What is heterogeneity and is it important?

Editor,
The heterogeneity documented by Tramer et al[1], is not between the
subgroups treated with steroids and anti-histamines. The authors
calculated the combined odds ratio for both groups as 0.36. These are
shown as combined results for each subgroup in Figure 1, but for some
reason the usual convention of a diamond for the combined results has not
been used.
The significant heterogeneity described in the legend refers to the within
subgroup differences in the anti-histamine trials, and did not defer the
original authors from combining the results for anti-histamines! The
suggestion that heterogeneity between the classes of drugs was significant
and that this dissuaded the authors from combining the two types of
treatment cannot be correct.

[1] Tramer MR, von Elm E, Loubeyre P, Hauser C. Pharmacological
prevention of serious anaphylactic reactions due to iodinated contrast
media: systematic review. 2006:675-8.

Competing interests:
None declared