It is usually assumed that serial measurement in clinical trials should be performed by the same assessor because of concern regarding interobserver variability. However, the high levels of prestandardization interobserver reliability observed in this study indicate that for these variables serial measurements in a clinical trial could be made by different assessors, assuming they were equally skilled. This observation has important implications for outcome measurement in AS clinical trials. Although high levels of prestandardization reliability precluded the demonstration of any significant effect, we speculate that the videotape might be effective in training less experienced assessors. Nevertheless, an alternative approach to standardization may be required for the chest excursion measurement. Reductions in observer variability have the potential to diminish sample size requirements for AS antirheumatic drug studies. The use of a videotape to achieve this goal offers cost and convenience advantages over one-on-one training procedures, and this method should be further assessed in a group of less experienced assessors.