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Research Christmas 2013: Research

The survival time of chocolates on hospital wards: covert observational study

BMJ 2013; 347 doi: https://doi.org/10.1136/bmj.f7198 (Published 14 December 2013) Cite this as: BMJ 2013;347:f7198

Re: The survival time of chocolates on hospital wards: covert observational study

Very nice.

But I'm not so sure that it's right to talk about an "initial rapid “grab” phase".

Insofar as the survival function is exponential, the distribution of survival times follows an exponential probability density function. That is characteristic of a Markov process in which the probability of an individual chocolate being eaten in the next second (say) is independent of how long it has already survived. In other words the process is entirely random.

The reason that the survival curve falls rapidly at first is is merely a consequence of the fact that random (exponentially-distributed) intervals have more short (below the mean) intervals than long intervals (though the latter occupy more of the time).

In fact the curves may not be exactly exponential. Could that perhaps be because many people prefer the red ones to the green triangles? I look forward to the follow-up study.

Reference

Colquhoun, D. Lectures on Biostatistics, Clarendon Press 1970 (available at http://www.dcscience.net/Lectures_on_biostatistics-ocr4.pdf )

Competing interests: No competing interests

16 December 2013
David Colquhoun
Research professor
UCL
Gower Street, London WC1E 6BT