Measures of Disease Burden

Dr. Ranadip Chowdhury M.D.Study Coordinator

CHRD-SAS

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Epidemiological Outcomes• Ratio: Relationship between two numbers

– Example: males/females

• Proportion: A ratio where the numerator is included in the denominator

– Example: males/total births

• Rate: A proportion with the specification of time

– Example: (Pneumonia cases in 2013/ total U-5 population in 2013) x 1,000

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In epidemiology, the occurrence of a disease or condition can be measured using rates and proportions. We use these measures to express the extent of these outcomes in a community or other population.

Rates tell us how fast the disease is occurring in a population.

Proportions tell us what fraction of the population is affected.

(Gordis, 2000)04/10/23 05:52 3

Concept of Risk

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Risk Versus Rate

Risk and rate are often used

interchangeably by epidemiologists but

there are differences

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Risk Versus Rate (cont.)

• Risk is a probability statement assuming an individual is not removed for any other reason during a given period of time

• As such, risk ranges from 0 to 1 (no chance to 100% probability of occurrence)

• Risk requires a reference period and reflects the cumulative incidence of a disease over that period

• Example: 1 in a million chance of developing cancer in a 70 year lifetime

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Risk Versus Rate (cont.)

• Rates can be used to estimate risk if the time period is short (annual) and the incidence of disease over the interval is relatively constant

• If however, individuals are in a population for different periods of time for any reason, then you should estimate risk by incidence density

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The Three Elements in Measures of Disease Incidence

• E = an event = a disease diagnosis

• N = number of persons in the population in which the events are observed

• T = time period during which the events are observed

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Morbidity Measures

• Incidence is always calculated for a given period of time

• An attack rate is an incidence rate calculated for a specific disease for a limited period of time during an epidemic

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Population at risk (PAR)

• The denominator for incidence is the population at risk

• Not including– Existing cases– Individuals who are not capable of developing

the disease.

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Factors affecting population at risk

• Populations are dynamic– births and deaths (with different risk)– immigration, emigration and other losses

• Affected by immunity status• Removal from at-risk population

– Cases– Lost to follow up– Death due to other causes (competing risks)– Change in risk status (hysterectomy and uterine

cancer)

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Calculating population at risk

• Exact – Sum each person’s contribution to

time-at-risk– Must know exact times of entry,

disease onset, or withdrawal for every individuals

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Calculating population at risk

Mid-point population– Assume onset/withdrawal random over

time – Use population at the mid-point of study to

estimate average PAR (most recent census figure)

– Mean of start and end populations

Incidence Density

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Concept of person-years

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c

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rates: year 1 = 3/7.083 = 42.4/100 person-years year 2 = 3/2.50 =120/100 person-years both yrs 6/9.583 = 62.6/100 person-years

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Dealing with recurrent cases

• Count only first event– remove case from PAR

• Count all events– don’t remove case from PAR

• Count secondary etc. events only after biologically based recovery period– remove temporarily from PAR

• Count all events - stratify – Remove case from PAR for primary case– enter case in PAR for secondary case

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Morbidity Measures

• Prevalence is not a rate

• Point prevalence measures the frequency of all current events (old and new) at a given instant in time

• Period prevalence measures the frequency of all current events (old and new) for a prescribed period of time

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Illustration of several concept of morbidity

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Calculation of incidence density

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Relationship Between Prevalence and Incidence

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Factors Influencing Prevalence

Increased by:• Longer duration of the

disease• Prolongation of life of

patients without cure

• Increase in new cases• (increase in incidence)

• In-migration of cases

• Out-migration of healthy people

• In-migration of susceptible people

• Improved diagnostic facilities

• (better reporting)

Decreased by:•Shorter duration of

disease

•High case-fatality rate from disease

•Decrease in new cases (decrease in

incidence)

•In-migration of healthy people

•Out-migration of cases

• Improved cure rate of cases

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Relation of incidence and prevalence

• Prevalence depends on incidence

• Higher incidence leads to higher prevalence if duration of cases does not change.

• Limitation of the bathtub analogy – flow rate needs to be expressed relative to the size of the source

• Introducing a new analogy . . .

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Population at risk

Existing cases

Deaths, cures, etc.

Relationship Between Incidence and Prevalence (cont.)

• Diarrhea– Incidence High– Duration short– Prevalence low

• Tuberculosis– Incidence low– Duration long– Prevalence high

• Pneumonia– Incidence high– Duration short– Prevalence low

• Malaria– Incidence high– Duration medium-

long– Prevalence low-

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Prevalence v/s incidence Point prevalence Period

prevalenceCumulative Incidence

Incidence Density

Numerator Existing cases(old and new)

Existing and new cases

New cases New cases

Denominator Initial Pop. Mid-point Pop.

Initial orMidyear Pop.

Person-time

Time One point A period A period A period

Unit None None None No./PT

Type proportion proportion proportion rate

Example question

Do you currently have asthma?

Have you had asthma during the last year?

Have you ever had asthma last year?

Study types Cross-sectional studiessurveillance

closedCohort studies

Open Cohort studies

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Calculation Practice

Skin Cancer on Sunny Beach:

1. Point prevalence on 9/9/20132. Period prevalence for year 20133. Incidence rate for year 2013

What information will you need?

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Diagnosed cases of Skin Cancer on Sunny Beach, 9/9/2013

Point Prevalence (9/9/2013)

= (10/450)*1000

= 22 per 1000

# of existing cases = 10

Total population at risk = 450

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Diagnosed cases of Skin Cancer on Sunny Beach, 2013

Average population at risk = 500

Incidence rate (year 2013)

= (5/500)*1000

= 10 per 1000

Period prevalence (year 2013)

= (15/500)*1000

= 30 per 1000

# of new cases = 5

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THANK YOU….

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