Nonparametric unfolding item response theory (IRT) model for dichotomous data (see W.H. Van Schuur (1984). Structure in Political Beliefs: A New Model for Stochastic Unfolding with Application to European Party Activists, and W.J.Post (1992). Nonparametric Unfolding Models: A Latent Structure Approach). The package implements MUDFOLD (Multiple UniDimensional unFOLDing), an iterative item selection algorithm that constructs unfolding scales from dichotomous preferential-choice data without explicitly assuming a parametric form of the item response functions. Scale diagnostics from Post(1992) and estimates for the person locations proposed by Johnson(2006) and Van Schuur(1984) are also available. This model can be seen as the unfolding variant of Mokken(1971) scaling method.
In this version of the package nonparametric ordinary bootstrap uncertairty estimates for the model fit and the diagnostic statistics have been added.
The R package "yesno" is not used anymore at the data checking step of the main mudfold() function.
Changes have been made at the generic functions prin.mdf() and summary.mdf() in order to incorporate bootstrap results.
an additional functionality has been added to the plot.mdf() function. Now by setting the argument plot.type= "persons" the distribution of the person parameters will be returned.