请问各位大神，有谁用过MASS包里的dose.p( )函数来计算LD50么？函数里的参数cf是什么意思呢？怎么用呢？dose.p(obj, cf = 1:2, p = 0.5)
obj         A fitted model object of class inheriting from "glm".
cf         The terms in the coefficient vector giving the intercept and coefficient of (log-)dose
p         Probabilities at which to predict the dose needed.