Using multiple daily pain ratings to improve reliability and assay sensitivity: how many is enough?

The Initiative for Methods, Measurement, and Pain Assessment in Clinical Trials (IMMPACT) has reported diminished assay sensitivity in pain treatment trials and recommended investigation of the causes. Specific recommendations included examination of outcome measure reliability and lengthening the baseline measurement period to allow more measurements to be collected. This secondary data analysis evaluated the minimum number of daily pain intensity ratings required to obtain a reliability of at least .90 and whether a composite of this smaller number of ratings was interchangeable with the composite of all ratings. Veterans Affairs medical center patients made 14 daily calls to an automated telephone system to report their average daily pain intensity rating. A single daily rating produced less than adequate reliability (intraclass correlation coefficient = .65), but a composite of the average of 5 ratings resulted in reliability above .90. A Bland-Altman analysis revealed that the differences between a 5-day composite and the composite of all ratings were small (mean .09 points, standard deviation = .45; 95% confidence interval = -.05 to .23) and below the threshold for a clinically meaningful difference, indicating that the 2 measurements are interchangeable. Our results support the IMMPACT recommendations for improving assay sensitivity by collecting a multiple-day baseline of pain intensity ratings.

This study examined the minimum number of pain ratings required to achieve reliability of .90 and examined whether this smaller subset of ratings could be used interchangeably with a composite of all available ratings. Attention to measure reliability could enhance the assay sensitivity, power, and statistical precision of pain treatment trials.