Figure 2. Dependence of the necessary sample size on sensitivity and specificity for p 0=0.05, with left- and right-sided alternative. Sensitivity is coded by letters (a-100%, b-95%, c-90%, d-85%, e-80%)
For p 0<0.5 specificity has a stronger effect on the sample size. It is strongest if p 0is close to 0 and decreases towards 0.5. Effect of specificity is stronger for left-sided than for right-sided alternative (Figure 2). Effect of sensitivity is about the same in the whole range ofp 0, for left- as well as right-sided alternative.
Due to symmetry, for p 0>0.5 it is the other way round, influence of sensitivity is stronger than that of specificity. The latter does not depend much onp 0, nor on the alternative, whereas the former has the most dramatic effects for p 0 near 1, with right-sided alternative.