Inclusion criteria for the review were clearly defined. A limited search of one relevant database was undertaken. There may have been the potential for language bias as only articles in English were included. Publication bias was reportedly assessed, although the meaningfulness of an analysis with less than ten studies was limited. Attempts were made to reduce reviewer error and bias throughout the review.
Quality assessment indicated that the quality of the evidence base was variable; although only limited details were presented, which made it difficult to interpret the quality assessment. Data were combined using suitable meta-analysis techniques. Statistical heterogeneity was explored, although it remained significant most of the time. The authors noted that none of the studies controlled for gastro-oesophageal reflux disease, and many studies did not control for alcohol or tobacco use.
The high levels of statistical heterogeneity and potential for confounding factors limit the reliability of the authors’ conclusions.