(iii) Variables with dichotomized P values>0.3 were added to the model, one by one, and any variable that reduced the deviance significantly was retained in the model.
This model can be written as follows: logit (FPC ) = 0.61 + 0.68 log (CPR ) + 3.57 CP R 2 Compared to the simpler linear model, this alternative is better both in terms of reducing the deviance (p=0.028) and visually (Fig. 2).
Next, we set sensitivity parameters to be 1 and fit for the systematic negative error parameters (Systematic Error Model in Additional File 2) to test their effect on reducing the deviance.
Our Risk Difference Model allows for the initial risk difference between the intervention groups and control groups and reduced the deviance from 23.54 in the Simple Model to 18.5, and thereby achieved the smallest deviance measure among all six models (ΔDev = 5.04, Δdf = 1, p = 0.02).
The tree construction process attempts to reduce the node deviance, defined as ∑ y − y ¯ 2, where y is the relative bias and y ¯ is the mean relative bias within a node.
The addition of the prognostic score to the model with clinical covariates reduced the residual deviance with a X1 = 2.96, p = 0.09.
The inclusion of the additional interaction term reduced the residual deviance by 0.38, 2.80 and 0.08 respectively on 4, 1 and 1 degrees of freedom; these reductions are all small in magnitude and none are statistically significant.
In particular, adding SSR type as an explanatory factor to the null model reduces the residual deviance by over 1500, which is clearly statistically significant (p < 0.0001 when compared to a chi-squared distribution with 18 degrees of freedom).
Splitting stops when the cases in a subset are either entirely, or almost entirely, of the same class or the same value, or when further splitting does not improve discrimination between cases (i.e. does not substantially reduce deviance).
For COPD the deviance reduces from 1,643.4 on1199 degrees of freedom to 433.3 on 103 degrees of freedom on fitting the basic model.
