The IRT Procedure

If one variable in your data set represents the frequency of occurrence for the other values in the observation, specify the variable’s name in a FREQ statement. PROC IRT then treats the data set as if each observation appears times, where is the value of the FREQ variable for observation i. Only the integer portion of the value is used. If the value of the FREQ variable is less than 1 or is missing, that observation is not included in the analysis. The total number of observations is considered to be the sum of the FREQ values.