The Numeric Rating Scale (NRS-11) has been widely used clinically for the assessment of pain. Its use for clinical research is controversial. Reports differ as to whether or not the NRS-11 should be treated as a ratio pain measurement tool. This study compared the NRS-11 to a ratio measure for pain assessment: the visual analog scale (VAS). Simultaneous pain measurements using these 2 scales were compared in clinical situations commonly encountered in a tertiary community hospital. Whereas linear relationships were noted in laboring patients and in postoperative patients with thoracic or abdominal incisions during cough, no such correlations were noted for the same postoperative patients at rest or for postoperative orthopedic patients. The NRS-11 should not be considered to be interchangeable with the VAS. Its use for clinical research should be limited to situations where it has specifically demonstrated linear properties.