Population health

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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 age and sex distribution in a population ^[2]

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 ^[2]
• Demographic Transition Model: describes, in stages, the evolving relationship between birth and death rates in a population over time ^[2]
  • Stage I: high birth rates and high death rates (low or no population growth)
  • Stage II: high birth rates and decreasing death rates (high population growth)
  • Stage III: decreasing birth rates and low death rates (slowed population growth)
  • Stage IV: low birth rates and low death rates (low or no population growth)

The goal of healthy demographic transitions is to lower death rates, lower birth rates, and ensure a healthy aging population.

Endemic, epidemic, and pandemic diseases

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

Overview

• Measures of disease frequency can be used to: ^[4]
  • 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

Incidence

• Description: number of new cases ^[5]
• 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

Prevalence

• Description: number of current (i.e., new and preexisting) cases
  • The proportion of all people with a disease to the total number of people in a population at a particular point in time or time period
  • Corresponds to disease frequency (how common a disease is)
  • Reflects the pretest probability of a disease
  • Correlates directly with the positive predictive value and inversely with the negative predictive value (see “Evaluation of diagnostic tests” below)
• Formula: total number of cases/total population during a specific time
  • The time period can be:
    • A specific point in time (point prevalence), e.g., a day
    • A specific period of observation (period prevalence), e.g., during puberty
  • 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.
• Prevalence ratio ^[6]
  • Description: The ratio of the prevalence of a condition in one population (P₁) to the prevalence of the same condition in another population (P₂)
  • Formula: P₁/P₂ (prevalence of population 1/prevalence of population 2)
  • Interpretation
    • Prevalence ratio > 1.0: The condition is more prevalent in P₁ than in P₂.
    • Prevalence ratio < 1.0: The condition is more prevalent in P₂ than in P₁.
    • A prevalence ratio of 1 indicates that the prevalence is the same in both populations.

Relationship between prevalence and incidence

• Formulas
  • Prevalence = (incidence) × (average duration of disease)
  • If 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
      OR
    • 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)
• Distribution
  • Chronic disease: prevalence is usually greater than incidence
  • Acute disease: prevalence and incidence are usually similar
• Reasons for changes in prevalence and incidence
  • Increased prevalence with stable incidence
    • Increased survival (e.g., improved quality of health care)
    • Prolonged duration of the disease
  • Decreased prevalence with stable incidence
    • Increased mortality
    • Decreased recovery time
    • Introduction of new effective treatment
  • Increased prevalence and incidence: higher diagnostic sensitivity
  • Decreased prevalence and incidence
    • Extensive vaccine administration
    • Removal of risk factors
    • Mitigation of infectious disease spread (e.g., social distancing, contact tracing, masks, quarantine)

Mortality

• Definition: the number of deaths in a population within a specific time interval

Morbidity ^[10]

• 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 ^[7]
• 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 ^[11][12]