# Item Parameters Conversion Converts item and person parameters using equating coefficients. ```r convert(A, B, coef = NULL, person.par = NULL) ``` ## Arguments - `A`: equating coefficient A. - `B`: equating coefficient B. - `coef`: vector of item parameters return by function `modIRT`. - `person.par`: vector of person parameters estimates. ## Details Difficulty parameters $b$ are converted using transformation $b*A+B$; discrimination parameters $a$ are converted using transformation $a/A$; guessing parameters c are not transformed. Person parameters $\theta$ are converted using transformation $\theta*A+B$. ## Returns A list with components - **coef**: vector of item parameters transformed. - **person.par**: vector of person parameters transformed ## References Battauz, M. (2015). equateIRT: An R Package for IRT Test Equating. **Journal of Statistical Software**, 68 , 1--22. Kolen, M.J. and Brennan, R.L. (2014). **Test equating, scaling, and linking: methods and practices**, 3rd ed., New York: Springer ## Author(s) Michela Battauz ## See Also `itm`, `modIRT` ## Examples ```r # conversion using direct coefficients # three-parameter logistic model # direct equating coefficients between forms 1 and 2 using the Stocking-Lord method data(est3pl) test <- paste("test", 1:5, sep = "") mod3pl <- modIRT(coef = est3pl$coef, var = est3pl$var, names = test, display = FALSE) l12 <- direc(mod1 = mod3pl[1], mod2 = mod3pl[2], method = "Stocking-Lord") convert(A = l12$A, B = l12$B, coef = coef(mod3pl$test1)) # the conversion of item parameters is obtained also using itm(l12) # conversion using bisector coefficients # two-parameter logistic model # direct equating coefficients using the "Haebara" method data(est2pl) test <- paste("test", 1:5, sep = "") mod2pl <- modIRT(coef = est2pl$coef, var = est2pl$var, names = test, display = FALSE) direclist2pl <- alldirec(mods = mod2pl, method = "Haebara") # compute chain equating coefficients for path 1,2,3,4 pth1 <- paste("test", 1:4, sep = "") chainec1 <- chainec(direclist = direclist2pl, pths = pth1) # compute chain equating coefficients for path 1,5,4 pth2 <- c(paste("test", c(1,5,4), sep = "")) chainec2 <- chainec(direclist = direclist2pl, pths = pth2) # create a list of objects of class ceqc ecall <- c(chainec1, chainec2) # compute bisector and weighted bisector coefficients allec <- bisectorec(ecall = ecall, weighted = TRUE, unweighted = TRUE) summary(allec) eqc14<-eqc(allec, link = "test1.test4", path = "bisector") convert(A = eqc14$A, B = eqc14$B, coef = coef(mod2pl$test1), person.par = seq(-3, 3, 0.5)) ```

convert function