Measures of disease frequency are used in order to determine the degree and impact of a disease with respect to a specific population. These measures describe the number of new cases within a defined period of time (incidence) or the total number of cases (prevalence) in a population. Morbidity and mortality characterize the degree to which illness and disease are present in the population and the risk and rate of death from particular diseases in the total population or age-specific groups.

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Incidence

Definition: Incidence is a measure of the number of new cases of a disease or condition that occurs in a population over a specific period of time.

  • Presented either as a rate or a ratio/percentage
  • Must be associated with a time interval 
  • Incidence-related measures:
    • Incidence proportion/cumulative incidence: 
      • Defined as the number of new cases of a disease/condition divided by the number of people in the population 
      • Expressed as a ratio or percentage and is a measure of the attack rate or risk (see Image 1)
    • Incidence rate (IR) /incidence density: 
      • A measure of the speed at which new cases of a disease/condition occur over a period of time in a population at risk. A high IR corresponds to a faster infection rate.
      • Defined as the number of new cases of a disease/condition that occurs divided by the total time the population observed was at risk and given in units of person-years (see Image 2)
      • The total amount of disease-free person-time a population is at risk is calculated as the sum of the disease-free time of each individual in the population over a specific time period. 
      • When calculating the total amount of disease-free time, if an individual is diagnosed during the time period, the amount of disease-free time for the individual before the diagnosis is added to the total time.

Videos

Measurements of Morbidity – Descriptive Epidemiology by Raywat Deonandan, PhD

Measurements of Morbidity – Descriptive Epidemiology by Raywat Deonandan, PhD

 

Image 1: Expression for the risk, or the cumulative incidence/incidence proportion, which is calculated as the number of new cases of a disease/condition in a population over a specific period of time divided by the size of the population at risk at the start of the time interval.

Image 2: Definition of the incidence rate (IR), which is calculated as the number of new cases diagnosed (A) during a specified period of time divided by the total amount of disease-free time the population (PT) observed is at risk (given in units of person-time).

Examples

Cumulative Incidence: Over the past year at a particular hospital, there were 500,000 patients admitted for hip or knee replacements. 5,000 of those patients over that year had a venous thromboembolism (VTE) during their admission. What is the risk of developing VTE over the past year at this hospital? 

  • The risk is also known as the cumulative incidence. There are 5,000 new cases over the year in a population at risk of 500,000.

  • Therefore, 1% of those admitted, or 1 out of 100 patients, experienced VTE during their hospitalization.

Incidence rate: Over the time period from 2000 to 2009, in Quebec, there were 91,000 cases of venous thromboembolism (VTE) over a population with a disease-free time over that period that totals 74,200,000 person-years. What is the incidence rate in VTE in Quebec over this time period?

  • The incidence rate (IR) is a measure of the speed at which VTE are occurring in Quebec. Over 2000-2009 in Quebec, there were 91,000 newly diagnosed cases of VTE and there were 74,200,000 person-years of disease-free time during this period.

  • Therefore, the IR is 0.0012 cases/person-year. Equivalently, this can be multiplied by 10,000 to get 12 cases/10,000 person-years of observation; it is often convention to express some of these measures for populations of 1,000 or 10,000. 

Prevalence

Definition: Prevalence is a measure of how many people are suffering from a disease at a particular point in time (point prevalence) or over a particular period in time (period prevalence).

  • Defined as all cases of a disease or condition in a population divided by the size of the population over the specific time period
  • Typically expressed as a proportion or percentage.

Video

Measurements of Morbidity – Descriptive Epidemiology by Raywat Deonandan, PhD

 

Image 3: Expression for prevalence (P), which is the total number of cases (C) of a disease or condition in a population divided by the size of the population (N) at a specific point in time (point prevalence) or over a period of time (period prevalence)

Example

Period prevalence: A group of epidemiologists conducted a year-long survey of the portion of the United States and found that there were 12 million patients who were in the Medicare database. 200,000 of these Medicare patients had venous thromboembolism (VTE). What is the period prevalence of VTE in Medicare patients in the United States for the year?

  • The period prevalence for the year is a measure of the proportion of Medicare patients who had VTE. There were 12 million patients in the Medicare database; 200,000 of these Medicare patients had VTE.

  • Therefore, the period prevalence of VTE in Medicare patients for the year was 1.6%.

Measures of Mortality

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

Mortality Rate (MR)

  • Also called the crude death rate and typically given per 1,000 or 100,000 people
  • Calculated as the ratio of the total number of people that 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 (see Image 4).
  • Some mortality rates are named after a specific population or age group, called age-specific mortality rates.  Common rates include:
    • Perinatal mortality rate (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 mortality rate (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 mortality ratio (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 mortality rate (per 1,000 live births): the total number of deaths for children age < 1-year-old divided by the total number of live births over a specific time period  x 1,000

Case-fatality rate (CFR)

  • 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 (see Image 5).
    • Typically expressed as a percentage or proportion and is a measure of the severity of a disease/condition.

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 (see Image 6).

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 the 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, given in units of person-years.
  • YPLL is calculated as the sum of the following time difference for all individuals who died prematurely in a population of size N: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 since 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 and is typically given per 1,000 individuals.

Video

Mortality Rate – Descriptive Epidemiology by Raywat Deonandan, PhD

 

Image 4: Expression for the annual death rate (ADR) per 100,000 people calculated as the total number of deaths over the year divided by the total population at the midpoint of the year.

Image 5: Expression for the case-fatality, which gives the proportion of people with a specific disease or condition who die (lethality). It is calculated as the number of individuals with a specific disease or condition who die during a specific period of time divided by the number of individuals in the population with that specific disease or condition.

 

Image 6: Expression for the proportionate mortality, which is the proportion of people in a population with a specific disease or condition who die divided by the total deaths in the population during a specific time period.

Morbidity

Definition: 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 time and 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.

Video

Measurements of Morbidity – Descriptive Epidemiology by Raywat Deonandan, PhD

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