Suppose we want to observe the woman over n visits, where n is sufficiently large so that there is less than a 5% chance that her observed mean SBP will not differ from her true mean SBP by more than 5 mm Hg. What is the smallest value of n to achieve this goal? (Note: Assume two readings per visit.) 

Hypertension

Blood pressure readings are known to be highly variable. Suppose we have mean SBP for one individual over n visits with k readings per visit (X̅ _n,k). The variability of (X̅ _n,k) depends on n and k and is given by the formula σ_w² = σ_A²/n + σ²/(nk), where σ_A² = between visit variability and σ² = within visit variability. For 30- to 49-year-old Caucasian females, σ_A² = 42.9 and σ² = 12.8. For one individual, we also assume that X̅ _n,k is normally distributed about their true long-term mean = μ with variance = σ_w².