Instructor Resource

Chapter 4

Copyright © Scott B. Patten, 2015.

Permission granted for classroom use with Epidemiology for Canadian Students: Principles, Methods & Critical Appraisal (Edmonton: Brush Education Inc. www.brusheducation.ca).

Chapter 4. Specialized mortality rates and composite measures of disease burden

Objectives

• Define and interpret key specialized measures of mortality: • cause-specific mortality rate, • age- and sex-specific mortality rate, • perinatal mortality rate, • infant mortality rate• case-fatality rate.

• Define and interpret composite health indicators: QALYs and DALYs.• Describe the role of these rates and indicators in

monitoring population health.

Cause-specific mortality

• This is among the oldest quantifiers of mortality (e.g., 17th century “Bills of Mortality” in England).• It is most commonly expressed as a proportion or

rate (but can also be a count).• Its numerator expresses the number dying of a

specific cause• Its denominator expresses the midyear population

at risk.• Note: as the midyear population represents person-

time at risk, this ratio can be regarded as a rate.

Cause-specific mortality (continued)

• Usually, information about the cause of death comes from medical certificates of death.• These are recorded in provincial mortality

databases and then shared nationally.• “Death Certificates” record underlying causes,

immediate causes, and the intermediate steps (antecedent causes).• The WHO ICD system is used to code the causes.

Cause-specific mortality (continued)

• Usually, information about the cause of death comes from medical certificates of death• These are recorded in provincial mortality

databases and then shared nationally• “Death Certificates” record underlying causes,

immediate causes and the intermediate steps (antecedent causes)

Top 10 causes of death in Canada(1) malignant neoplasms (cancer) (2) heart disease (3) cerebrovascular diseases (stroke) (4) chronic lower respiratory diseases(5) accidents(6) diabetes(7) Alzheimer disease (8) influenza and pneumonia (9) suicide(10) kidney disease

Cause-specific mortality (continued)

• Usually, information about the cause of death comes from medical certificates of death• These are recorded in provincial mortality

databases and then shared nationally• “Death Certificates” record underlying causes,

immediate causes and the intermediate steps (antecedent causes)

Cause-specific mortality (continued)

• Mortality rates are a kind of incidence rate.• Causes of death that are:• Sudden• Unexpected• Nonmedical

are investigated by medical examiners, providing another source of data that can be used in epidemiological studies.

Age- and sex-specific mortality• These are calculated within specified age and sex

strata.• Specialized examples include:• perinatal mortality rate• infant mortality rate

Perinatal mortality

• Perinatal death is death of a child less than one week of age (0 to 6 days) or a stillbirth of 28 or more weeks gestation. • Perinatal mortality rate is perinatal deaths

(numerator) divided by the number of total births (denominator), which consist of live births and late fetal deaths.

Infant mortality

• Infant death is death of child less than 1 year of age. • Infant mortality rate is infant deaths in a year

(numerator) divided by the live birth count for the same year.

Case fatality rates

• This is really a proportion that is labelled by convention as a rate.• The numerator expresses people who die from the

disease.• The denominator expresses people with a disease.

Note: this parameter only makes sense for a disease with a defined risk interval (e.g., an infectious or toxin-induced disease in an outbreak). An incidence proportion without a defined risk intervals is otherwise not meaningful.

Survival proportions and relative survival ratios• The proportion of people with a cancer who survive

for 5 years is an example of a survival proportion. • The relative survival ratio uses the proportion of

comparable people without the cancer who are expected to survive the time interval (usually 5 years).• The relative survival ratio is recommended by the

Canadian Cancer Society.

Potential years of life lost (PYLL)• Causes of death are strongly related to age. • Most of the top causes of death kill mostly older

people—a reality not reflected in crude mortality rates.• PYLL (usually called “YPLL” in the US) calculates the

potential years of life lost.• In Canada, PYLL is based on the number of years of

potential life not lived when a person dies before age 75.

PYLL (continued)

• For example, a death from a disease that occurs at age 65 results in 10 PYLL.• A death after age 75 results in no PYLL.• PYLL can be expressed as a number or rate. The

rate uses a person-time denominator. When this is done, the person-time in the rate is the midyear population of those younger than 75.

Quality adjusted life years (QALY)• QALY is pronounced “qually.”• Quality of life refers to a person’s self-appraisal of their

own health. • These are often assessed as “utility” in scales such as the

Health Utilities Index (HUI) or EQ-5D.• Typically, perfect health means utility = 1.• A state comparable to death means utility = 0.• Every disease leads to a state that is between these

values.• The impact of a disease or treatment on health can be

weighed as QALYs gained or lost.

Disabiltiy-adjusted life-years (DALY)• DALY is pronounced “dally.”• It is calculated as the sum of years lived with

disability and (potential) years of life lost. • The concept of disability is broad—and is usually

estimated using utility-defining strategies (e.g., “standard gamble” methods).• Mainly used by the Global Burden of Disease

project, but also in other burden of disease studies. (By contrast, QALYs tend to be used in health economic studies.)

Proportional mortality ratios• These are falling out of use—they are mainly of

historical interest.• They were popular when only “numerator”

mortality data (death counts) were available.• They are calculated as the number of deaths due to

a specific cause (numerator) divided by the total number of deaths (denominator).

End