Measures of Health Status

Epidemiologists use specific measures of morbidity and mortality to characterize the degree to which illness and disease are present in a population and in how much this affects the population’s health and death rate.

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Mortality

Measures of mortality are statistical measures that specify how many people are dying from a disease or particular exposure over a specific period of time and are scaled to the size of the selected population (usually per 1,000 or 100,000 people).

Mortality rate (MR)

  • Also called the crude death rate; typically specified as per 1,000 or 100,000 people
  • Calculated as the ratio of the total number of people who die due to all causes over a specific time period to the total number of people in the selected population: 

    MR = (# deaths / size of selected population) * 1000
  • Any period of time can be chosen but the annual death rate (ADR) is the most common. The expression for the ADR per 100,000 people is calculated as the total number of deaths over the year divided by the total population at the midpoint of the year:
Annual Death Rate Formula
  • Some MRs are named after a specific population or age group; these are called age-specific MRs. Common rates include:
    • Perinatal MR (per 1,000 births): the total number of neonatal deaths and fetal deaths (stillbirths) divided by the total number of live births for the year x 1,000 
    • Maternal MR (per 1,000 women): the total number of maternal deaths (defined as being due to pregnancy-related causes during pregnancy or within 42 days after the termination of pregnancy) divided by the number of women of reproductive age over a specific time period x 1,000
    • Maternal MR (per 100,000 live births): the total number of maternal deaths divided by the total number of live births over a specific time period x 100,000
    • Infant MR (per 1,000 live births): the total number of deaths for children < 1 year of age divided by the total number of live births over a specific time period  x 1,000

Case fatality rate

  • Defined as the number of deaths of individuals with a certain disease or condition divided by the total number of people diagnosed with that same disease or condition over a specific time period
  • Typically expressed as a percentage or proportion and is a measure of the severity of a disease/condition:
Case fatality rate equation

Proportionate mortality

  • Calculated as the number of deaths in individuals due to a specific disease/condition divided by the total number of deaths due to all causes over a specific time period:
Proportionate mortality equation

Standardized mortality ratio (SMR)

  • The ratio of the number of deaths in a study cohort to the expected number of deaths in a population
  • The number of deaths in the cohort is weighted based on age with respect to the general population.
  • If the SMR is above 1.0, it means there are more deaths in the cohort than expected.

Years of potential life lost (YPLL)

  • A measure of the impact of premature death on a population
  • Is given in units of person-years
  • YPLL is calculated as the sum of the following time difference for all i individuals who died prematurely in a population of size N:
    Years of Potential Life Lost formula
    where LE is the predetermined life expectancy in the population and ADi is the age of death of the ith individual in the population.      
    • For example, an individual who died at age 45 in a population with a life expectancy of 65 years would contribute 20 person-years to the YPLL.
  • The YPLL rate is used to compare premature deaths between populations of different sizes as the YPLL alone does not take into account population size. The YPLL rate is calculated as the YPLL for the population divided by the total size of the population that is younger than the predetermined life expectancy; typically given per 1,000 individuals.

Morbidity

  • Characterizes the number of those in a population who are or become ill over a specific time period 
  • Epidemiological description of the disease burden in a population 
  • Measures of morbidity: typically expressed either as a rate (number of cases of a disease per unit time) or a ratio (proportion or percentage of a disease in a population) 
    • Incidence: the number of new cases of a disease per unit of time; is a measure of disease risk. Calculated as the number of new cases of a disease or condition diagnosed during a specific period of time divided by the size of the population
    • Prevalence: the number of cases of a disease or condition in a population at a point in time or over a period of time. Corresponds to disease frequency

References

  1. Thacker, S. B., Stroup, D. F., Carande-Kulis, V., Marks, J. S., Roy, K., & Gerberding, J. L. (2006). Measuring the public’s health. Public health reports (Washington, D.C.: 1974), 121(1), 14–22. DOI:10.1177/003335490612100107. Retrieved from https://europepmc.org/article/med/16416694
  2. Celentano, David D., ScD., M.H.S., & Szklo, Moyses, MD, M.P.H., DrP.H. (2019). The occurrence of disease: I. disease surveillance and measures of morbidity. In Celentano, David D., ScD, MHS, & Szklo, Moyses, MD, MPH,DrPH (Eds.), Gordis epidemiology (pp. 41-64) doi://dx.doi.org/10.1016/B978-0-323-55229-5.00003-6. Retrieved from https://www.clinicalkey.es/#!/content/3-s2.0-B9780323552295000036
  3. Celentano, David D., ScD., M.H.S., & Szklo, Moyses, MD, M.P.H., DrP.H. (2019). The occurrence of disease: II. mortality and other measures of disease impact. In Celentano, David D., ScD, MHS, & Szklo, Moyses, MD, MPH,DrPH (Eds.), Gordis epidemiology (pp. 65-93) doi://dx.doi.org/10.1016/B978-0-323-55229-5.00004-8. Retrieved from https://www.clinicalkey.es/#!/content/3-s2.0-B9780323552295000048

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