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studies are unpublished or published only in the grey literature.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.