The problem with retrospective reports: change

When multiple measures are taken over intervals of time, change in those measures can be assessed. For example, pain intensity measures assessed before, during and after an intervention can be examined for change. However, when multiple measures are not available, researchers instead rely on patients to supply information on change by asking, for example, "how much has your pain improved since you began the treatment last month?"

The problem is that past experience is not recorded in memory as images are recorded in a video. People use a host of mental shortcuts (heuristics) to reconstruct the past. One of these heuristics is to use current experience as a guide to the past. If there is no reason to believe that the past should be any different from the now then the now becomes the past and that's what is reported. On the other hand, if there is some reason to believe the past my be different from the past then reports will be adjusted accordingly. For example, if people undergo a treatment for pain, their expectation of improvement will lead people to assume that their current pain should be less than their past pain. That is, pain should be improved after treatment compared to before treatment. According to Schwartz (2010), "asking patients whether they feel better now than before their treatment is the most efficient way to "improve" the success rate of medical interventions."