Signal refers to the the target of the measurement, noise to random error that obscures the signal. When trying to discriminate among people at a single point in time (who is better off, who is worse off) the signal comes from differences in scores between patients. The noise comes from variability or differences in score within patients over time. The greater the noise, the more difficult it is to detect the signal. When trying to evaluate change over time, the signal comes from the difference in scores in patients whose status has improved or deteriorated. The noise comes from the variability in scores in patients whose status has not changed.