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Biostatistics in Research Practice: The ESPRIT Trial, suggested answer 2

2. Mean follow-up was 3·5 years (SD 2·0).
One last time for an old favourite: what can we deduce about the
distribution of follow-up time?
Why should the follow-up times vary so much?

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Suggested answer

If the distribution were symmetrical, we would expect some observations
to be smaller than the mean minus two standard deviations.
Because the standard deviation is more than half the mean,
such observations would be negative.
Follow-up time cannot be negative, so the observations which are more
than two standard deviations from the mean must all be in
the upper tail of the distribution.
Therefore, the distribution must be positively skew or skew to the right.

Follow-up times vary because the patients are recruited over
the period of the trial, several years, and all followed up to the end of the trial.
Some patients therefore have much longer follow-up times than others.

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