**4.4 Responsiveness**

A large number of definitions and methods have been proposed for assessing responsiveness. A very good comprehensive definition for responsiveness is the ability of the instrument to detect clinically important changes in an individual's status over time even when these changes are small. This ability is the accuracy of the instrument that must be able to differentiate clinical observed changes from measurement error. Even though an instrument can capture very small changes, what really matters is to know if a change is clinically relevant. The Guyatt's responsiveness ratio (RR) does precisely this comparison by relating the variability found within the subject with between the subjects. The reference value for RR is 1.96 because this happens when the minimal important change equals the smallest detectable change.

Another adequate and common measure of responsiveness is the area under the receiver-operating characteristics (ROC) curve. It is very useful to define cutoff scores for discriminative purposes and to define injury severity. The reference value for the area under the curve is at least 0.70.

One point that impact negatively on responsiveness is the presence of floor or ceiling effects. They are considered to be present when more than 15% of respondents achieved the lowest or highest possible score. Thus, the responsiveness is limited because changes cannot be measured in these patients nor is it possible to distinguish one from another, which compromises reliability.

Limitations in measurements, such as ceiling or floor effects, can usually be avoided by selecting measures that have been demonstrated to provide meaningful information about people who are similar to those being measured. In other words, the target population of each measurement tool must be considered by matching the sample, e.g., patients with the appropriate questionnaire or functional scale.
