can i calibrate na in r if an item is missing is it calibrated which r functions can cope with na's what is nert value how to calibrate na's in an r dataframe what irt model does tt6 use r what is the irt item in a dataset what is the irt item on a dataset what is na in r can an r function calibrate a data frame what is an irt item what is itt item irt item calibration function when calibrating item, what is the value? irt how do i calibrate the dataset with na what model do i use for item calibration r function to calibrate data frame na irt data what is irt calibration what model is used for r calibration?