Agent (Chapter 1)
- Infectivity = infected / susceptible × 100
- Pathogenicity = clinical cases / exposed × 100
- Virulence (case fatality) = deaths / cases × 100
Frequency (Chapter 4)
- Prevalence = existing cases / population
- Cumulative incidence (risk) = new cases / population at risk at start
- Incidence rate = new cases / animal-time at risk
- Prevalence ≈ incidence rate × average duration
- True prevalence = (apparent + Sp − 1) / (Se + Sp − 1) (Rogan–Gladen)
Association (Chapter 5)
- Relative risk RR = risk_exposed / risk_unexposed
- Odds ratio OR = (a·d) / (b·c)
- Attributable risk (exposed) ARe = risk_exposed − risk_unexposed
- Attributable fraction (exposed) AFe = (RR − 1) / RR
- Population attributable fraction PAF = (risk_total − risk_unexposed) / risk_total
Diagnostic tests (Chapter 6)
- Sensitivity = TP / (TP + FN); Specificity = TN / (TN + FP)
- PPV = TP / (TP + FP); NPV = TN / (TN + FN) (depend on prevalence)
- LR+ = Se / (1 − Sp); LR− = (1 − Se) / Sp
- Youden’s index = Se + Sp − 1
Sampling (Chapter 8)
- Estimate a proportion: n = Z² · p(1 − p) / d²
- Finite-population correction: n_adj = n / (1 + n/N)
Interventions (Chapter 10)
- Vaccine effectiveness VE = 1 − RR
- Herd-immunity threshold HIT = 1 − 1/R₀
- Critical vaccination coverage Vc = HIT / VE
Surveillance (Chapter 12)
- System sensitivity CSe = 1 − ∏(1 − Seᵢ)
- Probability of freedom (one round): posterior = prior / [prior + (1 − prior)(1 − CSe)]
Evidence synthesis (Chapter 15)
- Random-effects weight wᵢ = 1 / (vᵢ + τ²)
- Heterogeneity I² = (Q − df) / Q × 100
Genetics (Chapter 16)
- Heritability (half-sib) h² = 4 · σ²_sire / σ²_total
- Genetic gain ΔG = i · accuracy · σ_a (breeder’s equation)