For the strain against which the flu shot was the most effective, approximately how effective was the shot overall when that strain was least prevalent?
A. 13%
B. 20%
C. 27%
D. 48%
Answer:D
For the strain against which the flu shot was the most effective, approximately how effective was the shot overall when that strain was least prevalent?
A. 13% B. 20% C. 27% D. 48%
Explanation: D The flu shot is most effective against Strain C, which is least prevalent in March. To determine the overall efficacy of the flu shot at this time, multiply the prevalence of each strain of flu by the efficacy of the flu shot against that strain, and then add those products to get a weighted average of the efficacy of the shot: (0.23 × 0.35) + (0.25 × 0.13) + (0.13 × 0.76) + (0.39 × 0.68) = 0.477 = 47.7%, which is closest to (D).