                         Changes in version 6.0                         

New functions

    o   Added `SimPolyDif()` and `SimDichoDif()` for simulating polytomous and dichotomous item responses with DIF.

    o   Added `difQuade()` for detecting DIF in ordinal items using Quade-type concordance indices (e.g., Goodman & Kruskal’s gamma, Kendall’s tau-a, Somers’ dyx).

    o   Added `Mantel.poly()` for detecting DIF in polytomous items using Mantel (1963).

    o   Introduced `LogistikPoly()` and `difPolyLogistic()` for ordinal logistic regression DIF detection (Zumb, 1999), with support for purification and flexible matching.

    o   Added `liu_agresti_ccor()` for computing the Liu & Agresti (1996) ordinal index.

    o   Integrated a suite of LASSO-based functions for DIF detection in dichotomous items (Magis et al., 2015):

    o   `lassoDIF()`, `lassoDIF.ABWIC()`, `lassoDIF.CV()`, and `lassoDIF.coef()`.

Enhancements

    o   Matching using the **restscore** is now supported in: `difQuade()`, `Mantel.poly()`, `LogistikPoly()`, `difPolyLogistic()`, as well as in updated versions of `difMH()` and `difLogistic()`.

    o   Added new `plot()` and `print()` S3 methods for objects returned by `difQuade()` and `difPolyLogistic()` (class `"Logistic"`).

    o   Improved modular structure and simulation capabilities to facilitate validation and comparison of DIF detection methods.

    o   Changes in difR 5.1 (2020-05-10) -

MAJOR UPDATES

    o   Option REML=FALSE as removed from LRT() function 
	 for compatibility.
	- Changes in difR 5.0 (2018-05-08) -

    o   The SIBTEST method is now available. It is 
	 implemented through the 'sibTest()' and 
	 'difSIBTEST()' functions that work similarly to
	 other DIF methods. These functions call the
	 'SIBTEST()' function from package 'mirt' (Chalmers,
	 2012).

    o   Functions 'selectDif()' and 'dichoDif()' were 
	 updated accordingly.

    o   All 'dif...()' functions return now an additional
	 argument called 'p.value' that directly provides
	 the p-values of the selected DIF method. This is
	 to simplify the extraction of the results for 
	 e.g., simulation studies. Many thanks to Phil
	 Chalmers (University fo Georgia, Athens, GA;
	 rphilip.chalmers@gmail.com) for suggesting this
	 improvement. See other poins below.

    o   For 'difMH()' function, the $p.value output
	 argument replaces the former $Pval argument (which
	 was returned for exact MH statistic only).

    o   For 'difStd()' function, the $p.value output
	 argument is not added since the DIF statistic has
	 no known NULL distribution.

    o   The 'difTID()' function has been thoroughly 
	 rewritten in order to call the 'deltaPlot()' 
	 function from package 'deltaPlotR' instead, as it
	 is more complete than the former 'difTID()' 
	 function.

    o   Consequently, the 'trItemDiff()' function was
	 removed and functions 'selectDif()' and 
	 'dichoDif()' were updated.

    o   The functions 'mantelHaenszel()', 'breslowDay()',
	 'genMantelHaenszel()', 'stdPDIF()', 'difMH()', 
	 'difBD()', 'difGMH()', 'difStd()', 'selectDif()',
	 'selectGenDif()', 'dichoDif()' and 'genDichoDif()'
	 were updated to allow the definition of the 
	 matching criterion from an external variable 
	 instead of the test score. This is specified 
	 through the new 'match' argument.

    o   The license for 'difR' package was updated to a
	 GPL license.

    o   Adjustement for multiple comparisons are now 
	 available for all DIF methods, except 'difStd()'
	 (standardized-P-DIF) and 'difTID()' (Transformed 
	 Item Difficulties). The argument 'p.adjust.method' 
	 was added to specify which adjustment to apply 
	 (e.g., "BH" for Benjamini-Hochberg). Adjusted 
	 p-values are then returned and DIF identification
	 is based on these adjusted p-values. Note that 
	 methods 'difStd()' and 'difTID()' do not return
	 p-values from null statistical distributions and
	 can therefore ot be updated for multiple 
	 comparisons. Many thanks to Patricia Martinkova
	 (Czech Academy of Sciences, Prague, Czech Republic; 
	 martinkova@cs.cas.cz) for suggesting this improvement.

