PU 520 Unit 3 Seminar (PART I) Chapter 3: Measures of Morbidity and Mortality Used in Epidemiology.

PU 520 Unit 3

Seminar(PART I)

Chapter 3: Measures of Morbidity and Mortality Used in

Epidemiology

Define and distinguish among ratios, proportions, and rates

Explain the term population at riskIdentify and calculate commonly used rates for morbidity, mortality, and natality

State the meanings and applications of incidence rates and prevalence

OUTCOMES

© 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Discuss limitations of crude rates and alternative measures of crude rates

Apply direct and indirect methods to adjust rates

OUTCOMES (CONT’D)


The simplest and most frequently performed quantitative measure in epidemiology.

Refers to the number of cases of a disease or other health phenomenon being studied.

Significant for rare diseases or symptom presentations (e.g., case of Ebola virus).

COUNT


Traffic fatalities in Manhattan in a 24-hour time period

College dorm students who had hepatitis

Foreign-born stomach cancer patients

EXAMPLES OF COUNTS


The value obtained by dividing one quantity by another

The most general form has no specified relationship between numerator and denominator.

Proportions, rates, and percentages are also ratios.

RATIO


Sex ratio (data from textbook)

EXAMPLE OF A RATIO CALCULATION


DEFINITION OF PROPORTION

A measure that states a count relative to the size of the group.

A ratio in which the numerator is part of the denominator.

May be expressed as a percentage.


Can demonstrate the magnitude of a problem.

Example: 10 dormitory students develop hepatitis. How important is this problem? If only 20 students live in the dorm, 50% are ill.

If 500 students live in the dorm, 2% are ill.

USES OF PROPORTIONS


Calculate the proportion of African-American male deaths among African-American and white boys aged 5 to 14 years.

EXAMPLE OF A PROPORTION


Definition: a ratio that consists of a numerator and a denominator and in which time forms part of the denominator.

Contains the following elements:disease frequencyunit size of populationtime period during which an event occurs

RATE


Example of Rate CalculationEXAMPLE OF RATE CALCULATION


The number of existing cases of a disease or health condition in a population at some designated time.

DEFINITION OF PREVALENCE


Provides an indication of the extent of a health problem.Example 1: Prevalence of diarrhea in a children’s camp on July 13 was 33% (point prevalence).

Example 2: prevalence of cancer in women during a specified time period (period prevalence)

INTERPRETATION OF PREVALENCE


Describing the burden of a health problem in a population.

Estimating the frequency of an exposure.Determining allocation of health resources

such as facilities and personnel.

USES OF PREVALENCE


POINT PREVALENCE


Number of persons ill

Average population

PERIOD PREVALENCE


The number of new cases of a disease that occur in a group during a certain time period.

DEFINITION OF INCIDENCE


Describes the rate of development of a disease in a group over a certain time period.

Contains three elements:Numerator = the number of new cases.Denominator = the population at risk.Time = the period during which the cases occur.

INCIDENCE RATE (CUMULATIVE INCIDENCE)


Help in research on the etiology/causality of disease.

Used to estimatethe risk of developing a disease.the effects of exposure to a hypothesized factor of interest.

APPLICATIONS OF INCIDENCE DATA


Incidence Rate Calculation (IWHS Data)


Alternative form of incidence rate.Used for diseases observed in a population for a

short time period.Not a true rate because time dimension often

uncertain.Example: Salmonella gastroenteritis outbreakFormula:

ATTACK RATE (AR)


An incidence measure used when members of a population or study group are under observation for different lengths of time.

INCIDENCE DENSITY


Formulas for Incidence Density


INCIDENCE DENSITY, EXAMPLE


INTERRELATIONSHIP BETWEEN PREVALENCE AND INCIDENCE


If duration of disease is short and incidence is high, prevalence becomes similar to incidence.

