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

Define and distinguish among ratios, proportions, and rates Outcomes Define and distinguish among ratios, proportions, and rates Explain the term population at risk Identify and calculate commonly used rates for morbidity, mortality, and natality State the meanings and applications of incidence rates and prevalence © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Apply direct and indirect methods to adjust rates Outcomes (cont’d) Discuss limitations of crude rates and alternative measures of crude rates Apply direct and indirect methods to adjust rates © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

Traffic fatalities in Manhattan in a 24-hour time period Examples of Counts Traffic fatalities in Manhattan in a 24-hour time period College dorm students who had hepatitis Foreign-born stomach cancer patients © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

Example of a Ratio Calculation Sex ratio (data from textbook) © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

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

Example of a Proportion Calculate the proportion of African-American male deaths among African-American and white boys aged 5 to 14 years. © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Rate 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 frequency unit size of population time period during which an event occurs © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Example of Rate Calculation © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Definition of Prevalence The number of existing cases of a disease or health condition in a population at some designated time. © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

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

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

Number of persons ill Average population Period Prevalence Number of persons ill Average population © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Definition of Incidence The number of new cases of a disease that occur in a group during a certain time period. © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

Applications of Incidence Data Help in research on the etiology/causality of disease. Used to estimate the risk of developing a disease. the effects of exposure to a hypothesized factor of interest. © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Incidence Rate Calculation (IWHS Data) © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Attack Rate (AR) 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 outbreak Formula: © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Incidence Density An incidence measure used when members of a population or study group are under observation for different lengths of time. © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Formulas for Incidence Density © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

Interrelationship Between Prevalence and Incidence © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Interrelationship Between Prevalence and Incidence (cont’d) 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 © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Outbreak of Meningococcal Infections in a Summer School Class of 10 Students © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Crude Rates, Measures of Natality Crude birth rate Fertility rate Infant mortality rate Fetal death rate Neonatal mortality rate Postneonatal mortality rate Perinatal mortality rate Maternal mortality rate © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

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

Fertility rates: United States, 1950-1992 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. © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

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

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

Neonatal Mortality Rate Reflects events happening after birth, primarily: congenital malformations prematurity (birth before gestation week 28) low birth weight (birth weight less than 2,500 g) © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Neonatal Mortality Rate Formula © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

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

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

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

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

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

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

Cause-Specific Rate © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Proportional Mortality Ratio (PMR) © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

The 10 Leading Causes of Death, 25-34 Years, All Races, Both Sexes, U The 10 Leading Causes of Death, 25-34 Years, All Races, Both Sexes, U.S., 2003 (Number in population = 39,872,598 © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Age-Specific Rate (Ri) © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Method for Calculation of Age-Specific Death Rates © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

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

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

Direct Method for Adjustment of Rates © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

Weighted Method for Direct Rate Adjustment © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

Standardized Mortality Ratio (SMR) © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)

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

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