OVERFLOW

            Question:

What can I do if RUMM2030 advises that 'overflow' has occurred?

            Explanation:

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:

• Poor targetting can result in overflow, especially with polytomous items, when too many responses are present in one category and little or no responses in the other categories.  The total, or almost complete, absence of data in many adjacent categories creates problems in estimating the  Rasch thresholds.  In this case, values are so small that the accumulative effect over all persons creates the situation in which the computer cannot cope with so many extreme values, and  overflow results.  If a large sample is also present, this problem will be magnified.

• Because RUMM2030 can handle data with zero frequencies in some categories, the presence of overflow means that the data must be very extreme.

            References:

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, .

            Action:

• 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.

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