Introduction: In areas where clinical trials are heterogeneous with respect to population, interventions and outcome measures, qualitative or criteria based meta-analyses are an effective and informative way for aggregation of clinical trial results. A criteria list is used for the standardised assessment and summarization of methodological quality of studies. The studies with the highest methodological quality contribute most to the outcome and conclusion of qualitative meta-analyses.

Objective: The assessment of methodological quality is hampered by omissions of pertinent details of design and conduct. These omissions can signal poor reporting but also disguise shortcomings in design and conduct. Therefore, the amount of omitted details could be a marker for methodological quality of studies.

Methods: Three meta-analyses about primary care interventions for musculoskeletal disorders have been conducted. Only when a paper provided all the necessary details criteria could be satisfied. The total number of criteria 'satisfied' provided a measure for the marginal study quality. Criteria were rated 'flawed' if bias was present or could be inferred. The total number of criteria 'flawed' was subtracted from the attainable maximum number of criteria. This provided a measure for the possible maximal study quality. The range between these 2 measures indicates the amount of omitted details.

Results: The overall outcomes of the 3 meta-analyses were influenced most by studies presenting much information about design and conduct, methodological shortcomings included. For these studies the ratio between the number of criteria 'satisfied' and criteria 'flawed' was large.

Discussion: The number of criteria 'satisfied' and 'flawed' proved to be effective markers for study quality. The range between these 2 quality markers can serve as surrogate measures for study quality. These 4 markers facilitate aggregation of clinical trials.