Perspective on Weibull proportional-hazards models

This note uses a paper of Elsayed & Chan (1990) to illustrate some of the advantages and some of the limitations of the proportional hazards approach. The role of proportional hazards as one of several tools for exploratory data analysis is described. The emphasis is on exploratory techniques as a way of: (1) measuring the importance of factors influencing system behavior; and (2) determining the form of the model. The semi-parametric version of proportional hazards shows the relative importance of explanatory factors in determining the failure behavior regardless of whether the model is strictly correct. Thus the relative chance of failure can be assessed, but not the absolute chance. The advantage of proportional hazards is that it always yields a quantitative measure of importance for each influence factor. Although Elsayed & Chan clearly establish the importance of temperature as the most critical factor in thin-oxide breakdown, the other analysis technique indicates that more needs to be done to validate a particular model of system behavior. In this case, the failure mechanism remains open, and the use of accelerated test data to predict performance under usual conditions needs further investigation