pltree  R Documentation 
Recursive partitioning based on PlackettLuce models.
pltree(formula, data, worth, na.action, cluster, ref = NULL, ...)
formula 
A symbolic description of the model to be fitted, of the form

data 
An optional data object containing the variables in the model.
Either a data frame of variables in 
worth 
A optional formula specifying a linear model for logworth.
If 
na.action 
how NAs are treated for variables in 
cluster 
an optional vector of cluster IDs to be employed for clustered
covariances in the parameter stability tests, see 
ref 
an integer or character string specifying the reference item (for which log ability will be set to zero). If NULL the first item is used. 
... 
additional arguments, passed to 
PlackettLuce trees are an application of modelbased recursive partitioning
(implemented in mob
) to PlackettLuce models for
rankings. The partitioning is based on ranking covariates, e.g. attributes of
the judge making the ranking, or conditions under which the ranking is made.
The response should be a grouped_rankings
object that groups
rankings with common covariate values. This may be included in a data frame
alongside the covariates.
Most arguments of PlackettLuce
can be passed on by pltree
.
However, PlackettLuce tree with fixed adherence are not implemented.
Arguably it makes more sense to estimate adherence or reliability within
the nodes of the PlackettLuce tree.
Various methods are provided for "pltree"
objects, most of them
inherited from "modelparty"
objects (e.g. print
,
summary
), or "bttree"
objects (plot
). The plot
method employs the node_btplot
panelgenerating function. The See Also
section gives details of separately documented methods.
An object of class "pltree"
inheriting from "bttree"
and "modelparty"
.
bttree
For fitting BradleyTerry trees
(equivalent to the PlackettLuce model for paired comparisons without ties).
coef
, vcov
, AIC
and predict
methods are documented on
pltreesummaries
.
itempar
, extracts the abilities or item parameters
in each node of the tree using itempar.PlackettLuce
.
fitted
, computes probabilities for the observed
choices based on the full tree.
# BradleyTerry example
if (require(psychotree)){
## Germany's Next Topmodel 2007 data
data("Topmodel2007", package = "psychotree")
## convert paircomp object to grouped rankings
R < as.grouped_rankings(Topmodel2007$preference)
## rankings are grouped by judge
print(R[1:2,], max = 4)
## Topmodel2007[, 1] gives covariate values for each judge
print(Topmodel2007[1:2, 1])
## fit partition model based on all variables except preference
## set npseudo = 0 as all judges rank all models
tm_tree < pltree(R ~ ., data = Topmodel2007[, 1], minsize = 5,
npseudo = 0)
## plot shows abilities constrained to sum to 1
plot(tm_tree, abbreviate = 1, yscale = c(0, 0.5))
## instead show logabilities with Anja as reference (need to used index)
plot(tm_tree, abbreviate = 1, worth = FALSE, ref = 6,
yscale = c(1.5, 2.2))
## logabilities, zero sum contrast
itempar(tm_tree, log = TRUE)
}
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