Quality assessment
Two authors independently assessed the quality of the methodology for
each study using the Newcastle-Ottawa scale adapted for cross-sectional
studies 13. The
assessment tool comprised of three scales; selection (maximum of five
stars), comparability (maximum of two stars), outcome (maximum of three
stars) and graded out of 10 points (stars). This assessment critically
appraised the internal (systematic error) and external validity of the
studies to reduce the risk of biases. The scale is composed of seven
items and classifies the study in four possible levels: very good (9-10
points), good (7-8 points), satisfactory (5-6 points) and unsatisfactory
(0-4 points). Any disagreement was resolved through consensus
(supplementary Table S3).