Cronbach's alpha was calculated as an internal consistency measure of precision of item sum scores for each construct.
Rather, for calibration purposes, a uniform distribution is useful in contributing to an equal degree of precision of item estimates across the metric, but is not a requirement.
The impact of the precision of item parameters values was modest and mainly perceptible for high values of ε (ε = 1.0), corresponding to a rather poor precision that might be rarely encountered in such a context.
It is more important to have a uniform distribution of persons such that the degree of precision of item estimates is similar across the whole of the construct (i.e. work instability) being measured.
Although this may simply reflect the distribution of symptoms in the general population, and is not a problem for a screening instrument, it can affect the precision of the item estimates, particularly items representing severe levels of anxiety and depression, where relatively few respondents were found.
While the number of respondents returning questionnaires exceeded the minimum sample size requirements, the skewed scores of some subscales indicated the lack of uniformity of sample distribution which would have given the greatest degree of precision for item and person estimates.
Therefore, when constructing an instrument fitting, the Rasch Model would be best, improving the precision of the items.
Emrich, S.M., Riggall, A.C., Larocque, J.J. & Postle, B.R. Distributed patterns of activity in sensory cortex reflect the precision of multiple items maintained in visual short-term memory.
The sample size is small, and only gives a degree of precision to item and person location within 0.5 logits.
A limited number of responses due to a small sample will also impact on the precision of the item calibration measures.
Indeed, the maximum decrease in power was observed for ESIRT = 0.2, N = 100 and J = 5: as compared with an expected power of 0.293, a 6%, 11%, and 16% decrease in power was noticed with a good, moderate or poor precision of the item difficulty parameters, respectively.
