🤒Intro to Epidemiology Unit 2 – Measures of Disease Frequency

Key Concepts and Definitions

• Epidemiology studies the distribution and determinants of health-related states or events in specified populations
• Disease frequency measures quantify the occurrence of a disease or health condition in a population
• Incidence measures the number of new cases of a disease that develop in a population over a specified time period
• Prevalence measures the proportion of a population that has a disease at a specific point in time
• Mortality rates quantify the number of deaths due to a specific cause in a population over a defined time period
• Risk refers to the probability of an individual developing a disease over a specified time period
• Rate is a measure of the frequency with which an event occurs in a defined population over a specified time period

Types of Disease Frequency Measures

• Incidence measures include incidence rate, cumulative incidence, and attack rate
  • Incidence rate calculates the number of new cases per population at risk per unit of time
  • Cumulative incidence measures the proportion of a population that develops a disease over a specified time period
  • Attack rate is a type of cumulative incidence used in outbreak investigations
• Prevalence measures include point prevalence and period prevalence
  • Point prevalence measures the proportion of a population that has a disease at a specific point in time
  • Period prevalence measures the proportion of a population that has a disease at any time during a specified time period
• Mortality measures include crude mortality rate, cause-specific mortality rate, and case fatality rate
  • Crude mortality rate measures the number of deaths from all causes per population per unit of time
  • Cause-specific mortality rate measures the number of deaths due to a specific cause per population per unit of time
  • Case fatality rate measures the proportion of individuals with a disease who die from that disease over a specified time period

Incidence: Calculation and Interpretation

• Incidence rate is calculated as: $\text{Incidence Rate} = \frac{\text{Number of new cases}}{\text{Population at risk} \times \text{Time period}}$
• The numerator includes only new cases of the disease that develop during the specified time period
• The denominator includes the population at risk of developing the disease during the specified time period
• Incidence rates are often expressed per 1,000, 10,000, or 100,000 population per year
• Higher incidence rates indicate a greater risk of developing the disease in the population
• Incidence rates can be used to compare disease occurrence across different populations or time periods
• Changes in incidence rates over time can indicate changes in disease risk factors or the effectiveness of interventions

Prevalence: Calculation and Interpretation

• Point prevalence is calculated as: $\text{Point Prevalence} = \frac{\text{Number of existing cases at a specific point in time}}{\text{Total population at the same point in time}}$
• Period prevalence is calculated as: $\text{Period Prevalence} = \frac{\text{Number of existing cases during a specified time period}}{\text{Average population during the same time period}}$
• Prevalence measures the burden of disease in a population at a given time
• Higher prevalence indicates a greater proportion of the population affected by the disease
• Prevalence is influenced by both the incidence of the disease and the duration of the disease
• Chronic diseases tend to have higher prevalence than acute diseases
• Changes in prevalence over time can reflect changes in incidence, disease duration, or population demographics

Mortality Rates and Ratios

• Crude mortality rate is calculated as: $\text{Crude Mortality Rate} = \frac{\text{Number of deaths from all causes}}{\text{Total population} \times \text{Time period}}$
• Cause-specific mortality rate is calculated as: $\text{Cause-Specific Mortality Rate} = \frac{\text{Number of deaths from a specific cause}}{\text{Total population} \times \text{Time period}}$
• Case fatality rate is calculated as: $\text{Case Fatality Rate} = \frac{\text{Number of deaths from a specific disease}}{\text{Number of individuals diagnosed with the disease}} \times 100\%$
• Mortality rates are often expressed per 100,000 population per year
• Standardized mortality ratios (SMRs) compare the observed number of deaths in a population to the expected number of deaths based on a standard population
• SMRs greater than 1 indicate higher mortality than expected, while SMRs less than 1 indicate lower mortality than expected

Comparing Disease Frequencies

• Relative risk (RR) compares the risk of disease in an exposed group to the risk in an unexposed group: $\text{Relative Risk} = \frac{\text{Incidence in exposed group}}{\text{Incidence in unexposed group}}$
• Odds ratio (OR) compares the odds of disease in an exposed group to the odds in an unexposed group: $\text{Odds Ratio} = \frac{\text{Odds of disease in exposed group}}{\text{Odds of disease in unexposed group}}$
• Attributable risk (AR) measures the excess risk of disease in an exposed group compared to an unexposed group: $\text{Attributable Risk} = \text{Incidence in exposed group} - \text{Incidence in unexposed group}$
• Population attributable risk (PAR) measures the proportion of disease cases in a population that can be attributed to a specific exposure: $\text{Population Attributable Risk} = \frac{\text{Incidence in total population} - \text{Incidence in unexposed group}}{\text{Incidence in total population}}$
• These measures help identify risk factors for disease and quantify their impact on disease occurrence in a population

Applications in Public Health

• Disease frequency measures are used to monitor trends in disease occurrence over time
• Incidence rates can be used to identify outbreaks or emerging health threats
• Prevalence estimates inform resource allocation and healthcare planning for chronic diseases
• Mortality rates help prioritize public health interventions and evaluate their effectiveness
• Comparing disease frequencies across populations can identify health disparities and target interventions to high-risk groups
• Measures like attributable risk and population attributable risk guide public health policy decisions and prevention strategies
• Disease frequency measures are essential for epidemiological research and evidence-based public health practice

Common Pitfalls and Limitations

• Misclassification of disease status or exposure can bias disease frequency estimates
• Incomplete or inaccurate data sources can lead to underestimation or overestimation of disease frequency
• Changes in diagnostic criteria or case definitions over time can affect the comparability of disease frequency measures
• Differences in population demographics, healthcare access, or disease reporting can confound comparisons across populations
• Measures like relative risk and odds ratio do not provide information about the absolute risk of disease
• The choice of an appropriate denominator is crucial for calculating and interpreting disease frequency measures
• Interpreting disease frequency measures requires considering the context, limitations, and potential sources of bias in the data