Fits an Extended NOminal Response Model (ENORM) using conditional maximum likelihood (CML) or a Gibbs sampler for Bayesian estimation.

```
fit_enorm(
dataSrc,
predicate = NULL,
fixed_params = NULL,
method = c("CML", "Bayes"),
nDraws = 1000,
merge_within_persons = FALSE
)
```

- dataSrc
a connection to a dexter database, a matrix, or a data.frame with columns: person_id, item_id, item_score

- predicate
An optional expression to subset data, if NULL all data is used

- fixed_params
Optionally, a prms object from a previous analysis or a data.frame with parameters, see details.

- method
If CML, the estimation method will be Conditional Maximum Likelihood; otherwise, a Gibbs sampler will be used to produce a sample from the posterior

- nDraws
Number of Gibbs samples when estimation method is Bayes.

- merge_within_persons
whether to merge different booklets administered to the same person, enabling linking over persons as well as booklets.

An object of type `prms`

. The prms object can be cast to a data.frame of item parameters
using function `coef` or used directly as input for other Dexter functions.

To support some flexibility in fixing parameters, fixed_params can be a dexter prms object of a data.frame. If a data.frame, it should contain the columns item_id, item_score and a difficulty parameter. Three types of parameters are supported:

- delta/beta
thresholds between subsequent item categories

- eta
item-category parameters

- b
exp(-eta)

Each type corresponds to a different parametrization of the model.

Maris, G., Bechger, T.M. and San-Martin, E. (2015) A Gibbs sampler for the (extended) marginal Rasch model. Psychometrika. 80(4), 859-879.

Koops, J. and Bechger, T.M. and Maris, G. (in press); Bayesian inference for multistage and other incomplete designs. In Research for Practical Issues and Solutions in Computerized Multistage Testing. Routledge, London.

functions that accept a prms object as input: `ability`

, `plausible_values`

,
`plot.prms`

, and `plausible_scores`