Thor Osborn, Principal Systems Research Analyst, Sandia National Laboratories

Parametric survival analysis is often used to characterize the probabilistic transitions of entities — people, plants, products, etc. — between clearly defined categorical states of being. Such analyses model duration-dependent processes as compact, continuous distributions, with corresponding transition probabilities for individual entities as functions of duration and effect variables. The most appropriate survival distribution for a data set is often unclear, however, because the underlying physical processes are poorly understood. In such cases a collection of common parametric survival distributions may be tried (e.g., the Lognormal, Weibull, Frechét and Loglogistic distributions) to identify the one that best fits the data. Applying a diverse set of options improves the likelihood of finding a model of adequate quality for many practical purposes, but this approach offers little insight into the processes governing the transition of interest. Each of the commonly used survival distributions is founded on a differentiating structural theme that may offer valuable perspective in framing appropriate questions and hypotheses for deeper investigation. This paper clarifies the fundamental mechanisms behind each of the more commonly used survival distributions, considering the heuristic value of each mechanism in relation to process inquiry and comprehension.