The VITamin D and OmegA-3 TriaL (VITAL): Rationale and Design of a Large Randomized Controlled Trial of Vitamin D and Marine Omega-3 Fatty Acid Supplements for the Primary Prevention of Cancer and Cardiovascular Disease

. Author manuscript; available in PMC: 2013 Jan 1.

Published in final edited form as: Contemp Clin Trials. 2011 Oct 2;33(1):159–171. doi: 10.1016/j.cct.2011.09.009

Table 3. Power for interaction effects on total cancer in VITAL, a 2×2 factorial trial of 10,000 men aged ≥50 and 10,000 women aged ≥55, with 5 years of follow-up

RR		RR	Power
Single agent^a	Interaction^b	Both agents	Main Effect	Interaction
0.90	1.0	0.81	49.9
	0.9	0.73	80.6
	0.8	0.65	95.9	51.5
	0.7	0.57	99.6	87.6
0.85	1.0	0.72	83.4
	0.9	0.65	96.2
	0.8	0.58	99.5	48.8
	0.7	0.51	>99.9	85.2
0.80	1.0	0.64	97.4
	0.9	0.58	99.6
	0.8	0.51	>99.9	46.0
	0.7	0.45	>99.9	82.5

RR = intent-to-treat RR, including noncompliant participants (assuming 80% compliance). Represents the effect among those not assigned to the other intervention and assumes the same effect for both agents.

The interaction is the RR for the combined group divided by the product of risks for the two separate groups—i.e., RRint=RRboth/(RR vitamin D alone*RR omega-3 fatty acids alone). An interaction=1 implies additive effects (no interaction).

RR, rate ratio.