MNLogit.prepare_X_matrix

MNLogit.prepare_X_matrix(df, utility_formulas, depvar)

Prepare the X matrix for the utility equations.

The X matrix is a tensor with dimensions: (N observations) x (Alternatives) x (Covariates). Assumes that the utility formulas specify an equal number of covariates per alternative; these can be zero-valued if an alternative lacks a specific attribute.

Utility formulas should express the relationship between the choice outcome (dependent variable) and the attributes of each alternative that incentivize that choice. The left-hand side (LHS) of each formula must correspond to a value of the dependent variable, while the right-hand side (RHS) should define an additive combination of the available covariates.

We also allow the incorporation of fixed covariates that do not vary across alternatives. For these, an alternative-specific parameter is used to allow the contribution to utility to vary by alternative.