The package permits the covariate effects of trinomial regression models to be represented graphically by means of a ternary plot. The aim of the plots is helping the interpretation of regression coefficients in terms of the effects that a change in regressors’ values has on the probability distribution of the dependent variable. Such changes may involve either a single regressor, or a group of them (composite changes), and the package permits both cases to be handled in a user-friendly way. Theoretical and methodological details are illustrated and discussed in Santi, Dickson and Espa (2019).

The package can read the results of **both categorical and ordinal trinomial logit** regression fitted by various functions (see the next section) and creates a `field3logit`

object which may be represented by means of functions `gg3logit`

and `stat_3logit`

.

The `plot3logit`

package inherits graphical classes and methods from the package `ggtern`

(Hamilton 2018) which, in turn, is based on the package `ggplot2`

(Wickham 2016).

Graphical representation based on **standard graphics** is made available through the package `Ternary`

(Smith 2017) by functions `plot3logit`

and `TernaryField`

, and by the `plot`

method of `field3logit`

objects.

See the help of `field3logit`

for representing composite effects and `multifield3logit`

for drawing multiple fields.

Function `field3logit`

of package `plot3logit`

can read trinomial regression estimates from the output of the following functions:

`multinom`

of package`nnet`

(logit regression);`polr`

of package`MASS`

(ordinal logit regression);`mlogit`

of package`mlogit`

(logit regression).

Moreover, explicit matrix of regression coefficients can be passed to `field3logit`

. See the help of the package (type `? 'plot3logit-package'`

) for furhter details.

Fit a trilogit model by means of package `nnet`

where the student’s employment situation is analysed with respect to all variables in the dataset `cross_1year`

:

The gender effect is analysed by means of a ternary plot which is generated in two steps, however, package `plot3logit`

should be loaded:

Firstly, the vector field is computed:

Secondly, the field is represented on a ternary plot, using either `gg`

-graphics:

or standard graphics:

Hamilton N. E., M. Ferry (2018) “ggtern: Ternary Diagrams Using ggplot2”, *Journal of Statistical Software, Code Snippets*, **87**(3), pp. 1-17. https://doi.org/10.18637/jss.v087.c03

Santi F., M. M. Dickson, G. Espa (2019) “A graphical tool for interpreting regression coefficients of trinomial logit models”, *The American Statistician*, **73**(2), pp. 200-207. https://doi.org/10.1080/00031305.2018.1442368

Smith M. R. (2017). “Ternary: An R Package for Creating Ternary Plots”, *Zenodo*. https://doi.org/10.5281/zenodo.1068996

Wickham, H. (2016) *ggplot2: Elegant Graphics for Data Analysis.* Springer-Verlag, New York.