The randomised controlled trial is
regarded by many as the most appropriate research design to evaluate the
effectiveness of an intervention (Littlewood 2011).
Two features of a basic randomised
controlled trial will be considered in this blog, and the clue is in the name –
randomisation and control.
Firstly, in a randomised
controlled trial, the research participants are allocated to two or more groups
randomly, meaning by chance.
Secondly, in its most basic form, research participants are allocated to the
intervention or control group. Figure
1 offers a schematic representation of the most basic randomised controlled
trial.
Figure 1 The randomised controlled
trial
Taking the components of a
randomised controlled trial in sequence, random allocation will be considered
first. Imagine a physiotherapist who has designed a new exercise intervention
for the treatment of low back pain and wants to establish the effectiveness of
the treatment. She receives the first patient in the clinic; a young male with mild
low back pain who continues to work and enjoys exercising. She decides that the
patient should receive the new treatment and allocates him accordingly. The
second patient of the day is also a young male but in contrast has not worked
due to severe low back pain for a number of years and does not appreciate the
benefits of exercise. She decides that this patient would probably not respond
well to the new exercise intervention and so allocates him to the usual
physiotherapy treatment. This non-random process of selection continues until
all 100 research participants are recruited. Upon completion of the trial the
physiotherapist notices that patients in the intervention group were more
active and, on average, reported milder levels of low back pain. This means
that the groups being compared are different to begin with, even before they
are exposed to the treatment.
After the data is collected, the
analysis suggests that the new exercise intervention is far superior to usual
physiotherapy. Is this a valid conclusion? Probably not; because as
physiotherapists we know that people with less severe pain, who maintain good
levels of activity, tend to perform better than those with more severe pain and
greater functional deficit, irrespective of the treatment they receive. This is
an example of selection bias where the process of allocation of participants to
groups has been adversely influenced. In contrast, a process of random
allocation would serve to distribute participants between the group in a more
balanced manner according to the factors we know, for example age, gender,
condition severity; but importantly also the factors we don’t know, for example
psychological status, attitude to treatment etc. This provides a more
appropriate basis upon which to judge the effectiveness of an intervention
because the only significant difference between the groups should be the
treatment received rather than other factors that can influence the outcome of
treatment.
I often reflect that in a fair
race Usain Bolt would beat me over a 100-metre race, but give me a 80 metre
start and I think I would cross the line first. Seems obvious when put in those
terms, but we also need to apply this thinking to research; if the race isn’t
fair, i.e. if one group is different to begin with, it is perhaps unsurprising
that one treatment appears superior to another in skewed conditions.
The second component, the control
group, will now be considered. It is easiest to recognise the value of a
control group by considering a situation where it is not used. Another example;
following a day’s lectures, my students often complain of headaches.
So, I ask
them to rate the severity of their headache on a scale of zero to ten where
zero equates to no pain and ten equates to the worst pain imaginable. I then
instruct them regarding an exercise intervention that I have spent many months
developing; repeatedly elevating their left arm. I’m not sure why this
intervention works but I’ve spent so much time thinking about it that it had to
be worth something, surely?! I ask the students to repeat the exercise two or
three times before returning the next morning. Consistently, I find that the
severity of the students’ headaches had reduced significantly by the following
morning. Should I conclude that my exercise intervention is effective based
upon this data? Clearly not, it is much more likely that not having to listen
to my voice, doing something more enjoyable, having a good rest and other
factors are more likely to explain the reduction in headache severity. If I had
randomly allocated students with headache to the intervention group, who
undertook the exercise, or control group where students continued as usual
without the exercise then a more valid evaluation would be possible. When the
students returned the following morning we could establish the headache
severity of both groups knowing that the only difference between the groups was
that one received the intervention and the other didn’t because the control
group would also be exposed to all these other factors, for example rest, that
might be responsible for a reduction in headache severity. In this situation it
is highly unlikely that my exercise
intervention would prove its worth and hence the value of a control group is
highlighted, and I need to go back to the drawing board…developing an
intervention with a plausible mechanism of action which is based on current
evidence.
An important point to recognise in relation to physiotherapy research
is that the control group does not usually consist of no treatment; this is one
of the most common misconceptions relating to a control group – so much so that
I have a standard response paragraph for peer reviewers when they make this
point. The reason for this is that it is generally accepted that it is
unethical to withhold treatment of potential benefit from patients. So most
randomised controlled trials involving physiotherapy compare the new
intervention against a placebo, for example de-tuned ultrasound, or usual care,
for example usual medical care. Hence the new intervention is expected to
demonstrate that it is at least equivalent but usually superior to existing
treatment before it would be considered for implementation into practice.
Hope this background information
was useful; on the back of this my next post, hopefully, will be a critical
appraisal of a randomised controlled trial I was involved with, the SELF study.
Thanks for reading, would really appreciate your constructive thoughts.
