On 7/1/2013 7:04 PM, Mark Ayson wrote:
> I was wondering if you could help me. I am working on a report about
> autologous blood injections and have an RCT that describes their
> randomisation technique as follows: "We assigned participants to two
> groups by randomized sequential allocation. The first patient was
> randomly assigned to the LC group by tossing a coin, the second
> patient to the AB group, and the other patients were assigned to the
> two groups, sequentially." Is this techniques an adequate method of
> randomisation?
This is alternating (or sequential) assignment, and the only thing
random about it is that an initial coin flip determines whether the
alternating sequence is LC, AB, LC, AB, LC,... or AB, LC, AB, LC, AB,...
It has one nice property in that any particular patient has an equal
probability of being in LC or AB. What it has problems with are temporal
trends.
Have you ever noticed how plants grown in a garden? There is an
alternating pattern of big cabbage, little cabbage, big cabbage, little
cabbage. They all start out as a seed, but one plant will, for various
reasons, grow slightly faster. Its roots will extend towards its
neighbor, stealing some of the water and nutrients. It will then grow
even faster at its neighbor's expense.
Alternating assignment would be a total disaster in this setting. A
fertilizer applied to every other plant in a row would either end up
looking really really good or really really bad, just because of the
big/little pattern in plant growth.
It's a bit harder to imagine a problem with alternating assignment in a
medical context, but it could happen here as well. Suppose you are
measuring the amount of time that a physician spends with a series of
patients. If the first patient runs a bit long, the physician will try
to rush the next patient a bit to try to get things back on schedule. If
one patient exits quickly, on the other hand, the physician may be more
generous with his/her time for the next patient.
You could wave your hands around and argue that this type of temporal
trend could not occur in your particular research setting, but there is
no practical way to prove or disprove this assertion using the data from
an alternating assignment. Thus, alternating assignment will always have
a cloud over its head.
In theory, alternating assignment could be blinded from the patient and
from the physician. Just keep the original coin flip blinded, and then
unless there are some characteristic side effects or other aspects of
the treatment that unblind the study, the blind could be maintained. In
reality, anyone who does not take the trouble to use randomization over
alternating assignment is probably not going to take the trouble to
properly blind the trial either.
Concealed allocation in a setting where total blinding is impossible, of
course, is a big waste of time, as anyone with half a brain will figure
out that this is alternating assignment rather than randomization after
just a handful of patients.
Using alternating assignment instead of randomization is enough to get
your study excluded from most systematic overviews, so you could argue
that alternating assignment is unethical. Why run a study if you know it
won't contribute any data to the meta-analysis? You know it is going to
have a miserable risk/benefit ratio before you even start the study.
There are a few settings where logistics mandate alternating assignments
over randomization, but these are very rare.
I have a nice example of an epic fail involving lack of randomization.
Although it is not really related to alternating assignment, it is still
amusing. Note that even though the pictures look like they were done by
a professional artist, I did them myself.
--> http://www.pmean.com/12/fishy.html
In general, treat alternating assignment as equivalent to a
non-randomized study. You could try to make some arguments that it is
almost as good as randomization, but most researchers will not buy those
arguments.
Steve Simon, [log in to unmask], Standard Disclaimer.
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