Improving Scoring Precision and Internal Construct Validity of the Bath Ankylosing Spondylitis Disease Activity Index Using Rasch Measurement Theory.

Applying the Rasch transformed scores simplifies completion and scoring of the measure and confirms internal construct validity. It also ensures linear measurement and justifies the use of parametric statistical analyses when analyzing datasets. The transformation table can be used with existing BASDAI datasets to allow direct comparisons of disease activity scores with those generated from future studies.