It has been said more than once in psychology that one person's effect is another person's error term. By minimising and occasionally ignoring individual and group variability cognitive psychology has yieled many fine achievements. However, when investigators are working with special populations, the subjects, and the unique nature of the sample, come into focus and become the goal in itself. For developmental psychologists, gerontologists and psychopathologists, research progresses with an eye on their target populations of study. Yet every good study in any of these domains inevitably has...