US EPA

The multistage Weibull (MSW) Time-to-tumor model and related documentation were posted on December 2009, principally (but not exclusively) for conducting time-to-tumor analyses to support risk assessments under the IRIS program. This latest R-based graphical tool [gofplot_msw()] generates diagnostic plots with MSW outputs and assist users to assess goodness-of-fit for models fitted using MSW.

This MSW plotting tool includes several plot types found useful for evaluating goodness-of-fit of survival functions (Collett 1994, 2003; Klein and Moeschberger 2003), including:

• Probability vs. Time plot,
• Dose-Response plot,
• Hazard plot,
• Quantile-Quantile plot
• and Probability-Probability plot.

The general objective of this R-based tool is to support the peer reviewed EPA version of the multistage Weibull (MSW) Time-to-tumor model. For all MSW users, both internal and external of EPA, the tool can be used to assess the goodness-of-fit of the (parametric) MSW model by comparing it to a nonparametric model fitted to the same data. The nonparametric model imposes only the most necessary restrictions (esp. monotonicity) on the relationship between time, dose, and probability of tumor onset or death, with no assumption made on the specific distributional form of the data. By minimizing the restrictions on the structure of the model, the empirical nonparametric model fits the data as “closely” as possible. Comparisons between the parametric and nonparametric models provide a subjective assessment for goodness-of-fit of the multistage Weibull Time-to-tumor model to the data. 

U.S. EPA. Producing Plots to Assess Goodness-of-Fit for the Multistage Weibull (MSW) Time-to-Tumor Model [gofplot_msw() (Version1.0)]. U.S. Environmental Protection Agency, Washington, DC, 2023.