Ergodicity (or its absence)
Why a group average usually does not tell us what one person needs.
Ergodicity is a term from statistical physics that has come to play an important role in psychology as well, mainly because psychological processes usually do not seem to meet its conditions.
A system is called ergodic if the behaviour observed over time in one element shows the same pattern as the behaviour observed at one moment across many elements. In other words, the average patient and the individual patient over time then show the same dynamics. That would be convenient, because the results of group research could then be translated directly into individual practice.
Psychological systems, however, seem rarely to meet that condition.
Molenaar's reversal
The Dutch methodologist Peter Molenaar (2004) worked this out in an influential paper: what holds for the group need not hold for the individual. An example often used in this literature is typing. In group studies, typing speed and accuracy are positively correlated: those who type faster also type more accurately. If one person is followed over a longer period, however, the association within that person often turns out to be negative: when she starts typing faster, it comes at the expense of her precision. The group picture and the individual picture point in opposite directions here.
The assumption that the psychological dynamics of individuals equal the dynamics of the group average is the assumption of ergodicity, and according to Molenaar it rarely holds. (Freely after Molenaar, 2004)
For mental health care this is an essential point. A large part of our evidence-based practice rests on randomised controlled trials: group studies showing that treatment X works better on average than treatment Y. The outcome for the group, however, does not tell us which patient benefits from X, which from Y, and which from neither. And within one person, the relation between factors (sleep and mood, exercise and anxiety) may follow a different pattern from that in the group.
The idiographic complement
Non-ergodicity is no reason to abandon group research, but it is a reason to complement it with research at the level at which we also act: the individual. Idiographic research, with many measurements of one person over time, shows how a specific patient functions, which variables in his or her system are related to one another and how that relation changes. It thus forms a natural counterpart to the group research we already know.
In practical terms, this means that repeated measurements over time (ESM, diaries, IROC, questionnaires administered several times) are more than a by-product of treatment. Methodologically, they are needed to bring into view the dynamics that apply to this specific patient.
For the consulting room the implication is modest, but important. A statement such as "this works for patients like these" deserves some caution, because a group of comparable patients offers no guarantee that the result also applies to this one person. It therefore remains necessary to follow how this specific system responds.