Population health

Last updated: March 11, 2022

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The study of population health is mostly rooted in classical epidemiology, which focuses on the distribution and determinants of disease in populations. Population health studies have a wide range of applications that include guiding public health policy and interventions, evaluating the importance and potential impact of PICO questions for clinical research, and informing clinical practice (e.g., using prevalence measures to estimate the pretest probability of a disease). The most common types of epidemiological studies used in population health are observational studies. These can be descriptive studies (e.g., ecological studies), analytical studies (e.g., cohort studies), and studies that can be both observational and analytical (e.g., cross-sectional studies). They typically seek to identify populations with particular outcomes and assess the association between potential exposures and these outcomes. This article focuses on basic concepts of population health (e.g., population pyramids, and endemic, epidemic, or pandemic diseases) and measures of disease frequency, e.g., incidence, prevalence, birth rates, mortality rates, case-fatality rates, morbidity, and disease burden.

The following concepts are discussed separately: causal relationships in research studies, other reasons for observed associations (e.g., random errors, systematic errors, confounding), measures of association commonly encountered in studies of clinical interventions (e.g., relative risk, absolute risk reduction), measures used in the evaluation of diagnostic research studies (e.g., sensitivity, specificity), precision and validity, and foundational statistical concepts (e.g., measures of central tendency, measures of dispersion, normal distribution, confidence intervals).

See also “Epidemiology,” “Statistical analysis of data,” and “Interpreting medical evidence.”

Population pyramid

  • Definition: a graphical representation of the age and sex distribution of a population [1]
  • Examples
    • Expansive pyramid
      • Represents a population in which there is a higher percentage of young people of both sexes
      • For example, a population with a high birth rate and low life expectancy
    • Stationary pyramid
      • Represents a population in which age and sex distribution remains relatively constant over time
      • For example, a population with a low birth rate, low mortality, and increasing life expectancy
    • Constrictive pyramid
      • Represents a population in which there is a higher percentage of elderly people of both sexes
      • For example, a population with a low birth rate and high life expectancy

Demographic transition

  • Definition: changes that occur in a population that goes from having high birth rates and high death rates to having low birth rates and low death rates [1]
  • Demographic Transition Model: describes, in stages, the evolving relationship between birth and death rates in a population over time [1]

Endemic, epidemic, and pandemic diseases

Diseases can be classified according to their pattern of occurrence across time and geographic area.

Types of diseases
Endemic Epidemic Pandemic
Definition [2]
  • A disease that affects individuals at a relatively constant rate within a specific population in a given region
  • A disease that affects individuals at a rapid rate within a specific population in a given region
  • A disease that affects a wide geographic area, e.g., multiple countries or continents
  • Unlimited
  • Limited
  • Limited


  • Limited
  • Limited
  • Unlimited
Possible contributing factors
  • Spread of disease vectors and pathogen reservoirs
  • Geographical conditions
  • Climate
  • Living conditions (e.g., sewage systems, housing, work)
  • Infectivity of a pathogen: increased ability to multiply in a host
  • Living conditions (e.g., living in crowded areas)
  • Spread/introduction of the pathogen to a new geographical area


  • Measures of disease frequency can be used to: [3]
    • Quantify mortality and/or morbidity (i.e., incidence and prevalence of disease)
    • Describe population characteristics (e.g., at-risk populations, life expectancy)
    • Determine the association between two factors (e.g., exposure and disease)
  • Data is systematically collected and analyzed and then used for planning, implementing, and evaluating public health practices (i.e., public health surveillance), in order to:
    • Detect changes in health behavior, health issues, and identify potential outbreaks and epidemics
    • Estimate the magnitude of a health issue (e.g., disease or risk factor), measure trends, and characterize a disease
    • Identify individuals with infectious diseases or exposures to environmental agents and their contacts
    • Determine the effectiveness of control measures and public health programs
    • Develop future research hypotheses

