Retrodiction

Retrodiction (also known as postdiction—although this should not be confused with the use of the term in criticisms of parapsychological research) is the act of making a “prediction” about the past.

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Temporal representation of retrodiction or postdiction.[1]

The activity of retrodiction (or postdiction) involves moving backwards in time, step-by-step, in as many stages as are considered necessary, from the present into the speculated past to establish the ultimate cause of a specific event (for instance, in the case of reverse engineering, forensics, etc.).

Given that retrodiction is a process in which “past observations, events and data are used as evidence to infer the process(es) that produced them” and that diagnosis “involve[s] going from visible effects such as symptoms, signs and the like to their prior causes”[2] the essential balance between prediction and retrodiction could be characterized as:

retrodiction : diagnosis :: prediction : prognosis[3]

regardless of whether the prognosis is of the course of the disease in the absence of treatment, or of the application of a specific treatment regimen to a specific disorder in a particular patient:

“We consider diagnostic inference to be based on causal thinking, although in doing diagnosis one has to mentally reverse the time order in which events were thought to have occurred (hence the term “backward inference”). On the other hand, predictions involve forward inference; i.e., one goes forward in time from present causes to future effects. However, it is important to recognize the dependence of forward inference/prediction on backward inference/diagnosis. In particular, it seems likely that success in predicting the future depends to a considerable degree on making sense of the past. Therefore, people are continually engaged in shifting between forward and backward inference in both making and evaluating forecasts. Indeed, this can be eloquently summarized by Kierkegaard’s observation that, “Life can only be understood backwards; but it must be lived forwards”.[4]

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