt-------------------------------------gt
2 2 lt--------------------------------------D
2 3 lt-----------------WD 1
4 lt-----------------------------------------------
--------gt 3 5 lt-------------------------
------------gt 2 1995 1996 1997 1998
Jan. Jan. Jan. Jan.
Study Period 3 Years Study Participants 5 P
erson Years of Observation 10 Average Duration
of Follow-Up 2.0 Years
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Incidence Rate (IR)
No. new cases of disease during a given
period IR -------------------------------------
----------------------- Total
person-time of observation So, 1 case IR
----------- 1 case per 10 years
follow-up 10 years Whereas, 1 case CI
------------ 0.20 20.0 5 persons
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Comparison of IR and CI
If we multiply by 0.2, the IR of 1 case per 10
years is equivalent to 0.2 cases per 2
years which suggests a 20 risk of disease
development within 2 years of
follow-up. Whereas, the CI risk estimate of 20
(1 case per 5 persons) was based on a period of 3
years of follow-up. The CI calculation of risk
of disease development differs from the IR
calculation, in part, because it assumed that for
incomplete follow-up, no cases of disease
occurred.
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Discussion Question 3
Previously, we said that the incidence rate can
range from 0 to infinity! How can this be?
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Discussion Question 3
Consider the following McDonalds shooting
lasting 1/2 hour with 50 patrons in the
restaurant. 29 survivors at risk period of 1/2
hr 14.5 person hrs. 21 deaths at risk period
of avg. 1/4 hr 5.25 person hrs. 21 deaths /
20 person hours This translates to 919,800,000 /
100,000 person years Therefore, as time
increases, IR approaches infinity.
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Incidence Rate (IR)
NOTE The selection of the time unit for
the denominator is arbitrary, and is
not directly interpretable Example 100 cases
/ person year can also be expressed
as 10,000 cases / person century 8.33 cases
/ person month 1.92 cases / person week 0.27
cases / person day
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Incidence Rate (IR)

• Incidence rate - Incidence density -
  Force of morbidity
• Measure of the instantaneous rate of development
  of disease in a population

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Comparison of IR and CI
In general Risk estimates derived from IR and CI
calculations will be similar when Follow-up
loss is minimal The disease of interest occurs
infrequently. CI is most useful if interest
centers on the probability than an individual
will become ill over a specified period of
time. IR is preferred if interest centers on how
fast the new cases are occurring in the
population.