Randomisation (if done properly) ensures no systematic differences between the intervention group in factors, known and unknown, that may affect the outcome.
The following is an example of a randomised control trial.
Mothers about to give birth too early were randomised to either a short inexpensive course of corticosteroids or placebo and the outcome that they were interested in was the number of babies dying from complications of immaturity.
This is an interesting example as many RCTs were undertaken over a period of 10 years comparing a short inexpensive course of corticosteroids or placebo in mothers about to give birth too early. These studies all gave different results some said that treatment was better than placebo, some said there was no difference between treatment and placebo for the outcome of interest.
It was not until a systematic review was undertaken a decade later that it indicated strongly that corticosteroids reduce the risk of babies dying from the complications of immaturity.
Difference between efficacy study and effectiveness studies
It is worth mentioning here the difference between efficacy studies and effectiveness studies. They are both important when evaluating interventions but they serve different purposes and have different study designs. All too often people get these two confused.
Efficacy studies investigate the benefits and harms of an intervention under highly controlled conditions. These studies create high internal validity, they often however require substantial deviations from clinical practice, including restrictions on the patient sample, control of the provider skill set and limitations on provider’s action. The intervention is often compared to placebo.
Effectiveness studies (also known as pragmatic studies) examine interventions under circumstances that are closer to what happens in practice, with more heterogeneous populations, less-standardised treatment protocols and they take place in routine clinical settings. The intervention is most often compared to usual care.
The efficacy study reveals efficacy, the magnitude of the effect may well be different in practice, and depending on who is being treated (i.e. which patients).
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