# Rearrange the data matrix for the Detection of DIF using the Lasso Approach (Magis et al. (2015) A Function that rearrange the matrix to use the lasso DIF detection for dichotomous items. ```r LassoData(Data, group) ``` ## Arguments - `Data`: numeric: either the data matrix only, or the data matrix plus the vector of group membership. - `group`: numeric or character: either the vector of group membership or the column indicator (within Data) of group membership. ## Returns A matrix of five columns where, respectively,: - **SCORE**: is the total score. - **GROUP**: is the group membership. - **PERS**: is the number of the respondent. - **Y**: is the dichotomous answer to the item. Only "0" and "1" are allowed. - **ITEM**: is the item name (must be a character). ## Details This function rearranges the data matrix for use in lasso-based DIF detection with dichotomous items. It requires a matrix of dichotomous item responses and a vector indicating group membership. ## References Magis, D., Tuerlinckx, F., & De Boeck, P. (2015). Detection of Differential Item Functioning Using the Lasso Approach. Journal of Educational and Behavioral Statistics, 40(2), 111–135. https://doi.org/10.3102/1076998614559747 ## Author(s) Sebastien Beland Faculte des sciences de l'education Universite de Montreal (Canada) sebastien.beland@umontreal.ca Carl F. Falk Department of Psychology McGill University (Canada) carl.falk@mcgill.ca , [https://www.mcgill.ca/psychology/carl-f-falk](https://www.mcgill.ca/psychology/carl-f-falk) ## Examples ```r ## Not run: # Example with the verbal data data(verbal) LassoData(Data=verbal[,1:24], group=verbal[,26]) # Example with SimDichoDif to generate uniform DIF It <- 15 # number of items ItDIFa <- NULL ItDIFb <- c(1,3) NR <- 100 # number of responses for group 1 (reference) NF <- 100 # number of responses for group 2 (focal) a <- rep(1,It) b <- rnorm(It,1,.5) Gb <- rep(2,2) # Group value for U-DIF Ga <- 0 # Group value for NU-DIF: need to be fix to 0 for U-DIF Out1 <- SimDichoDif(It,ItDIFa,ItDIFb,NR,NF,a,b,Ga,Gb) Data<-Out1$data[,1:15] Member<-Out1$data[,16] LassoData(Data=Data, group=Member) ## End(Not run) ```

LassoData function