Incidence and prevalence


  • Description: number of new cases [4]
  • Population at risk: the group of people that are at risk of developing the condition being studied (individuals at risk cannot have the condition at the time the study period starts)
  • Incidence study
    • Used to determine the incidence of a particular event in a population during a certain time period (usually a year). If the event in consideration is death, the study is called a mortality study.
    • Usually performed as a cohort study to compare the incidence of an event (e.g., disease) between two groups
Measures of incidence
Incidence rate Cumulative incidence
  • The number of new cases of a disease per unit of time
  • The proportion of new cases of disease (in an initially disease-free population) over a defined period of time
  • Measures risk of disease (using the same calculation as absolute risk)
  • In the setting of disease outbreaks, cumulative incidence is referred to as attack rate: the proportion of people who become ill in relation to the total number of people exposed.
  • Number of new cases/person-time units
  • Person-time units (denominator)
    • Describes the total time that the population was at risk
    • The time units can be in days, months, years, etc.
  • Number of new cases in a specific time period/number of people in a given population at risk
  • Individuals in a community are followed for 130,000 person-years of observation. 12 people develop lung cancer during that time. The incidence rate of lung cancer in that community would be 12 per 130,000 person-years (or 9 per 100,000 person-years)
  • Individuals in a community with a population of 150,000 are followed for 10 years. 12 people develop lung cancer during that time. The cumulative incidence of lung cancer in that community would be 12 per 150,000 individuals (or 8 per 100,000 individuals) over 10 years.


  • Description: number of current (i.e., new and preexisting) cases
  • Formula: total number of cases/total population during a specific time
    • The time period can be:
    • For example, if a survey conducted in 2020 in a town with a population of 120,000 reveals that 451 people have lung cancer, the prevalence of lung cancer in that town would be 451 per 120,000, which would be expressed as 375.8 per 100,000.

Relationship between prevalence and incidence

  • Formula: (Incidence) x (average duration of disease) = prevalence
  • In a chronic disease, the prevalence is usually greater than the incidence, while in an acute disease, the values are usually similar.
  • An increased prevalence with a stable incidence can be explained by factors that result in increased survival and/or prolonged duration of the disease (e.g., improved quality of care of patients).
  • A decreased prevalence with a stable incidence can be the result of factors that increase mortality and/or speed of healing time.
  • Extensive vaccine administration and/or the removal of risk factors can cause prevalence and incidence to decrease together.
  • the population is in a steady state, the relationship between incidence rate (IR), prevalence (P), and the average duration of the disease (T) can be described mathematically as follows:
    • P/(1 - P) = IR × T
    • IR = (P/(1 - P))/T
  • If the disease is extremely rare, P ≈ IR × T
  • Number of new cases per unit time = IR × (population at risk)


  • Definition: the number of deaths in a population within a specific time interval
Overview of other measures of mortality
Measure Description Formula [5]
Mortality rate (crude death rate)
  • Frequency of deaths in a population within a specific time interval
  • MR = (number of deaths during a specific time period)/(population size) × 100
  • Typically measured for one year and expressed as the number of deaths per 1000 individuals/year or 100,000 individuals/year
Case fatality rate (lethality)
  • Percentage of cases (patients with a specific condition) that result in death within a specific time interval
  • CFR = (number of deaths from a specific condition)/(number of cases with the same specific condition) × 100
Proportionate mortality rate
  • Percentage of deaths due to a specific condition within a specific time interval
  • PMR = (number of deaths from a specific cause in a specific time period)/(total deaths from all causes in that time period) × 100 or 1000
Fetal mortality rate
  • Rate of fetal deaths within a specific time interval
  • FMR = (number of fetal deaths of ≥ 20 weeks of gestation)/(total number of live births + fetal deaths) × 1,000
Neonatal mortality rate
  • Rate of neonatal deaths within a specific time interval
Postneonatal mortality rate
  • Rate of postneonatal deaths within a specific time interval (from 28 to 364 days of age)
  • PNMR = (number of infant deaths between 28 and 364 days of age)/(total number of live births) × 1,000
Infant mortality rate
  • Rate of total infant deaths within a specific time interval (from birth to 1 year of age) [5]
  • IMR = (number of infant deaths during the first year after birth)/(total number of live births) × 1,000
Perinatal mortality rate
  • The rate of fetal deaths (stillbirths) and early neonatal deaths within a specific time interval [6]
  • PMR = (number of fetal + neonatal deaths)/(total number of live births + fetal deaths) × 1,000
Under-five mortality rate
  • Rate of death of children younger than 5 years within a specific time interval [7]
  • U5MR = (number of deaths of children younger than 5 years)/(total number of live births) x 1,000
Maternal mortality rate
  • The rate of maternal deaths attributed to pregnancy within a specific time interval.
  • MMR = (maternal deaths)/(live childbirths) × 100,000
Standardized mortality ratio
  • The ratio of deaths observed in a specific population to deaths expected in the general population
  • SMR = (number of deaths observed in population)/(number of deaths expected in population)
Years of potential life lost
  • A measure of premature mortality; estimates the number of years a person in a given population would have lived if they hadn't died prematurely
  • Individual YPLL = (standard year) - (age at individual's death)
  • Population YPLL = sum of individual YPLL across the population

