Great pointŠ.Love the fishŠ
Best
Amy

On 7/2/13 5:27 PM, "Work-related" <[log in to unmask]> wrote:

>Thought I would point out that in some circumstances alternating
>assignment could do much better than simple random (individual, fixed
>probability) assignment.  This will be the case if patients being
>randomised close to each other in time are likely to be similar to each
>other in ways that affect outcome (positive autocorrelation).  Eg,
>ironically, Steve's fish experiment - taking the sequentially caught
>fish.  Or maybe if weather might affect the outcome (cold snaps or
>heatwaves in a trial with elderly patients ?), or the disease change over
>time (flu ?).  Problem is that under other circumstances it can do really
>badly (cabbages example). You could make a judgement that positive
>autocorrelation was very likely, but you will not know for sure - & only
>the simple random assignment will allow theoretically well based
>calculation of confidence intervals.
>Steve's point about meta-analysts' attitudes to it is highly relevant,
>and maybe this is largely because it does make subversion / un-blinding
>rather easy ... 
>
>
>PS I think systematic reviewers / meta-analysts do usually accept some
>practices such as balanced randomisation which could suffer some of same
>problems, if to much lesser extent.
>
>
>
>----- Original Message -----
>From: "Steve Simon, P.Mean Consulting" <[log in to unmask]>
>To: [log in to unmask]
>Sent: Tuesday, 2 July, 2013 7:53:15 PM
>Subject: Re: randomisation
>
>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.
>Sign up for the Monthly Mean, the newsletter that
>dares to call itself average at www.pmean.com/news
>
>