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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