Comparisons require stability. Examinees have changed. Item difficulties have drifted or been modified by instructional effects. The raters have changed, even if minutely. The empirical definitions of the rating scale categories have altered.
Comparisons require a stable frame of reference.
In order to compare performance across time, all other changes across time must be eliminated or controlled. Most test items retain approximately stable difficulties over the testing period. The data set can be reformatted so that each of these items is split into two items. Rating scales often present a challenge. To an examinee unfamiliar with the Greek alphabet, all Greek words are exceedingly difficult.