A Study of Distributional Shape in Life Testing

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Abstract
Exponential, Weibull, gamma and log-normal models are frequently used in the analysis of failure time data. By extending the generalized gamma model of Stacy [l5]. Prentice [13] showed that the models listed are all embraced by a single parametric family. Likelihood methods were described for discrimination among the special cases and for assessment of each within the more comprehensive framework. Here, such methods are applied to several recent data sets from the industrial and medical literature in order to study distributional shape. New results are given for the accommodation of censoring and regression variables. The appropriateness of Weibull and lognormal models is emphasized.