The development of an instrument typically requires a reliability study. For formative assessment instruments, however, reliability is less of a concern unless a formative instrument gets used as a research tool or for summative purposes. When that happens, the formative instrument needs to be evaluated for reliability. The POSTT is one of those instruments that was developed for formative purposes and then subsequently used in research; but little attention has been paid to reliability. Since only a few researchers have reported the reliability of the POSTT, it is important to investigate its reliability. The primary purpose of the study reported in this paper was to determine the reliability of the POSTT using a test and retest method. The study was a secondary analysis of existing POSTT data. The results suggest that the items are relatively stable although there is some intra-individual variability. We conclude that the instrument is sufficiently reliable for research purposes.