What can I do if RUMM2030 advises that 'overflow' has occurred?
Overflow is the term used to indicate that a computer is unable to proceed with the processing of data. It occurs when values become so small that it is impossible for the computer to conduct normal algebraic processes due to serious lack of precision. Overflow can occur with Rasch analysis using RUMM2030 during the estimation of item and person parameters and for test of fit statistics:
- Because RUMM2030 can handle data with zero frequencies in some categories, the presence of overflow means that the data must be very extreme.
Andrich, D. & Luo, G. (2003). Conditional Pairwise Estimation in the Rasch Model for Ordered Response Categories using Principal Components. Journal of Applied Measurement, 4(3), 205-221.
Andrich, D. & Luo, G. (2004). Estimating parameters in the Rasch model in the presence of null categories. Journal of Applied Measurement, .
- Examine your data response file carefully and especially any extended scored items. The simplest, and usually most comprehensive, method is to load the data response file into a spreadsheet like EXCEL and check the response frequencies across all items. If a heavy bias is detected in the response frequencies as described above, then re-think the targetting issue, that is, have you considered a wide enough range of locations of persons so that most categories have a non-zero frequency.