Intended for healthcare professionals


Effect of fibre, antispasmodics, and peppermint oil in the treatment of irritable bowel syndrome: systematic review and meta-analysis

BMJ 2008; 337 doi: (Published 14 November 2008) Cite this as: BMJ 2008;337:a2313

Another unhappy twist to this statistical saga

There is a general problem with these sort of forest plots for
relative risk.

Skimming uneasily through medical literature, one finds that relative
risk is often formatted as in this article on irritable bowel syndrome (1)
, viz with a logarithmic x- axis.

Such representation verges on medically misleading, because of the
contrary nature of logarithms.

An increase of relative risk of, for example, one half, would occupy
a small section on the side above one, but a much larger section on the
side below one. Logarithms are not neat and linear. By definition, and
most inconveniently here, they expand exponentially.

So we have a medically skewed picture through such a logarithmic
framework for relative risk. Fractional figures below one are stretched
out; figures above one are (relatively) squeezed in. Presumably,
logarithms are deployed because the data- always guaranteed to be
irregular and nasty- would otherwise be scattered too far off the page.
Alas, one might misuse such a logarithmic basis to almost misrepresent
data on a graph. Relative risk above one would be rendered less important
in terms of the graph, of the visual set- up.

The transcription error partly happened because the logarithmic
transformation of the data was not being made explicit.

Yet again, the mathematics and the medicine are capable of being at
cross purposes. These two monsters just do not want to trot prettily in


(1) Effect of fibre, antispasmodics and peppermint oil in the
treatment of irritable bowel syndrome: systematic review and meta-
analysis. Alexander C Ford et al. BMJ 2008; 337: a2313

Competing interests:
None declared

Competing interests: No competing interests

13 May 2009
Zekria Ibrahimi
psychiatric patient
Coombs Library UB1 3Eu