Method 3: SMDs, weighting by number of health care
professionals
Where the population of interest is the health care professional, it may
be desirable to weight the results by the number of health care
professionals included6,21to aid population inference. This method utilises commonly reported
summary information without adjustment for clustering. In this method of
meta-analysis the studies are weighted by the number of health care
professionals as an alternative to the commonly used inverse variance
method which uses weights based on standard errors. As pointed out in
table 4 there are weaknesses to this approach which we discuss below.
Not every trial in the SOCIAL review reported the number of health care
professionals – for example a cluster trial where an intervention was
directed at all staff on a hospital ward. Where no information was given
about the number of health care professionals, Ivers et al.6 used the number of
practices/hospitals/communities instead, and we followed that method
here. An alternative might be to estimate the number of health care
professionals using data from similar studies – e.g. using mean number
of GPs per surgery or mean number of nursing staff on a hospital ward.
Note that this method needs to be combined with method 1 or method 2
above or an alternative way of summarising mixed outcome measures; here
we combined it with method 1 to summarise standardised mean differences.