    o   The functions 'selectDif()', 'selectGenDif()',
	 'dichoDif()' and 'genDichoDif()' were updated
	 accordingly. 

    o   The 'Logistik()' function returns now in addition 
	 the matrices 'seM0' and 'seM1' with the standard
	 error values of the parameter estimates in 'parM0'
	 and 'parM1' matrices (with the same format). 

    o   The 'difLogistic()' function returns now in addition 
	 the matrices 'logitSe', wth the standard error values
	 of the parameters hold in matrix 'logitPar', as well
	 as matrix 'seM0' from the output of 'Logistik()' 
	 function. 

    o   The functions 'Logistik()' and 'difLogistic()' have
	 a new argument 'all.cov' to specify whether all 
	 covariance matrices of model parameters should be 
	 returned (as lists), for both nested models and all 
	 items. Returned arguments are called 'cov.M0' and
	 'cov.M1'. Default is FALSE so these lists are 
	 returned as NULL values.

    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.

    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.

    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.                                                                                                                  

    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).

    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.

    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.

    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).

    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   DOI numbers were added to the references.

    o   Some typos were fixed.
	- Changes in difR 4.8 (2018-04-28) -

    o   Affiliations and email addresses were updated.

    o   Some typos were fixed.
	- Changes in difR 4.7 (2016-11-22) -

    o   A minor bug was fixed in the 'print.Lord' method. 
	 When the 3PL was used with fixed guessing parameter,
	 the 'print.Lord' function was reporting p-values 
	 computed with 3 degrees of freedom, while in this
	 case it should be 2 degrees of freedom. Many thanks
	 to Patricia Martinkova (Czech Academy of Sciences,
	 Prague, Czech Republic; martinkova@cs.cas.cz) for
	 noticing this issue.

    o   A minor bug was fixed in the 'difMH' method. In case
	 of item purification with asymptotic (not exact)
	 statistics, the DIF detection threshold returned by
	 the function was always the one from the chi-square
	 distrib ution, even when the 'logOR' statistic was
	 used (the computations were however correct).

    o   The CITATION file was updated.

    o   Several typos were fixed.
	- Changes in difR 4.6 (2015-04-29) -

    o   The DESCRIPTION file was updated to meet CRAN policy.

    o   Several typos were fixed.
	- Changes in difR 4.5 (2013-04-07) -

    o   The authors' affiliations were updated.

    o   Several typos were fixed.
	- Changes in difR 4.4 (2013-01-29) -

    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.
	- Changes in difR 4.3 (2012-06-13) -

    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.
	- Changes in difR 4.2 (2012-04-01) -

    o   The difR package was updated to compile under versions
	 2.14.0 and later of R.

    o   Several typos were fixed.
	- Changes in difR 4.1 (2011-06-22) -

    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!
	- Changes in difR 3.3 (2010-10-21) -

    o   Several help files have been updated to describe the 
	 corresponding 'plot' functions.

    o   Several typos have been fixed.
	- Changes in difR 3.2 (2010-10-04) -

    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. 
	- Changes in difR 3.1 (2010-06-22) -

    o   Some typos have been fixed. 

    o   The reference 'Aguerri et al.' in the 'breslowDay' and 'difBD'
	 commands has been updated.
	- Changes in difR 3.0 (2010-04-20) -

    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.
	- Changes in difR 2.2 (2010-01-23) -

    o   The system files LICENSE and NEWS have been rewritten for an 
	 easier display in Rgui. Several non-ASCII characters have also
	 been removed.
	- Changes in difR 2.1 (2010-01-13) -

    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.
	- Changes in difR 2.0 (2009-12-18) -

    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).
	- Changes in difR 1.1 (2009-10-27) -

    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.

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.
	- Changes in difR 4.0 (2011-03-04) -
