Whether the patient gets A or B is entirely random; the choice has been made by a random number generator on a computer (but it's just like tossing a coin).
Why is it done this way?
Is the surgeon a gambler who likes mixing business with pleasure? Are the patients all reckless risk takers? Is it all an elaborate practical joke?
Surprisingly (!), the answer is none of these. It is done because it removes bias in the allocation of patients to valve A or B.
Random allocation of patients to treatments usually sounds very strange to people who have never heard of it. The reason it is done is to get a fair comparison between the two treatments; in the example, valve A and valve B. If sicker patients tend to get allocated to valve A, then comparing the outcome of the two groups after the operation would not tell which valve was better. We usually know some of the reasons for better or worse outcomes, and we could try to ensure that the two groups were similar as far as those things go; these might be age, severity of condition, etc. But by randomizing, we make sure that, on average, both groups will be similar in all ways (even similar with respect to unknown influences on the outcome) except the thing we're really interested in: the two treatments.
Why doesn't anybody know which valve will be used until the last possible minute?
This certainly seems weird, doesn't it? The whole aim is to make the two treatment groups as similar as possible. Even if they're randomized it's still possible for biases to creep in. For example:
- The surgeon, anaesthetist and other theatre staff might make more careful preparations for the experimental valve;
- The surgeon might change his/her schedule to avoid giving one of the valves to a patient he/she was concerned about;
- and so on.
In general, it is desirable, if possible, for neither the patient nor the doctor to know which treatment the patient is allocated. This prevents any bias in the evaluation of the trial; the outcome for the patient is measured before anyone knows which group they were in. This is done in drug trials, where the two drugs can be made to look alike.
Obviously, with an operation, the surgeon has to know which procedure is being carried out!
How come they're experimenting on humans? Isn't that illegal?
Of course it would be illegal (and immoral) to experiment on humans in this way without their consent. These studies obtain informed consent from the subjects; but the subjects have to consent to being randomized. That is, they consent to their treatment being chosen by the flip of a coin.
The ethical justification for such a study is that we really don't know whether valve A or valve B is better. If we did, we could not reasonably withhold from the patient the better option.