RT Journal Article SR Electronic T1 Cardboard floor: about the barriers for social progression and their impact on the representativeness of epidemiological studies JF Journal of Epidemiology and Community Health JO J Epidemiol Community Health FD BMJ Publishing Group Ltd SP 105 OP 106 DO 10.1136/jech-2020-214978 VO 75 IS 2 A1 Arias-de la Torre, Jorge A1 Valderas, Jose M A1 Benavides, Fernando G A1 Alonso, Jordi YR 2021 UL http://jech.bmj.com/content/75/2/105.abstract AB