Potential biases in the review process
We are aware that there was a risk of introducing bias at all stages
in the review process. In this updated version of the review two
authors independently assessed eligibility for inclusion and carried
out data extraction, and data checks were carried out by a third
author. However, assessing risk of bias is not an exact science and
involves judgement. We would encourage readers to consider the
tableswhere the characteristics of included studies are summarised,
to assist them in interpreting results.