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Pharmacoeconomics: Open Access

ISSN: 2472-1042

Open Access

When do Rayleigh and Weibull parametric models predict the same results? A short note

Abstract

Martin Connock, Mubarak Patel, Daniel Gallacher and Aileen Clarke

Health Technology Appraisals (HTA) model survival data using a conventional set of six parametric distributions. Alternative and potentially useful distributions
exist and could also be exploited.
Rayleigh 2 parameter (R2P) and Weibull models are both defined with two parameters and exhibit monotonic hazard; the Rayleigh model is relatively unused in
HTA and should not be regarded as just a special case of the Weibull model.
Aim: To explore circumstances where the predictions from R2P and Weibull models may differ or coincide.
Method: Stata software was used to model sample survival data with R2P and Weibull models and to compare the predicted hazard and survival from each
parametric.
Results: R2P models generate different predictions to Weibull models except in the special case where the Weibull shape parameter equals or closely approaches
two and the Weibull hazard prediction is linearly increasing. When compared to Weibull models R2P models may generate a better fit to observed data according
to conventionally used indicators of goodness of fit.
Conclusion: Rayleigh modelling of survival warrants inclusion in the survival modelling undertaken in Health Technology Appraisals.

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