Differences between the estimation of critical
for two
factor's levels.
1  criticaldiff(model, level1 = NULL, level2 = NULL, der = NULL)

model 
Parametric or nonparametric regression model
obtained by 
level1 
First factor's level at which to perform the differences between critical points. 
level2 
Second factor's level at which to perform the differences between critical points. 
der 
Number which determines any inference process. By default

Differences are calculated by subtracting a factor relative to
another (level2  level1). By default level2
and
level1
are NULL
, so the differences calculated are for all
possible combinations between two factors. Additionally, it is obtained
the 95% confidence interval for this difference which let us to make
inference about them.
An object is returned with the following elements:
critical.diff 
a table with a couple of factor's level where it is used to calculate the differences between the critical points, and their 95% confidence interval (for the estimation, first and second derivative). 
Marta Sestelo, Nora M. Villanueva and Javier RocaPardinas.
Sestelo, M. (2013). Development and computational implementation of estimation and inference methods in flexible regression models. Applications in Biology, Engineering and Environment. PhD Thesis, Department of Statistics and O.R. University of Vigo.
1 2 3 4 5 6  library(npregfast)
data(barnacle)
fit2 < frfast(DW ~ RC : F, data = barnacle, seed = 130853, nboot = 100) # with interactions
criticaldiff(fit2)
criticaldiff(fit2, der = 1)
criticaldiff(fit2, der = 1, level1 = "lens", level2 = "barca")

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