Table 2.

Large randomised trials conducted by mail

Study (year completed)	BDS 1988 [18]	PHS 1989 [19]	WHS 2005 [17]	WACS 2007 [20]	ASCEND 2017 [14,15]	VITAL 2018 [21]
Intervention	Aspirin	Aspirin, β carotene	Aspirin, vitamin E	Vitamin C, vitamin E, β carotene	Aspirin, n-3 FA	Vitamin D n-3 FA
Population	UK doctors	US doctors	Female health professionals	Women with, or at high risk of CVD	Diabetes	General population
Invitations	~20K	0.26M	1.7M	53Ka	0.4M	~9M
Randomised	5139	22,071	39,876	8171	15,480	25,871
Mean or median follow-up (years)	~5	4.8	10.1	9.4	7.4	5.3

^aInvited female health professionals who had already expressed interest in research

BDS, British Doctors Study; CVD, cardiovascular disease; FA, fatty acid; PHS, Physicians Health Study; VITAL, VITamin D and OmegA-3 TriaL; WACS, Women’s Antioxidant Cardiovascular Study; WHS, Women’s Health Study