Risk of bias – Allocation concealment, lack of blinding and
incomplete accounting for fate of participants. Serious risk of
bias was indicated by poor allocation concealment, blinding or
follow-up, or lack of reporting of any two of these elements.
Very serious risk of bias was indicated by poor, or lack of
reporting of, more than one of the elements of study validity.
Selective outcome reporting is also an important element of
study validity, but was omitted from this assessment as it is
difficult to assess and would result in almost all validity
assessments suggesting serious risk of bias. In observational
studies confounding and similarity of the different groups at
baseline needed to be assessed.