Hyperbaric Oxygen Therapy - Hyperbaric Chamber Information ...

need to insulate patients from outside influences or choose outcomes which cannot
plausibly be affected by external factors. This is most evident when therapies span several
years as in those for thermal burns and acute myocardial infarction.
Regression to the mean
Regression to the mean occurs in studies that have asymmetrically sampled from the
population A result of this sampling effect is regression of the sample mean toward the
population mean from pre- to post-test. For example, patients who exhibit a randomly
high score at the beginning of a trial will show a lower value post intervention and
patients with extremely low scores will present with higher scores at the end of the trial.
A selection regression threat can also occur when the intervention and control groups are
not comparable. For example, if the HBOT group had a disproportionate number of
subjects with extremely high values compared to controls we would expect the HBOT
group to regress a greater distance towards the overall population mean. The HBOT
group would therefore appear to have better outcomes than controls. Under these
circumstances it would be wrong to attribute such differential change to HBOT since it
may be a result of statistical regression. Comparative studies in cluster headaches and
sudden deafness might be predisposed to this threat.
Placebo effect
The placebo effect arises from the tendency of individuals to report a favourable
response regardless of the physiological efficacy of treatment. For studies that do not use
a placebo control group it is impossible to determine whether subjective outcomes are
due to the treatment effect or to other factors such as differential treatment of the study
group or the patients’ belief that the treatment is beneficial. Blinding is also necessary to
negate the placebo effect. Participants in a trial may change their behaviour (change of
lifestyle or withdraw from the study) in a systematic way if they are aware of which group
they have been allocated. This also applies to researchers who may change their
treatment practices for patients in the group that is presumed to produce a more or less
favourable result depending on their hypothesis. Therefore the use of a placebo group
with blinding will ensure that all aspects of the intervention offered to patients are
identical except for the actual experimental treatment.
Maturation
The effect of maturation can arise from differential rates of normal growth between preand
post-test. Thus the changes that would occur as a result of the ongoing
developmental process (such as healing and mortality) may be confused with the impact
of treatment. In the event that intervention and control groups are maturing at different
rates, with respect to outcome, it cannot be assumed that post-test differences are a result
of treatment.
Testing effects
A testing effect can be the result of familiarity from repeated testing. Patients can modify
their responses or behaviour in order to achieve more favourable test results. The extent
to which this validity threat applies to HBOT is small since the majority of outcomes
were objective measures.
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