of hits, even at the expense of also increasing false alarms. Thus, my performance is characterized by my trade-off of hits against false alarms, which can be shown graphically by the receiver operating characteristic curve (ROC), which is a plot of the probability of detection (hits) against the probability of false alarms. Guyatt et al. have applied this type of thinking to health measurements; the signal represents true differences in health that one wishes to detect; the noise represents measurement error over which the signal must be detected (86, p1343). They then link these ideas to the purpose of the measurement, noting that for evaluative instruments, the relevant signal concerns change over time, so the signal-to-noise ratio is represented by a measure of responsiveness. For a discriminative measure, signal represents the ability to distinguish between people, so that a signal-to-noise ratio is represented by a reliability coefficient (86, Table 2).