= Solution
<Publication bias> occurs when the availability or publication of a study depends on its results, for example their direction or statistical significance. The published studies then need not represent all studies meeting the review's eligibility criteria. This is different from unbiased studies simply having large sampling errors.
If mortality reductions favouring transfusion are more likely to be published than null or unfavourable findings, the observed <log risk ratios> will tend to be too negative and the pooled <risk ratio> too small. The apparent benefit will then be exaggerated. An ordinary <meta-analysis> interval ignores uncertainty about the missing evidence and may have poor coverage even if its sampling-<variance> calculation is correct for the studies included. \b[The direction of bias depends on which results are preferentially made available]; <publication bias> does not invariably favour a particular treatment.
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