Abstract
Pain relief, or a decrease in self-reported pain intensity, is frequently the primary outcome of pain clinical trials. Investigators commonly report pain relief in one of two ways: using raw units (additive) or using percentage units (multiplicative). However, additive and multiplicative scales have different assumptions and are incompatible with one another. In this work, we describe the assumptions and corollaries of additive and multiplicative models of pain relief to illuminate the issue from statistical and clinical perspectives. First, we explain the math underlying each model and illustrate these points using simulations, for which readers are assumed to have an understanding of linear regression. Next, we connect this math to clinical interpretations, stressing the importance of statistical models that accurately represent the underlying data; for example, how using percent pain relief can mislead clinicians if the data are actually additive. These theoretical discussions are supported by empirical data from four longitudinal studies of patients with subacute and chronic pain. Finally, we discuss self-reported pain intensity as a measurement construct, including its philosophical limitations and how clinical pain differs from acute pain measured during psychophysics experiments. This work has broad implications for clinical pain research, ranging from statistical modeling of trial data to the use of minimal clinically important differences and patient-clinician communication.
Highlights
Pain is highly prevalent, burdensome, and a common reason for doctor visits [1,2,3,4]
Self-reports of pain intensity are remarkably easy and inexpensive to collect. These pragmatic and measurement properties make a reduction in self-reported pain, which we define as pain relief, the gold standard for assessing pain improvement
Treatments that yield appreciable pain relief will be employed in the clinic, and findings from these trials may be communicated to patients
Summary
Burdensome, and a common reason for doctor visits [1,2,3,4]. In an attempt to understand the severity of patients’ pain, doctors and researchers ask patients about the intensity of the their pain, requiring patients to condense and transmute their subjective experience to a single number. Self-reports of pain intensity are remarkably easy and inexpensive to collect. These pragmatic and measurement properties make a reduction in self-reported pain, which we define as pain relief, the gold standard for assessing pain improvement. Clinical studies of pain commonly quantify pain relief as the primary outcome. How pain relief is quantified and reported roughly falls into one of two categories: absolute reductions in pain and relative (or percent) reductions in pain. Studies that report absolute reduction may state that a drug decreased pain by 2/10 numerical rating scale (NRS) units or 23/100
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