Thomas A, Danon L, Christensen H, Northstone K, Smith D, Nixon E, Trickey A, Hemani G, Sauchelli S, Finn A, Timpson N, Brooks-Pollock E. Limits of lockdown: characterising essential contacts during physical distancing. Wellcome Open Res. 2021 May;6:116. doi: 10.12688/wellcomeopenres.16785.1
Britt (DiBenedetti) DM, Collins FL, Cohen LM. Cigarette smoking and chewing gum use among college students. J Appl Biobehav Res. 2007 Jun 4;4(2):85-90. doi: 10.1111/j.1751-9861.1999.tb00057.x