The mxPath function creates MxThreshold objects. These consist of a list of ordinal variables and the thresholds that define the relationship between the observed ordinal variable and the continuous latent variable assumed to underly it. This function directly mirrors the usage of mxPath, but is used to specify thresholds rather than means, variances and bivariate relationships.The vars argument specifies which variables you wish to specify thresholds for. Variables are referenced by name, and these names must appear in the manifestVar argument of the mxModel function if thresholds are to be correctly processed. Additionally, variables for which thresholds are specified must be specified as ordinal factors in whatever data is included in the model.
The nThresh argument specifies how many thresholds are to be specified for the variable or variables included in the vars argument. The number of thresholds for a particular variable should be one fewer than the number of categories specified for that variable.
The free argument specifies whether the thresholds created by the mxThreshold function are free or fixed parameters. This argument may take either TRUE for free parameters, FALSE for fixed parameters, or a vector of TRUEs and FALSEs to be applied in order to the created thresholds.
The values is a numeric vectors containing the starting values of the created thresholds. values gives a starting value for estimation. The labels argument specifies the names of the parameters in the resulting MxThreshold object. The lbound and ubound arguments specify lower and upper bounds for the created threshold parameters.
Thresholds for multiple variables may be specified simultaneously by including a vector of variable names to the vars argument. When multiple variables are included in the vars argument, the length of the vars argument must be evenly divisable by the length of the nThresh argument. All subsequent arguments (free through ubound) should have their lengths be a factor of the total number of thresholds specified for all variables.
If four variables are included in the vars argument, then the nThresh argument should contain either one, two or four elements. If the nThresh argument specifies two thresholds for each variable, then free, values, and all subsequent arguments should specify eight values by including one, two, four or eight elements. Whenever fewer values are specified than are required (e.g., specify two values for eight thresholds), then the entire vector of values is repeated until the required number of values is reached, and will return an error if the correct number of values cannot be achieved by repeating the entire vector.