## Understanding Disease Frequency Measures **Key Point:** The cross-sectional survey captures existing cases at a single point in time, which is the definition of **prevalence**, not incidence. ### Distinction Between Incidence and Prevalence | Measure | Definition | Timing | Formula | Use Case | |---------|-----------|--------|---------|----------| | **Prevalence** | Proportion of population with disease at a specific point in time | Point-in-time snapshot | Cases at time t / Total population at time t | Cross-sectional surveys, burden of disease | | **Incidence** | Rate of new cases occurring in disease-free population over a period | Prospective, over time | New cases / Person-time at risk | Causation, risk factors, prognosis | | **Attack Rate** | Proportion of exposed persons who develop disease during an outbreak | Outbreak setting | New cases / Exposed population | Foodborne illness, epidemic investigation | | **Case Fatality Rate** | Proportion of diagnosed cases that result in death | Outcome measure | Deaths from disease / Total cases with disease | Disease severity, outcome assessment | **High-Yield:** In this scenario: - **Prevalence = 75/500 = 0.15 or 15%** — this is what the cross-sectional survey directly measures - Incidence cannot be calculated without follow-up data over time - Attack rate applies to outbreak investigations, not endemic disease surveys - Case fatality rate requires mortality data, not just disease presence **Clinical Pearl:** Cross-sectional studies are the gold standard for measuring **prevalence** because they capture the disease burden at a single point in time. This is why prevalence is also called "point prevalence." **Mnemonic:** **PICA** — **P**revalence = **P**oint in time (cross-sectional); **I**ncidence = **I**nterval over time (prospective follow-up).
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