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

BMJ 2008; 337 doi: https://doi.org/10.1136/bmj.a2313 (Published 14 November 2008) Cite this as: BMJ 2008;337:a2313

Re: Forest plot: not seeing the wood for the logs

It would be unfortunate if Zekria Ibrahimi’s comments (1)
mislead anyone into thinking that forest plots are unreliable. It is of
course true that logarithms to base 10 or base 2 give different values.
But when a confidence interval is calculated for a quantity on a log
scale (such as log relative risk) the values obtained are then back-
transformed to the original scale (here, the relative risk). Thus the
answer will be the same whether using logs to base 2, 10, e or anything
else. So the forest plot for a meta-analysis will look identical
regardless of the base used as long as the scale shows the relative risk,
not the log relative risk, as is good practice. It is not correct to say
that "different log bases would manipulate the visual lay-out, perhaps to
misrepresent data."

In the paper by Ford et al (2) the relative risk was indeed plotted on a
log scale rather than the log relative risk being plotted. Unfortunately
the original axis labelling incorrectly showed some negative values, but a
correction was posted on 8 May.

The comment about number needed to treat (NNT) is also misleading as the
NNT should never be used as the basis for a meta-analysis.

Douglas G Altman

1 Ibrahimi Z. Forest plot: not seeing the wood for the logs.
http://www.bmj.com/cgi/eletters/337/nov13_2/a2313#213704

2 Ford AC, Talley NJ, Spiegel BMR, Foxx-Orenstein AE, Schiller L, Quigley
EMM, Moayyedi P. Effect of fibre, antispasmodics, and peppermint oil in
irritable bowel syndrome: Systematic review and meta-analysis. BMJ
2008;337:a2313.

Competing interests:
None declared

Competing interests: No competing interests

18 May 2009
Douglas G Altman
Professor of Statistics in Medicine
Centre for Statistics in Medicine, University of Oxford, Wolfson College, Linton Rd, Oxford OX2 6UD