S D t_{s} t_{d} t_{s} < t_{d}

M(V, t)

V t M_{e}(S, t_{s}, D, t_{d})

S t_{s} D t_{d}

ε(S, t_{s}, D, t_{d}) = \frac{M_{e}(S, t_{s}, D, t_{d})}{M(S, t_{s})M(D, t_{d})}

(1) M(S, t_{s})ε(S, t_{s}, D, t_{d})