This study examined the effects of increasing the number of assessments on the reliability and validity of measures of average pain intensity. Two hundred chronic pain patients completed 2 weeks of hourly pain ratings. A series of regression analyses were performed, and test-retest stability, internal consistency and validity coefficients were computed to address 4 questions. (1) Are chronic pain patients' reports of pain similar from one day to another? (2) What is the reliability and validity of a single rating of pain intensity when used as an indicant of average pain? (3) How many assessments (data points) are required to obtain estimates of average pain intensity with adequate to excellent psychometric properties? (4) How important is it to sample pain from different days? The results were consistent with predictions based on patients' self-reports of their pain and on psychometric theory. First, the majority of patients did not report similar levels of pain from one day to another, and average pain scores calculated from ratings obtained from a single day were less stable than those calculated from ratings obtained from multiple days. Also, and as expected, the results indicate that a single rating of pain intensity is not adequately reliable or valid as a measure of average pain. However, a composite pain intensity score calculated from an average of 12 ratings across 4 days demonstrated adequate reliability and excellent validity as a measure of the average pain in this sample of chronic pain patients.(ABSTRACT TRUNCATED AT 250 WORDS)