Short duration--cases recover rapidly or are fatal.Example: common cold

If duration of disease is long and incidence is low, prevalence increases greatly relative to incidence.Example: many chronic diseases

INTERRELATIONSHIP BETWEEN PREVALENCE AND INCIDENCE

(CONT’D)


OUTBREAK OF MENINGOCOCCAL INFECTIONS IN A SUMMER SCHOOL CLASS OF

10 STUDENTS


Crude birth rateFertility rateInfant mortality rate

Fetal death rate

Neonatal mortality rate

Postneonatal mortality rate

Perinatal mortality rate

Maternal mortality rate

CRUDE RATES, MEASURES OF NATALITY


CRUDE BIRTH RATE


GENERAL FERTILITY RATE


Fertility rates: United States, 1950-1992. Source: Reprinted from National Center for Health Statistics, Annual Summary of Births, Marriages, Divorces and Deaths, United States, 1992, Monthly Vital Statistics Report, Vol 41, No 13, p. 3. Hyattsville, MD: National Center for Health Statistics; 1993.


INFANT MORTALITY RATE


FETAL DEATH RATE


FETAL DEATH RATIO


Reflects events happening after birth, primarily:congenital malformationsprematurity (birth before gestation week 28)low birth weight (birth weight less than 2,500 g)

NEONATAL MORTALITY RATE


NEONATAL MORTALITY RATE FORMULA


POSTNEONATAL MORTALITY RATE


Infant, neonatal, and postneonatal mortality rates: United States, 1940-2003. Source: From Hoyert DL, Heron MP, Murphy SL, Kung H. Deaths: Final Data for 2003. National Vital Statistics Reports, Vol 54, No 13, p. 12. Hyattsville, MD: National Center for Health Statistics; 2006.


PERINATAL MORTALITY RATE


PERINATAL MORTALITY RATIO


MATERNAL MORTALITY RATE


Use crude rates with caution when comparing disease frequencies between populations.

Observed differences in crude rates may be the result of systematic factors (e.g., sex or age distributions) within the population rather than true variation in rates.

CRUDE RATES


Specific rates refer to a particular subgroup of the population defined in terms of race, age, sex, or single cause of death or illness.

SPECIFIC RATES


CAUSE-SPECIFIC RATE


PROPORTIONAL MORTALITY RATIO (PMR)


THE 10 LEADING CAUSES OF DEATH, 25-34 YEARS, ALL RACES, BOTH SEXES, U.S., 2003

(NUMBER IN POPULATION = 39,872,598


AGE-SPECIFIC RATE (RI)


METHOD FOR CALCULATION OF AGE-SPECIFIC DEATH RATES


Summary measures of the rate of morbidity and mortality in a population in which statistical procedures have been applied to remove the effect of differences in composition of various populations.

ADJUSTED RATES


Direct method may be used if age-specific death rates in a population to be standardized are known and a suitable standard population is available.

DIRECT METHOD


Year 2000 population

Replaces the standard based on 1940 population.

Results in age-adjusted death rates that are much larger.

Affects trends in age-adjusted rates for certain causes of death.

Narrows race differentials in age-adjusted death rates.

Reduces the three different standards into one acceptable standard.

NEW STANDARD POPULATION


DIRECT METHOD FOR ADJUSTMENT OF RATES


WEIGHTED METHOD FOR DIRECT RATE ADJUSTMENT


Indirect method may be used if age-specific death rates of the population for standardization are unknown or unstable, for example, because the rates to be standardized are based on a small population.

The standardized mortality ratio (SMR) can be used to evaluate the results of the indirect method.

INDIRECT METHOD


STANDARDIZED MORTALITY RATIO(SMR)


If the observed and expected numbers are the same, the SMR would be 1.0, indicating that observed mortality is not unusual.

An SMR of 2.0 means that the death rate in the study population is two times greater than expected.

INTERPRETATION OF SMR


ILLUSTRATION OF INDIRECT AGE ADJUSTMENT: MORTALITY RATE CALCULATION FOR A FICTITIOUS

POPULATION OF 230,109 PERSONS


SMR is (502/987.9) X 100 = 50.8%.

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PU 520 Unit 3 Seminar (PART I) Chapter 3: Measures of Morbidity and Mortality Used in Epidemiology.

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