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* Changes and Developments in the difR Package *
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- Changes in difR 4.6 (2015-04-24) -
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MAJOR UPDATES
o The 'Logistik' function was rewritten. It now allows
to specify a matching criterion that is not the test
score (through the 'match' argument). It also permits
to specify a continuous group membership variable
instead of a binary group membership. This is done by
the 'member.type' argument. Finally, the item
responses are directly modeled in the logistic
regression, not as a two-way table anymore. This is
to fix an issue related to the computation of
Nagelkerke's R^2 coefficients (which are now returned
separately for the two fitted models). Many thanks to
Bruno Facon (Department of Psychology, Universite
Lille-Nord de France, France) for noticing this issue.
o Similarly, the 'genLogistik' function was rewritten.
It now allows to specify a matching criterion that is
not the test score (through the 'match' argument).
Also, the item responses are directly modeled in the
logistic regression, not as a two-way table anymore.
Nagelkerke's R^2 coefficients are also now returned
separately for the two fitted models.
o The 'difLogReg' function was created as a meta-function
for logistic regression DIF analysis. It encompasses
all options from 'difLogistic' and 'difGenLogistic'
and returns the corresponding output for simplicity of
use.
o The 'difLogistic', 'difGenlogistic', 'selectDif',
'selectGenDif', 'dichoDif' and 'genDichoDif' functions
were updated according to the previous modifications.
o The 'difMH', 'difStd', 'difBD', 'difTID', 'difLogistic',
'difGMH', 'difGenLogistic', 'difLogReg', 'difLord',
'difRaju' and 'difGenLord' methods now allow specifying
a set of anchor items (that are assumed to be DIF-free
by the user). The 'anchor' argument was added. If
supplied, anchor items are used to compute the test
score together with the tested item) and anchor items
are not tested for DIF. Item purification is disabled if
anchor items are provided.
o The functions 'selectDif', 'selectGenDif', 'dichoDif'
and 'genDichoDif' have been updated with the 'anchor'
argument.
MINOR UPDATES
o The DESCRIPTION file was updated to meet CRAN policy.
o Several typos were fixed.
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- Changes in difR 4.5 (2013-04-07) -
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MAJOR UPDATES
o In the 'itemPar2PL', 'itemPar3PL' and 'itemPar3PLconst'
functions, the extraction of the standard errors and
covariances between the item parameters was not
performed accurately from the output of the 'ltm' and
'tpm' functions. Actually, the values were extracted
from the linear parametrization of the model, not from
its IRT form. This was corrected by an appropriate
application of the Delta method. A note was also added
in the 'difR/doc' folder with mathematical details and
notations for the Delta method. Many thanks to Emily
Price (Department of Educational Research and Evaluation,
Ohio University) for noticing this issue, and to Dimitris
Rizopoulos (Erasmus University Rotterdam, The Netherlands)
for providing key information to fix this problem.
MINOR UPDATES
o The authors' affiliations were updated.
o Several typos were fixed.
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- Changes in difR 4.4 (2013-01-29) -
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MINOR UPDATES
o The 'mantelHaenszel' function was updated to avoid a
technical issue that was occuring in extreme cases.
When all items but one are flagged as DIF, and item
purification process is run, then 'mantelHaenszel' was
trying to compute 'rowSums' on a vector and not on a
one-column matrix, thus leading to an error. This was
fixed. Many thanks to Yung Chih Ou (Department of
Educational Psychology and Counseling, National Taiwan
Normal University) for noticing this issue.
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- Changes in difR 4.3 (2012-06-13) -
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MINOR UPDATES
o The computation of the R^2 coefficients in the
'Logistik' function was slightly modified to overcome
a mathematical issue. With large data sets, the null
and model deviances are very large and the ratio of
exponentials of the deviances might return NaN values
and affect the output of the 'difLogistic'. Many thanks
to Marco Pomati (University of Bristol, UK) for
noticing this issue.
o Several typos were fixed.
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- Changes in difR 4.2 (2012-04-01) -
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MAJOR UPDATES
o The Raju method was improved. Two ways of computing
the area between the ICC are available: the "unsigned"
one and the "signed" one. See Raju (1990) for further
details. Both the 'RajuZ' and the 'difRaju' have now
an additional 'signed' logical argument to select
which type of area to compute. Many thanks to Dylan
Molenaar (Department of Psychology, University of
Amsterdam, The Netherlands) for noticing this
alternative.
o Because of the previous update, the functions 'difRaju',
'plot.Raj', 'selectDif' and 'dichoDif' were updated.
o The exact Mantel-Haenszel test can now be carried out.
