This is the full systematic review protocol registered with PROSPERO (review number: 42016048393).
The protocol provides information on the rationale, the inclusion/exclusion criteria for the studies, the search strategy, and planned synthesis of results.
An amendment was made from the original protocol to make the inclusion criteria stricter. The original protocol specified that studies that include only a single smoking sub-group will be included. This would have made any comparisons between studies incredibly difficult, therefore the inclusion criteria now specifies that each study must compare two or more smoking sub-groups. This will create a stricter inclusion criteria and make comparisons more meaningful.