Measures of Disease Frequency COURTNEY D. LYNCH, PhD MPH ASSISTANT PROFESSOR DEPT. OF OBSTETRICS...

Measures of Disease Frequency

COURTNEY D. LYNCH, PhD MPH

ASSISTANT PROFESSOR

DEPT. OF OBSTETRICS & GYNECOLOGY

Courtney.Lynch@osumc.edu

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

On passing, 'Finish' button: Goes to Next SlideOn failing, 'Finish' button: Goes to Next SlideAllow user to leave quiz: At any timeUser may view slides after quiz: At any timeUser may attempt quiz: Unlimited times

Thank you!

If you have any questions, please contact:

Courtney D. Lynch, PhD MPH

Courtney.Lynch@osumc.edu