Morbidity [8]

  • Definition: the number of individuals in a population with a disease at a specific point in time or specific time interval (i.e., disease frequency)
  • Description
    • Estimated using incidence and/or prevalence data
    • Data sources include: health surveys, registries (e.g., cancer registries), hospital or general practice statistics (e.g., electronic health records)

Disease burden

The “Global Burden of Disease (GBD) Study” provides estimates of the burden of diseases globally (see “Tips and Links”).

  • Definitions: estimated impact of a disease on a given population
  • Can be estimated using, e.g.:
    • Disability-adjusted life years (DALYs): years of life lost due to disease, disability, and/or premature death
    • Quality-adjusted life years (QALYs): number of years a person is expected to live corrected for loss of quality of life caused by diseases and disabilities
    • Other indicators: financial cost, mortality, and/or morbidity

Other measures

  • Birth rate: the number of live births divided by the number of people in a population within a specific time interval [5]
  • Fertility rate: the number of live births among women of childbearing age (15–44 years) in a population within a specific time interval
  • Health-adjusted life expectancy: average number of years a person is expected to live in full health

Leading causes of death in the US [9]

Leading causes of death by age in the US
Age (in years) 1st 2nd 3rd
< 1 Congenital anomalies Low birth weight Maternal complications
1–4 Accident Congenital anomalies Homicide
5–14 Cancer Suicide
15–34 Suicide Homicide
35–44 Cancer Heart disease
45–64 Cancer Heart disease Accident
> 65 Heart disease Cancer Chronic respiratory disease
Leading causes of death by sex in the US
Female Individuals Male Individuals
1 Heart disease (21.8%) Heart disease (24.2%)
2 Cancer (20.7%) Cancer (21.9%)
3 Chronic lower respiratory disease (6.2%) Unintentional injury (7.6%)
4 Cerebrovascular disease (6.2%) Chronic lower respiratory disease (5.2%)
5 Alzheimer disease (6.1%) Stroke (4.3%)
6 Unintentional injury (4.4%) Diabetes mellitus (3.2%)
7 Diabetes mellitus (2.7%) Suicide (2.6%)
8 Influenza/pneumonia (2.1%) Alzheimer disease (2.6%)
9 Kidney disease (1.8%) Influenza/pneumonia (1.8%)
10 Sepsis (1.6%) Chronic liver disease (1.8%)
  1. Haupt A, Kane TT. Population Reference Bureau’s Population Handbook (Sixth Edition). Population Reference Bureau ; 2011
  2. Jekel JF, Katz DL, Elmore JG. Epidemiology, Biostatistics, and Preventive Medicine. Saunders ; 2007 : p. 3-4
  3. Alexander et al.. Epidemiologic Research and Information Center Notebook. Epidemiologic Research and Information Center (ERIC) Notebook. 2015 .
  4. Raymond S. Greenberg. Medical Epidemiology: Population Health and Effective Health Care, 5th Edition. McGraw-Hill ; 2015
  5. An Introduction to Applied Epidemiology and Biostatistics.
  6. Barfield WD. Standard Terminology for Fetal, Infant, and Perinatal Deaths. Pediatrics. 2016; 137 (5): p.e20160551. doi: 10.1542/peds.2016-0551 . | Open in Read by QxMD
  7. Under-five mortality. https://data.unicef.org/topic/child-survival/under-five-mortality/. Updated: August 1, 2021. Accessed: November 15, 2021.
  8. Wunsch G, Gourbin C. Mortality, morbidity and health in developed societies: a review of data sources. Genus. 2018; 74 (1). doi: 10.1186/s41118-018-0027-9 . | Open in Read by QxMD
  9. Xu et al.. Mortality in the United States, 2018. Centers for Disease Control and Prevention (CDC). 2020 .

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