module 3a and 3b

Introduction to Epidemiological Methods

• The module focuses on defining mathematical terms related to epidemiology.

• It emphasizes the difference between incidence and prevalence in describing disease occurrence.

Key Mathematical Terms in Epidemiology

• Ratio: A major mathematical term in epidemiology, defined as the division of one number by another.

  • Examples include various ratios like risk ratio and odds ratio.

• Proportions, Rates, and Percentages:

  • Proportion: A type of ratio where the numerator is part of the denominator.

  • Rate: A ratio that includes a measure of time in the denominator, often used to express disease occurrences over a period (e.g., incidence rate).

  • Percentage: A proportion multiplied by 100, commonly used in reporting incidence and prevalence.

Example Calculations of Ratios

• Sex Ratio of AIDS:

  • Male deaths: 450,451; Female deaths: 89,895.

  • Male to female ratio ≈ 5 (for every woman who died, five men also died).

• Proportion of AIDS Deaths Among Males:

  • Proportion = Male deaths / Total deaths = 450,451 / (450,451 + 89,895) = 0.83.

  • Percentage = 0.83 * 100 = 83% of deaths occurred among males.

Incidence and Prevalence

• Incidence: Refers to new cases of disease or mortality in a population over a defined period of time.

  • Calculated by dividing new cases by the population at risk.

• Prevalence: Refers to all existing cases of a health condition at a specific time.

  • Variants include point prevalence, period prevalence, and lifetime prevalence.

Variants of Prevalence

• Point Prevalence: All existing cases at a specific time.

  • Example: 5% of cattle showing signs of bovine respiratory disease upon arrival at feedlot.

• Period Prevalence: Existing cases within a specific time frame (e.g., asthma cases within the past year).

• Lifetime Prevalence: Any diagnosed cases throughout a person's life (e.g., asthma in Texas and New York populations).

Relationship Between Incidence and Prevalence

• Understanding the difference is crucial:

  • Incidence relates to new cases, while prevalence concerns all existing cases.

  • Increased incidence leads to higher prevalence, but duration of disease also affects prevalence levels.

Reporting Health Events

• Importance of defining numerator and denominator clearly:

  • For example, in COVID case definitions, a positive test for SARS-CoV-2 is essential beyond symptoms.

  • Considerations of morbidity (severity reported) and mortality rates over time.

Quantitative Measures in Epidemiology

• Count: Total number of cases, but potentially misleading without context of population size.

• Incidence Rate: New cases over a specified time, expressed as a percentage to include only individuals at risk.

  • Example: Incidence rate for prostate cancer would only include males.

• Attack Rate: Calculated in specific exposures, such as foodborne outbreaks.

Measures of Mortality

• Mortality Measurement Types:

  • Crude rates (overall observations) vs. specific rates (based on subgroups).

  • Crude Death Rate: Total number of deaths in a year divided by the reference population.

  • Case Fatality Rate: Deaths due to specific illnesses among the ill.

• Proportional Mortality: Deaths within a population due to specific diseases (e.g., causes of death by heart disease).

Specific Rates

• Cause-Specific Rates: Deaths due to specific causes in the entire population.

• Age-Specific Rates: Deaths or illnesses within specific age groups, important for interpreting mortality data.

• Sex-Specific Rates: Death data analyzed by sex (e.g., calculating death rates for males).

Adjusting Mortality Rates

• Importance of adjusting rates to accurately compare populations, especially considering age factors.

• Understanding crude versus adjusted rates is essential to avoid misinterpretations.

Key Epidemiological Measures of Natality and Mortality

• Maternal Mortality Rate: Deaths due to childbirth problems per 100,000 live births.

• Infant Mortality Rate: Proportion of infants dying within their first year per live births.

• Fetal Mortality Rate: Fetal deaths beyond 20 weeks per total births (including live births and fetal deaths).

• Birth Rate: Births during a defined period relative to population size.

• General Fertility Rate: Births per women aged 15-44, useful for understanding demographic trends.

Conclusion

• The module provides fundamental insights into interpreting epidemiological data and establishes a foundation for further discussions on mortality, morbidity, and health outcomes in future modules.