To test whether extraction of the 2 subscales in the Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) affected the subscale score reliability and whether scores from the RDC/TMD subscales are comparable to the same scales when the whole Symptom Check List-90 (SCL-90R) is administered.
The full SCL90-R and a modified version containing only the depression and somatization scales were administered in counterbalanced order to 103 subjects. As another test of context, a subset of participants completed the modified and full versions as part of a larger battery of instruments relevant to facial pain. Statistical analyses included internal reliability for item analysis and intraclass correlation (ICC) and Lin's concordance correlation coefficient (CCC) for total scale score reliability.
Internal reliability was approximately 0.95 for depression and 0.87 for somatization, independent of test form. Total scale scores were reliable across test versions, with both ICC and CCC approximately 0.95 for depression and 0.91 for somatization. Permutation tests using the CCC indicated a mild influence on the somatization score but not the depression score due to order effects, but these effects were not significant when considering the 95% CIs based on resampling methods.
Whether items from other subscales are present or not does not affect the internal reliability or parallel forms reliability of the total scores from either depression or somatization. Context of administration, via order of forms completion, does not alter total score or reliability of depressive items but may alter total scores for somatization.