measures of disease frequency courtney d. lynch, phd mph assistant professor dept. of obstetrics...
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Measures of Disease Frequency
COURTNEY D. LYNCH, PhD MPH
ASSISTANT PROFESSOR
DEPT. OF OBSTETRICS & GYNECOLOGY
Learning Objectives
• Discriminate between the various scales of disease measurement• Differentiate between a rate, ratio and a proportion
• Define and calculate incidence• Define and calculate prevalence
• Describe the relation between incidence and prevalence
Measures of disease frequency
Counts Ratios Proportions Rates
Count
What is the problem with counts?
When is a count useful?
Example: Counts of H1N1 influenza
Ratio
Shows the relative size of two values Demonstrates how many times larger (or
smaller) one group is compared to another Example:
Sex ratio = boys/girls at birth
= 52/48
Proportion
A proportion is a ratio in which the numerator is a subset of the denominator A / A+B
Dimensionless Example:
Proportion African American (AA)=
AA/AA + other races
Proportion vs. ratio
Medical school class with 60 males and 40 females
Proportion male: 60/100
Ratio of males/females: 60/40
Rate
A ratio that takes the form a/a+b during some period of time
Epidemiologic rates contain Count of disease frequency Size of the population at risk Time period during which the disease
occurred in the reference population Example:
CVD mortality rate = CVD deaths/population at risk in 2012
Some important properties
Ratios May or may not have units Always > 1
Proportions Unitless Takes values between 0 and 1
Rates The time period in the numerator and denominator must be the
same. A rate may or may not be a proportion but is always a ratio
Population dynamics
In fixed populations, membership is permanent and defined by an event/characteristic Babies born at Wexner Medical Center in
2012 In dynamic populations, membership is transient
and defined by being in or out of a ‘state’ Citizens of Franklin County
Measures of disease frequency
Incidence
Prevalence
Incidence
The frequency of the occurrence of new cases over a specified period of time
Measures the appearance of disease Two types: cumulative incidence and incidence
rate Useful to etiologic researchers because changes
in incidence suggest a change in the balance of causal factors
Cumulative incidence
Risk or probability of an individual getting a disease
A proportion # of new cases of disease / # at risk at
beginning of follow-up over a specified time period
Usually used in fixed populations
Cumulative incidence
Cumulative incidence (CI) assumes that you have followed the entire population for the entire period
But, many, if not most, populations are dynamic Thus, CI is not an appropriate measure
Incidence density (incidence rate) takes into account differences in periods of follow-up
Incidence rate (density)
# of new cases / sum of disease-free person-time over a specified time period
Incidence rate (density) denominator
The IR denominator is the person-time at risk, defined as the sum of disease-free time in the population. Units for denominator = person-years
The IR denominator allows for the population to be dynamic
Approaches to the denominator
1. Add the individual risk periods Exact, preferred method
2. Use the average number of people in the study multiplied by the study duration 500,000 persons * 1 year = 500,000 person-
years 50 persons * 50 years = 2,500 person-years
3. Use the average duration per person 100 persons * 2.5 years = 250 person-years
Calculating person-time
1999 2000 2001 2002 2003
MI
Lost
MI
2
5
4
5
3
Exact person-
time = 19
Estimate person-time
5 * 2.5 = 12.5
IR= 2/19
IR= 2/12.5
Prevalence
The proportion of people in a population with the disease, at a specified point (or period) in time
Measures existing disease Two types of prevalence:
Point prevalence Period prevalence
Point prevalence
# of existing cases / total population at a specified point in time
A proportion
Period prevalence
(# of existing cases + number of cases that occur during the interval) / population at midpoint of interval (or average population size)
Also a proportion
Value of prevalence
Describing a health burden Health planning/allocation of resources –
treatment, hospital beds, manpower, etc.
Relation between prevalence and incidence
Prevalence depends on incidence and disease duration
Prevalence is similar to incidence*duration of disease [P=ID]
If a disease is of short duration, I~P If a disease is chronic, prevalence is
higher than incidence
Incident cases
Deaths and recovery
Prevalent cases
Incidence and Prevalence
Why do epidemiologists prefer incidence?
We’re usually interested in etiology Don’t want to vary a number of factors at
the same time Possible risk factors for disease Factors associated with survival
The problem of birth defects
Summary
The burden of disease can be quantified by counts, ratios, proportions, and rates.
Incidence measures new cases Prevalence measures existing cases Prevalence = incidence * duration of disease
Measures of Disease Quiz
PROPERTIES
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Thank you!
If you have any questions, please contact:
Courtney D. Lynch, PhD MPH