FlexParamCurve (version 1.3)

extraF.nls: Compare Two $nls$ Models Using Extra Sum-of-Squares F-Tests

Description

Function to compare two nested nls models using extra sum-of-squares F-Tests.

Usage

extraF.nls(submodel,

genmodel)

Arguments

submodel

$nls$ model with fewer curve parameters (reduced model)

genmodel

$nls$ model with more curve parameters (general model)

Value

• A data.frame listing the names of the models compared, F, numerator degrees of freedom, demonimator degrees of freedom, P value and the residual sum of squares for both the general and reduced models

Details

Models must be entered in the correct order with the reduced model appearing first in the call and the more general model appearing later. These must be nested models, i.e. the general model must contain all of the curve parameters in the reduced model and more. Entering models with the same number of parameters will produce NAs in the output, but the function will produce seemingly adequate output with non-nested models. The user must check that models are nested prior to use. This function is not promoted for use in model selection as differences in curves of different grouping levels in the dataset may be obscured when curves are fitted to the entire dataset, as in nls. Extra sum-of-squares is obtained from: F = (SS1 - SS2)(df1 - df2) / (SS2 / df2) where SS = sum-of-squares and df = degrees of freedom, for the more reduced model (1) and the more general model (2), respectively. To account for missing individuals for different fits df are scaled in all models to the value they would be if all individuals fit successfully (note that if all individuals had the same fit, this would not influence extra sum of squares). If the F value is significant then the more general model provides a significant improvement over the reduced model, but if the models are not significantly different then the reduced parameter model is to be preferred.

References

Ritz, C. and Streibigg, J. C. (2008) $Nonlinear regression with R.$ Springer-Verlag, New York.

See Also

extraF nls pn_modselect_step pn_mod_compare

Examples

#fit and compare two nested nls models (7 vs 8 parameter models)

data(posneg_data)

   modpar(posneg_data$age, posneg_data$mass) #create pnmodelparams for fixed parameters

   richardsR1.nls <- nls(mass ~ SSposnegRichards(age, Asym = Asym, K = K,

   Infl = Infl, M = M, RAsym = RAsym, Rk = Rk, Ri = Ri, RM = RM, modno = 1)

                        , data = posneg_data)

   richardsR2.nls <- nls(mass ~ SSposnegRichards(age, Asym = Asym, K = K,

   Infl = Infl, M = M, RAsym = RAsym, Rk = Rk, Ri = Ri, modno = 2)

                        , data = posneg_data)

   extraF.nls(richardsR2.nls, richardsR1.nls)