help for pcmodel

Estimation of the parameters of a Partial Credit Model

pcmodel (varlist) [if],
[categorical(varlist)
continuous(varlist)
difficulties(matrix list)
iterate(#)
adapt
robust
from(matrix)
rsm
estimateonly]

Description

pcmodel allows estimating the parameters of a random effect
partial credit model or a random effect rating scale model
(the item difficulties, and the covariates
that may influence the considered latent trait are considered as fixed
effects, and the individual latent traits are considered as a normally distributed
random effect)

Two situations are possible:

- The item difficulties can be considered as already known
(for example provided by the scale developer).
In this case, they do not have to be estimated during the analysis.

- The difficulties are considered as unknowns and will
be estimated during the analysis.

pcmodel allows including covariates that
can possibly influence the individual latent traits in the considered model
(partial credit or rating scale). These covariates
can either be categorical or continuous.

pcmodel provides assistance for interpreting both the quality
of model fit (by estimating the marginal McFadden's pseudo R2) and the
contribution of covariates to the model
(by estimating the the type III sum of squares, the percentage
of variance explained with the introduction of each covariates and the percentage
of McFadden's pseudo R2 explained with the introduction of each
covariate).

It is finally possible to test the fit using pcmtest after
estimating parameters of the model with pcmodel

Options

categorical List of the categorical covariates included in
the Partial Credit model or the Rating Scale model.

continuous List of the continuous covariates included in
the Partial Credit model or the Rating Scale model.

difficulties Row vectors containing the known values
of each item difficulty (if they are known).
A row vector must match with each item, and have the
same name as the corresponding item. If the option difficulties
is not filled, the item difficulties are
considered as unknown, and they are estimated during the analysis.
(this option cannot be used with the rsm option

iterate specifies the (maximum) number of iterations. With the
adapt option, use of the iterate(#) option will cause pcmodel to skip the
"Newton Raphson" iterations usually performed at the end without updating
the quadrature locations.

adapt causes adaptive quadrature to be used instead of
ordinary quadrature.

robust specifies that the Huber/White/sandwich estimator of the
covariance matrix of the parameter estimates is to be used.

from specifies a row vector to be used for the initial values.
It is not necessary to specify column-names or equation-names for this line vector,
but this vector must have exactly the number of parameters to be estimated,
starting with the difficulties parameters, the parameters associated
with the covariates, and ending with the estimated standard deviation
of the latent trait.

rsm performs a Rating Scale model instead of a Partial Credit model.

estimateonly Do not perform the Marginal McFadden's pseudo R2
nor the type III sums of square computations

Outputs

e(mll): marginal log-likelihood

e(cn): Condition number

e(N): Number of observations

e(Nit): Number of items

e(Ncat): Number of categorical covariates

e(Ncont): Number of continuous covariates

e(sigma): Estimated standard deviation of the latent trait

e(Varsigma): Variance of the estimated standard deviation of
the latent trait

e(b): coefficient vector of the parameters associated with
the latent trait covariates (if no covariate is included in the model, value
of the average latent trait).

e(V): Covariance matrix for the latent trait covariates.

e(delta): Estimated difficulty parameters

e(Vardelta): Covariance matrix for the estimated difficulty
parameters

Example

Simulation of the data (using simirt):

. simirt, nbobs(200) dim(5) rsm1(0.2) group(0.5) deltagroup(0.4) clear(1)

Estimating a Partial Credit Model with pcmodel:

. pcmodel item*(2)

Estimating a Rating Sacle Model with pcmodel:

. pcmodel item*, rsm(3)

Testing the fit of the previously performed model with pcmtest:

. pcmtest, si (4)

Estimating a Partial Credit Model with pcmodel, considering that the item difficulties
are provided by the scale developer (-1 & -0.8 for item1, -0.5 & -0.3 for item2, 0 & 0.2 for
item3, 0.5 & 0.7 for item4 and 1 & 1.2 for item5) and that the group covariate may
influence the individual latent trait:

1/ Defining the row vectors containing the known values of each item difficulty, with the
same name as the corresponding item:

. matrix item1=(-1,-0.8)(5)
. matrix item2=(-0.5,-0.3)(6)
. matrix item3=(0,0.2)(7)
. matrix item4=(0.5,0.7)(8)
. matrix item5=(1,1.2)(9)

2/ Estimating the Partial Credit Model with item difficulties already known, including
the group covariate as a categorical covariate:

. pcmodel item*, difficulties(item1 item2 item3 item4 item5) cat(group)(10)

Author

Jean-François Hamel

Email:
jeanfrancois.hamel@chu-angers.fr

Also see

Online: help for pcmtest, gllamm, simirt,
raschtest.