The argument 'exact' was added to the 'mantelHaenszel'
function. By default, the asymptotic test (Holland and
Thayer, 1988) is performed.
o Because of the previous update, the functions 'difMH',
'plot.MH', 'print.MH', 'selectDif' and 'dichoDif' were
updated.
MINOR UPDATES
o The difR package was updated to compile under versions
2.14.0 and later of R.
o Several typos were fixed.
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- Changes in difR 4.1 (2011-06-22) -
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MINOR UPDATE
o The R code used in 'mantelHaenszel' and 'difMH'
functions was modified to overcome a technical issue.
With some large data sets, the computation of MH DIF
statistics was impossible due to integer overflow. The
'mantelHaenszel' function was then returning NA values,
'difMH' was stopping with an error message because of
these NAs. The R code was modified to avoid integer
overeflow and ensure stable computation of MH DIF
statistics. Many thanks to Shih-Ying Yao (University
of California - Berkeley) for noticing this issue and
helping in solving it.
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- Changes in difR 4.0 (2011-03-04) -
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MAJOR UPDATES
o An important mistake in the R code has been fixed. Under
the 3PL model, Lord's chi-squared statistics were
were wrongly computed (see the 'LordChi2' help file for
further details). This has now been fixed. Many thanks
to J. Patrick Meyer (Curry School of Education,
University of Virginia) for having discovered this
mistake.
o The non-IRT methods of TID, Mantel-Haenszel, Breslow-
Day, standardization and generalized Mantel-Haenszel
can now handle data sets with missing values. They must
be coded as NA values into the data set. Basically,
they are withdrawn from the computation of sum-scores.
o The non-IRT methods of logistic regression and
generalized logistic regression can now handle data sets
with missing values. They must be coded as NA values
into the data set. Basically, they are withdrawn from
the computation of sum-scores and the fitting of
logistic models.
o The IRT methods of Lord, Raju, LRT and generalized Lord
can now handle data sets with missing values, as both
'ltm' and 'lme4' support for missing data. They must
be coded as NA values into the data set. They are
withdrawn from the fitting of the IRT models.
o The Transformed Item Difficulties (TID) method, also
known as Angoff's Delta method, is now available. The
function 'trItemDiff' computes the perpendicular
distances and the 'difTID' function returns the full
output, similarly to other DIF methods.
o An effect size measure has been added for the Lord and
Raju methods when the 1PL model is used. It is based
on the difference in item difficulties. See the help
files of the methods 'difLord' and 'difRaju'.
o The 'itemPar1PL' has a new argument, called 'discr', to
fix the common discrimination parameter. By default it
is equal to 1, and can be changed to any other value.
Alternatively, fixing it to 'NULL' will lead to the
estimation of this common parameter (only with the 'ltm'
package).
o The 'difLogistic' and 'difGenLogistic' functions return
now the fitted parameters of the null model M_0 (see
the help file for details on this null model). This is
to propose other graphical displays of the item curves
(see below).
o The 'plot.Logistic' and 'plot.genLogistic' functions
have been updated. The new argument 'itemFit' permits
to select whether the item curves are displayed
according to the best model (i.e. model M_0 if the item
is flagged as DIF and model M_1 otherwise) or the null
model (i.e. M_0).
MINOR UPDATES
o The functions 'selectDif' and 'dichoDif' have been
updated to allow for the TID method.
o The functions 'itemParEst', 'difLord', 'difRaju',
'difGenLord', 'selectDif', 'selectGenDif', 'dichoDif'
and 'genDichoDif' have been updated to incorporate the
new 'discr' argument of 'itemPar1PL' function.
o The former argument 'thr' of 'difStd' method has been
renamed to 'thrSTD', as there is now an equivalent one
for the TID method that is called 'thrTID'.
o The 'plot.Logistic' and 'plot.Lord' functions have a new
argument, called 'group.names', to provide the names of
the reference and focal group for the legend.
o The 'plot.genLogistic' and 'plot.GenLord' functions have
a new argument, called 'ref.name', to provide the name
of the reference group for the legend.
o The help files have been checked, typos have been fixed
and some descriptions have been rewritten for sake of
consistency.
o The 'print.dichoDif' function has been slightly
modified: the DIF methods are now printed as a list and
not one after each other.
o A minor bug was fixed in the 'print.dichoDif' function:
options of the Breslow-Day method were printed even
when this method was not requested!
