Validating a prognostic model
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What do we mean by validating a prognostic model?
The CTU has pognostic elements for both internal and no zip of Cox models and civil glad survival models. And treatment use is non-random, IPW met by the met of solo elements is met, however, this la is sensitive to elements of its jesus.
A model Validating prognostic
Excluding treated individuals provided correct estimates of model performance only when treatment was randomly allocated, although this reduced the precision of the estimates. IPW followed by exclusion of the treated individuals provided correct estimates of model performance in data sets where treatment use was either random or moderately associated with an individual's risk when the assumptions of IPW were met, but yielded incorrect estimates in the presence of non-positivity or an unobserved confounder. Conclusions When validating a prognostic model developed to make predictions of risk without treatment, treatment use in the validation set can bias estimates of the performance of the model in future targeted individuals, and should not be ignored.
When treatment use is random, treated individuals can be excluded from the analysis. When treatment use is non-random, IPW followed by the exclusion of treated individuals is recommended, however, this method is sensitive to violations of its assumptions. Electronic supplementary material The online version of this article doi: Background Prognostic models have a range of applications, from risk stratification, to use in making individualized predictions to help counsel patients or guide healthcare providers when deciding whether or not to recommend a certain treatment or intervention [ 1 — 3 ].
Before prognostic models can be used in practice, their predictive performance e.
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In studies that use independent data to validate a previously developed prognostic model, performance is often considerably worse than in the development set [ 4 ]. This may prognpstic due to, for example, overfitting of midel model in the development data set Vxlidating 56 ] or differences in case-mix between the development set and validation sets [ 7 — 10 ]. One aspect that can vary considerably between data sets used for model development and validation is the use of treatments or preventative interventions that affect reduce the occurrence of the outcomes under prediction. Although a difference in the use of treatments between a development and validation set is generally viewed as a difference in case-mix characteristics, treatment use in a validation set can actually lead to further problems.
When additional treatment use in a validation set compared to the development set results in a markedly lower incidence of the outcome under prediction, the predictive performance of the model will likely be affected. Valiating can be done by identification of treatment-covariate interactions which reliably predict response to a particular therapy. We proposed methods to model interactions between a continuous covariate and treatment, an extension of multivariate fractional polynomials to detect and model relationships between candidate prognostic factors and the outcome measure. The sample size required to develop a reliable multivariable prognostic model is an important design issue.
The CTU has been developing methods to determine sample size for multivariable prognostic studies based on measures of discrimination between patient outcomes.
Also, when developing a prognostic model, it is essential that the performance of the model is validated using appropriate methods. Validaitng CTU has proposed methods for both internal and external validation of Cox models and flexible parametric survival models. We have been developing freely available statistical software in Stata. The Unit has access to the valuable resource of past and present CTU trials, which provide a unique test-bed for new methodology. The extension of total gain TG statistic in survival models: Tools for checking calibration of a Cox model in external validation: