Many
studies are unpublished or published only in the grey literature;
There
is poor reporting of studies (e.g. information presented is inadequate,
selective or ambiguous);
A high
proportion of individuals has been excluded from published analyses;
Obtaining additional longer-term
outcome data beyond that reported may provide useful insights (e.g. for
mortality or child development outcomes);
Outcome
measures have been defined differently across studies;
Time-to-event
outcome measures are required;
Multivariate
or other complex analyses are required;
Exploration of interactions between
interventions and patient-level characteristics is important.