Epidemiology: Understanding Disease Patterns and Risk Factors

Posted on Jan 16, 2025 in Statistics

Epidemiology

Epidemiology is the study of the distribution and determinants of disease or other health-related outcomes in human populations. It is also the application of that study to control health problems.

Etiology

  • Etiology refers to all the determinants of a disease.
  • These determinants can be physical, psychological, or behavioral.
  • Rarely is there just one determinant.

Prevalence

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  • Period Prevalence: The proportion of cases over a length of time.
  • Point Prevalence: The proportion of cases at one specific time.

Incidence

  • Incidence refers to the number of new cases over a set amount of time.
  • Incidence Proportion:
    • Out of all the people who could have the disease, how many actually did?
    • Incidence proportion is the number of new cases of the disease per person at risk.

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  • Incidence Rate:
    • This measure uses ‘person-time’ at risk.
    • It is the sum of individual units of time that a person at risk for developing a disease has been followed.
    • Formula: (New cases of disease in a period of time) / Total person-time at risk.
    • Units: per-person-month (PM), per-person-year (PY), etc.

Prevalence vs. Incidence

  • The continuous addition of new cases (incidence) increases prevalence, while deaths or cures decrease it.
  • Prevalence = Incidence * Duration

Measures of Morbidity

  • Mortality Rate:
    • Deaths from all causes / Population at a point in time.
  • Case Fatality:
    • Deaths by the disease / Population at a time with the disease.

Risk Ratio

  • Risk ratio is used for comparing incidence with different population sizes.
  • Example:
    • Group A: (3 have disease / 12 population) = 0.25
    • Group B: (6 have disease / 10 population) = 0.6
    • RR = 0.25 / 0.6 = 0.416
    • For every 1 new case in Group B, we have 0.416 new cases in Group A.
    • Group A has the lower risk ratio, therefore a lower risk of disease.

Contingency Tables

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Quantifying Risk Ratio

  • Risk ratio ranges from 0 to infinity.
  • RR > 1: Exposure is positively associated with the outcome.
  • RR = 1: No association between exposure and outcome.

Interpreting Risk Ratio

  • Those who are in the [exposed group] are [more or less] likely to experience [the outcome]. The risk of [disease] was [RR] times as likely in [exposed] compared to [unexposed] over [x] days, months, or years.
  • Example: Among young adults, those who hiked had a relative risk of 1.2 for Lyme disease compared to non-hikers over one hiking season.
    • Hikers have a 1.2 times higher likelihood, a 1.2-fold risk, or a 20% increased risk.

Formulas

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