# Plot the item characteristic (expected score) curve for focal and reference groups for the iiItem along with a representation of the focal group density. ```r PlotNcdif( iiItem, itemParameters, irtModel = "2pl", logistic = TRUE, plotDensity = FALSE, focalAbilities = NULL, focalDistribution = "norm", focalDistrExtra = list(mean = 0, sd = 1), from = -5, to = 5, thetaInt = 0.01, colour = TRUE, highColour = "blue", main = "", xlab = "Ability", ylab = "Probability", iccText = "Group ICCs", focalIccText = "Focal group ICC", referenceIccText = "Reference group ICC", focalDensityText = "Focal group density" ) ``` ## Arguments - `iiItem`: Item (row) number for the item in each of the itemParameter matrices to plot. - `itemParameters`: A list containing "focal" and "reference" item parameters. Item parameters are assumed to be on the same scale. Item parameters for each group should me a matrix with nrow equal to the number of items. - `irtModel`: A string stating the irtModel to be used. Should be one of "1pl", "2pl", "3pl", "grm" or "pcm". - `logistic`: A logical value stating if the IRT model will use the logistic or the normal metric. - `plotDensity`: logical indicating if the focal distribution density should be plotted as a density curve (TRUE) or if it should be represented as an area gradient (FALSE). Defaults to gradient. - `focalAbilities`: If NULL, density is calculated theoretically from focal distribution. If not NULL, it must be a numerical vector containing the abilities for the individuals in the focal group. - `focalDistribution`: A string stating the distribution name to be used for density calculation. Only used if focalAbilities is NULL. - `focalDistrExtra`: Extra parameters for the focal group distribution function if needed. - `from`: value on the x-axis to serve as minimum for the plot - `to`: value on the x-axis to serve as maximum for the plot - `thetaInt`: value for the x-axis step for probabilities and density evaluation. Only used if focalAbilities is NULL. - `colour`: logical value indicating if the area gradient should be presented in colour when plotDensity is FALSE, or if the different lines should be presented in colour when plotDensity is TRUE. - `highColour`: character indicating the colour text name that should be used for high density regions. - `main`: text for plot main title. - `xlab`: text for x-axis label. - `ylab`: text for y-axis label. - `iccText`: text for legend title related to ICC curves. - `focalIccText`: legend for focal group ICC curve. - `referenceIccText`: legend for reference group ICC curve. - `focalDensityText`: legend for focal group density curve when plotDensity is TRUE. Text for legend title related to the colour gradient when plotDensity is FALSE. ## Returns plotNCDIF A ggplot object for the plot ## Examples ```r data(dichotomousItemParameters) threePlParameters <- dichotomousItemParameters isNot3Pl <- ((dichotomousItemParameters[['focal']][, 3] == 0) | (dichotomousItemParameters[['reference']][, 3] == 0)) threePlParameters[['focal']] <- threePlParameters[['focal']][!isNot3Pl, ] threePlParameters[['reference']] <- threePlParameters[['reference']][!isNot3Pl, ] threePlParameters[['focal']][, 3] <- threePlParameters[['focal']][, 3] + 0.1 threePlParameters[['reference']][, 3] <- threePlParameters[['reference']][, 3] + 0.1 threePlParameters[['focal']][, 2] <- threePlParameters[['focal']][, 2] + 1.5 threePlParameters[['reference']][, 2] <- threePlParameters[['reference']][, 2] + 1.5 threePlParameters[['focal']] <- threePlParameters[['focal']][-c(12, 16, 28), ] threePlParameters[['reference']] <- threePlParameters[['reference']][-c(12, 16, 28), ] # # Non Uniform - != guess DIF item PlotNcdif(iiItem = 22, itemParameters = threePlParameters, irtModel = "3pl", plotDensity = FALSE, main = "Item 22 Non uniform and different guessing DIF. 3PL") # # Uniform - != guess DIF item PlotNcdif(iiItem = 15, itemParameters = threePlParameters, irtModel = "3pl", plotDensity = FALSE, main = "Item 15 Uniform and different guessing DIF. 3PL") ``` ## Author(s) Victor H. Cervantes

PlotNcdif function