On Farmer’s Line, Probability Density Functions, and Overall Risk

Limit lines used to define quantitative probabilistic safety goals can be categorized according to whether they are based on discrete pairs of event sequences and associated probabilities, on probability density functions (pdf ‘s), or on complementary cumulative density functions (CCDFs). In particular, the concept of the well-known Farmer’s line and its subsequent reinterpretations is clarified. It is shown that Farmer’s lines are pdf ’s and, therefore, the overall risk (defined as the expected value of the pdf) that they represent can be easily calculated. It is also shown that the area under Farmer’s line is proportional to probability, while the areas under CCDFs are generally proportional to expected value.