This document is intended for readers who are familiar with the international safety standard IEC 61508 [Ref. 1] in general and with that document’s Part 7: Annex D [Ref. 2] in particular. As currently written, Annex D provides “initial guidelines on the use of a probabilistic approach to determining safety integrity for pre-developed software” (SW) included in safety instrumented functions. It further states that “the annex provides an indication of what is possible, but the techniques should be used only by those who are competent in statistical analysis.” If these guidelines are to be used effectively in the testing and certification of safety-related SW it is essential that individuals involved in testing and certifying such SW understand how to interpret these guidelines correctly. To this end, this document explains the possibilities and limitations inherent in the information contained in IEC61508-7 Annex D.