Monday, 25 October 2010

How big a sample?

I am always interested in how people reach a decision which involves a numerical answer, because this is often related to O.R.. What follows may be thought to be slightly distasteful -- you have been warned.

One reason for not writing any blogs for several weeks has been travels on holiday, to which I may return later. In the middle of these, I passed a milestone birthday, which was duly celebrated. Soon after that, I received a letter from the National Health Service telling me that they screen all men of my age and over for cancer of the bowel. To do this required me to take samples from material which had passed through that part of my body. (Do I need to say more? No? Good.)

The sample scheme asked for samples from three pieces of material (produced at different times) and two smears from each one, giving six specimens.

So, questions: why two smears? why three pieces? Why not fewer from each and more pieces? I don't know the reasoning, but suspect that the answer is psychological as well as physical. Two eliminates some possibility of contamination. Three is enough for the average person to cope with. Put together, there are enough specimens for the analysts to look at, and reduce the risk of false positives and overlooking the symptoms by accident.

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