Each Patient as His or Her Own Control Pre Post Study Design

The very simplest, but unacceptable, proposal is to compare patients' conditions prior to and after treatment with T, with no control group and no randomization (hence not an RCT). Any change in condition beyond that consistent with random variation is taken to indicate a treatment effect. Researchers unable to detect a statistically significant difference between T and C groups in an RCT often report such pre-post comparisons as indicating an equally beneficial "effect" in the groups, a misinterpretation of statistical significance.

With any treatment, even a completely inert placebo, it is very unusual, given a large enough sample size, to not see a statistically significant change in patients' conditions pre- and posttreatment. Such a change is attributable not to the effect of that treatment but rather to artifacts of various kinds that a control group is meant to "control" for, as discussed below:

1. The first artifact is statistical regression to the mean (Blomqvist 1986; Campbell and Kenny 1999; Davis 1976; Senn 1997; Stigler 1997). Patients are selected for participation because they have symptoms of a disorder severe enough to require treatment (especially if there is an explicit cutoff point on severity required for inclusion), and frequently the outcome measures evaluate severity of the same symptoms. However, assessment of symptom severity at entry is never completely reliable. With error of measurement due to day-to-day inconsistencies in patients' conditions, coupled with assessor bias and random error, patients with a false-positive assessment at entry are included in the study, but patients in the population with a false-negative assessment at entry are excluded. In absence of any effect of treatment on their clinical condition, those patients with a false-positive assessment at study entry are likely to later have their symptoms be assessed as closer to their true level, creating the false appearance of overall improvement.

2. Second are expectation effects. Both patients and assessors hope and expect to see improvement after onset of treatment. As Yogi Berra is reported to have said: "If I hadn't believed it, I wouldn't have seen it!" The power of suggestion is very strong, particularly in the process of recruitment into an RCT. The informed consent process, as well as patients' awareness of physicians' sworn obligation to do no harm, combined with the patients' pain, needs, and hopes, tends to bolster expectation effects.

3. Third are possible secular trends both in the condition of the patients and in the measurements of response. In some cases, patients with a disorder, receiving only the close and caring attention given in both T and C groups, spontaneously improve. At the same time, how assessors view a patient's response can change over time. With blinding, these effects are the same for patients in the T and C groups.

The cumulative effect of these and other artifactual influences constitutes the placebo response, which is experienced by those in both the T and C groups, whatever C might be, but which is expressed in its pure form in a placebo control group. With randomization to the T and C groups and blinded assessment, the effect attributable to T is the differential response of the T and C groups, not the responses to T or to C separately. Thus, pre-post comparisons within a single treatment group may be of some clinical interest, and worth reporting, but are not adequate to establish the efficacy or effectiveness of any treatment.

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