Although several interventions look promising, complex interventions, methodological weaknesses and conflicting results limit the validity and applicability of the findings.
Although most studies did provide a description of the cost of the intervention programme, methodological details were often not displayed, and savings resulting from the health effects of the intervention were not always included sufficiently.
Subgroup analyses and sensitivity analyses were conducted regarding patient characteristics, intervention period, methodological quality and assessment of diet compliance.
Associations may be confounded by study characteristics other than the intervention, particularly methodological quality and patient characteristics.
Evaluation of such a large scale national public health intervention presents methodological challenges in terms of trial design and implementation.
As sites began to implement the intervention, one methodological issue that arose was the extent to which sites should over-sample data from hospital areas receiving early versions of the intervention.
This multi-component, multi-interactive nature of a yoga intervention presents methodological challenges in the context of clinical research, such as defining the content and delivery of these interventions [ 34- 36].
The following characteristics of the population, intervention, and methodological quality of the trials were defined a priori as potential effect modifiers: sex, dose of aspirin, duration of treatment, allocation concealment, and compliance with treatment.
We plan to use the random effects model rather than the fixed effects model, because we assume that the included studies will not be functionally equivalent and will show considerable clinical (concerning population, intervention) and methodological (design, quality etc).
Possible reasons for missing evidence of efficacy may be inappropriate methods, too short intervention periods, methodological problems (e.g. lack of randomized controlled study design), unsuitable outcome variables [ 29].
