IN = individual responses recorded as vectors in the data file (one line represents one individual's responses to items 1 through k). Alternatively, one could use PA for a response pattern matrix or TA for a fixed-effects table of counts instead of a matrix of individual responses. A discussion of these alternative data files are discussed in the MULTILOG manual.
RA = indicates that marginal maximum likelihood item parameter estimation is to be carried out due to the fact that we do not know the theta for the given items. Alternatively, FI could indicate fixed-theta item parameter estimation or SCORE for the computation of individual scores for the respondents.
NI = number of items, k (for this sample data file we have 10 items).
NE = number of examinees (in this case, 1500).
NCHAR = number of characters in the ID field for each respondent (4 characters for our sample data files, e.g., 1001, 1002…).
NG = numbers of groups - for our discussion, we will only be dealing with a single group at a time.