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- Changes in difR 3.3 (2010-10-21) -
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MAJOR UPDATES
o It is now possible to capture and save the output of each
DIF method directly from the 'dif...' functions. The
arguments 'save.output' and 'output' control for saving
the output (as displayed by the corresponding 'print'
function) into a text file, whose name and path are
specified within the 'output' argument.
o The functions 'selectDif', 'selectGenDif', 'dichoDif'
and 'genDichoDif' have been adapted in order to allow
for saving their output too
o Similarly, the plots drawn from the 'plot' functions can
be captured into either PDF or JPEG format, by using the
newly added 'save.plot' and 'save.options' arguments.
They define the name of the figure, the path where it is
saved, and the format of the figure.
o In addition, the 'print' functions display a message to
specify whether the output was saved (and where the saved
file is located) or not, and the 'plot' functions display
a message into the R console to specify whether the plot
was not captured, the format is incorrect, or the plot was
captured and where it was stored.
MINOR UPDATES
o Several help files have been updated to describe the
corresponding 'plot' functions.
o Several typos have been fixed.
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- Changes in difR 3.2 (2010-10-04) -
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MINOR UPDATES
o The reference 'Magis, Beland, Tuerlinckx, and De Boeck (in
press)' has been updated as the paper is now published.
o Some typos have been fixed.
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- Changes in difR 3.1 (2010-06-22) -
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MAJOR UPDATES
o The variance of the log odds-ratio statistic (Philips and
Holland, 1987) has been added as output of the 'mantelHaenszel'
function.
o The 'difMH' method has a new input argument called 'MHstat'. It
specifies which DIF statistic must be considered. Default value
'MHChisq' sets the chi-square statistic, while 'logOR' imposes
the log odds-ratio statistic.
o The commands 'print.MH', 'plot.MH', 'selectDif' and 'dichoDif'
have been updated according to the changes in the 'difMH'
command.
o The 'breslowDay' method has a new input argument called 'BDstat'.
It specifies which DIF statistic must be considered. Default
value 'BD' sets the Breslow-Day statistic as recommended by
Aguerri et al. (2009), while 'trend' imposes the trend test
statistic as recommended by Penfield (2003).
o The commands 'print.BD', 'selectDif' and 'dichoDif' have been
updated according to the changes in the 'difBD' command.
MINOR UPDATES
o Some typos have been fixed.
o The reference 'Aguerri et al.' in the 'breslowDay' and 'difBD'
commands has been updated.
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- Changes in difR 3.0 (2010-04-20) -
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MAJOR UPDATES
o The Wald statistic can now be used to flag items as DIF with
both the logistic and the generalized logistic regression
method. The new argument 'criterion' permits to select between
'LRT', the usual likelihood-ratio test, and 'Wald' for the
Wald test. For the latter, an appropriate contrast matrix is
built up internally.
o The command 'subtestLogistic' has been added to identify DIF
items among a subset of groups of examinees, using the output
of the 'difGenLogistic' command and the Wald test. Subtests are
performed with appropriate contrast matrices built up
internally.
o The standardized P-DIF statistics can now be computed with
different weights. The default weights are based on the focal
group, but other options are weights based on the reference
group or on both groups together. The newly added argument
'stdWeight' in the functions 'stdPDIF' and 'difStd'.
o A second measure of effect size has been added for the
standardization method. It is a Delta measure, similar to
that of the Mantel-Haenszel method, and is described in Dorans
(1989, p.228, Eqn. 15).
MINOR UPDATES
o The DESCRIPTION and CITATION files have been updated.
o The help file references have been updated.
o Several typos have been fixed in the help files.
o As a consequence of the first aforementioned major update,
the first output argument of the 'Logistik' and 'genLogistik'
functions, previously called 'deviance', have been renamed to
'stat'.
o The functions 'selectDif', 'dichoDif', 'selectGenDif' and
'genDichoDif' have been updated to incorporate the different
changes of this version of difR.
o The coding of the effect size, which was previously done by
means of star symbols (*, ** and ***), has been replaced by
capital letters A, B and C for respectively negligible,
moderate and large DIF sizes. This is to match the usual
ETS conventions, and the change was made within the functions
'difMH', 'difStd', 'difLogistic' and 'difGenLogistic'.
o The TODO file has been added as a list of updates or adds-on
for further versions of difR.
o The general 'difR-package' Rd file has been created. It contains
a summary of the difR package, with a list of the methods
currently implemented and a list of related references.
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- Changes in difR 2.2 (2010-01-23) -
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MINOR UPDATES
o The system files LICENSE and NEWS have been rewritten for an
easier display in Rgui. Several non-ASCII characters have also
been removed.
