RT Journal Article SR Electronic T1 The chicken or the egg? The value of longitudinal research in an increasingly diverse tobacco product landscape JF Tobacco Control JO Tob Control FD BMJ Publishing Group Ltd SP s131 OP s133 DO 10.1136/tobaccocontrol-2020-055694 VO 29 IS Suppl 3 A1 Brian A King YR 2020 UL http://tobaccocontrol.bmj.com/content/29/Suppl_3/s131.abstract AB