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- Changes in difR 2.1 (2010-01-13) -
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MINOR UPDATES
o The file CITATION has been created and added to the difR package.
o The display of the output of Mantel-Haenszel, standardization,
logistic regression and generalized logistic regression have been
slightly modified: the codes for the effect sizes are now
returned as "Effect size codes" and not "Signif. codes" as in the
previous version of difR.
o Several issues regarding non-ASCII characters in the help files
have been fixed.
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- Changes in difR 2.0 (2009-12-18) -
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MAJOR UPDATES
o The logistic regression method has been improved. In addition to
the deviance statistics, the function 'Logistik' also returns the
fitted parameters of the two nested models which are compared by
means of the likelihood-ratio test. Moreover, the argument 'type'
has been added to test either nonuniform ('type="nudif"'),
uniform ('type="udif"') or both DIF effects simultaneously
('type="both"'). The 'difLogistic' function has been updated
acordingly, and returns the matrix with the fitted parameters of
the best model for each item.
o The 'plot.Logistic' function has been improved to allow for
plotting the logistic curves of any selected item according to
the best model retained in the 'difLogistic' function.
o The 'plot.Lord' function has been improved to allow for plotting
the item characteristic curves of any selected item, according to
the model fitted and the returned item parameter estimates by the
'difLord' function.
o The 'plot.GenLord' function has been improved to allow for
plotting the item characteristic curves of any selected item,
according to the model fitted and the returned item parameter
estimates by the 'difGenLord' function.
o The generalized logistic regression method is now available. It
extends the logistic regression to more than one focal group, by
an appropriate extension of the logistic models and the
comparison of model parameters. The functions 'genLogistik' and
'difGenlogistic' are the corresponding versions of 'Logistik' and
'difLogistic' for this generalized method.
o Generalized versions of the functions 'selectDif' and 'dichoDif'
have been developed for comparing the results of the generalized
tests that are currently available (generalized Mantel-Haenszel,
generalized Lord's chi-square test and generalized logistic
regression). These are the 'genSelectDif' and 'genDichoDif'
functions. They operate similarly to their "non-generalized"
versions.
o Some measures of effect size are now available. For
Mantel-Haenszel method, the effect size is the estimated common
odds ratio, up to a transformation, to get the Delta measure on
the ETS delta scale. For the standardization method, the
standardized P-DIF statistic is used as the measure of effect
size and the classification proposed by Dorans, Schmitt and
Bleistein (1992) is adopted. For logistic regression and
generalized logistic regression, the effect size is the
difference in nagelkerke's R^2 coefficients between the two
tested models. Both classification scales of Zumbo and Thomas
(1997) and Jodoign and Gierl (2001) are available. These measures
of effect sizes are displayed by the 'print' functions of the
different methods.
MINOR UPDATES
o The functions 'selectDif' and 'dichoDif' have been updated to fix
a minor bug occuring when Lord or Raju method was requested: the
arguments 'engine', 'irtParam' and 'same.scale' were not passed
correctly to thes functions.
o The examples taking a long time to run have been fixed using the
'Not run' approach.
o A minor bug was fixed in the Examples section of the
'mantelhaenszel' function: the definition of the data and group
variable were incorrect.
o The significance code for standardization method was corrected
in the 'difStd' function.
o The description of the package (in file DESCRIPTION) has been
slightly modified.
o The help files of the commands 'breslowDay' and 'difBD' have been
updated to mention that the currently implemented Breslow-Day
statistic is the one given by Aguerri et al. (in press) and NOT
the statistic for the test of trend in odds ratio heterogeneity
(Penfield, 2003).
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- Changes in difR 1.1 (2009-10-27) -
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MINOR UPDATES
o The output of 'itemPar2PL' function was corrected to fix a bug.
The parameter estimates of the item difficulties were actually
not returned as item difficulties (in the IRT sense) but as
additive intercept parameters as described in the help file of
the 'ltm' function of the eponymous package. Version 1.1 of
difR was corrected to return the correct item difficulty
estimates. Note that functions 'itemPar1PL', 'itemPar3PL' and
'itemPar3PLconst' were not affected by this bug.
o The listing of the authors was corrected in the reference manual
to fix a printing mistake.
o Several R codes in the Example sections have been preceded by the
'#' symbol, so that they are not executed when copy-pasting the
full R code from these sections. However, the R code itself
remains functioning but it was not executed during the building
of the difR package version 1.1. Anyone interested in seeing how
the command works has just to copy-paste the R code without the
'#' symbol. This is mentioned in the help files by the
'(remove #)' message.