Testing for baseline differences in randomized controlled trials: an unhealthy research behavior that is hard to eradicate

de Boer, Michiel R; Waterlander, Wilma E; Kuijper, Lothar DJ; Steenhuis, Ingrid HM; Twisk, Jos WR

doi:10.1186/s12966-015-0162-z

International Journal of Behavioral Nutrition and Physical Activity

Table 1 Results from four randomized controlled supermarket trials with different methods of adjustment for baseline covariates

From: Testing for baseline differences in randomized controlled trials: an unhealthy research behavior that is hard to eradicate

Study	Comparisons	Adjustment for covariates	Outcome	B	Lower 95% CI	Upper 95% CI	p-value
1. Effects of 25% discounts on fruits and vegetables in a web-based supermarket [10]	25% discount on fruits and vegetables versus regular prices	Adjusted for statistically significant characteristics only (= level of education)	Fruit (g)	126	−376	630	0.62
			Vegetables (g)	445	−105	995	0.11
			F&V (g)	571	−300	1443	0.20
		Fully adjusted (including for level of education)	Fruit (g)	481	−69	1,030	0.09
			Vegetables (g)	504	−64	1,071	0.08
			F&V( g)	984	97	1,872	0.03
2. Introducing taxes, subsidies or both in a web-based supermarket [11]	25% discount on healthy foods versus no discount	Adjusted for statistically significant characteristics only (= none)	Fruit (g)	−376	−1097	345	0.03*
			Vegetables (g)	88	−678	854	0.01*
		Fully adjusted	Fruit (g)	−382	−1105	341	0.10*
		Fully adjusted	Vegetables (g)	52	−665	769	0.05*
	50% discount on healthy foods versus no discount	Adjusted for statistically significant characteristics only (= none)	Fruit (g)	599	−126	1,323	0.03*
			Vegetables (g)	1034	264	1,804	0.01*
		Fully adjusted	Fruit (g)	420	−322	1,163	0.10*
		Fully adjusted	Vegetables (g)	821	85	1,556	0.05*
	10% versus 5% increase on unhealthy foods	Adjusted for statistically significant characteristics only (= none)	Fruit (g)	216	−500	933	0.83*
			Vegetables (g)	−128	−889	633	0.45*
		Fully adjusted	Fruit (g)	304	−421	1,029	0.69*
		Fully adjusted	Vegetables (g)	121	−598	840	0.59*
	25% versus 5% increase on unhealthy foods	Adjusted for statistically significant characteristics only (= none)	Fruit (g)	82	−643	807	0.83*
			Vegetables (g)	345	−425	1,115	0.45*
		Fully adjusted	Fruit (g)	83	−646	813	0.69*
		Fully adjusted	Vegetables (g)	368	−355	1,091	0.59*
3. The effects of price discounts on fruits and vegetables with or without health education in a real supermarket [12]	50% discount on fruits and vegetables versus no intervention	Adjusted for statistically significant characteristics only (= none)	F&V(g) at 1 month	2386	87	4,685	0.04
			F&V (g) at 3 months	1,226	−1,063	3,596	0.31
			F&V (g) at 6 months	5,252	2,836	7,668	<0.01
			F&V (g) at 9 months	−826	−3,284	1,632	0.51
		Fully adjusted	F&V(g) at 1 month	1,295	−1,031	3,621	0.28
			F&V (g) at 3 months	951	−1,293	3,194	0.41
			F&V (g) at 6 months	3,894	1,500	6,287	<0.01
			F&V (g) at 9 months	−1,397	−3,833	1,038	0.26
	Nutrition education versus no intervention	Adjusted for statistically significant characteristics only (= none)	F&V(g) at 1 month	−1,583	−4,049	884	0.21
			F&V (g) at 3 months	−886	−3,432	1,661	0.50
			F&V (g) at 6 months	35	−2,571	2,640	0.98
			F&V (g) at 9 months	−1,235	−3,747	1,277	0.34
		Fully adjusted	F&V(g) at 1 month	−913	−3,383	1,557	0.47
			F&V (g) at 3 months	−91	−2,584	2,402	0.94
			F&V (g) at 6 months	1,075	−1,468	3,617	0.41
			F&V (g) at 9 months	−481	−2,937	1,976	0.70
	50% discount on fruits and vegetables + Nutrition education versus no intervention	Adjusted for statistically significant characteristics only (= none)	F&V(g) at 1 month	1,912	−354	4,179	0.10
			F&V (g) at 3 months	978	−1,386	3,343	0.42
			F&V (g) at 6 months	5,383	2,958	7,808	<0.01
			F&V (g) at 9 months	−1,176	−3,582	1,230	0.34
		Fully adjusted	F&V(g) at 1 month	1,290	−1008	3,587	0.27
			F&V (g) at 3 months	1,213	−1,078	3,504	0.30
			F&V (g) at 6 months	5,556	3,188	7,925	<0.01
			F&V (g) at 9 months	−1,157	−3,533	1,220	0.34
4. Effects of different discount levels on healthy products coupled with a healthy choice label in a web-based supermarket [13]	25% versus 10% discount on healthy foods	Adjusted for statistically significant characteristics only (= none)	Fruit (g)	398	−384	1,180	0.33*
			Vegetables (g)	−83	−887	721	<0.01*
		Fully adjusted	Fruit (g)	393	−442	1,228	0.52*
		Fully adjusted	Vegetables (g)	39	−808	885	0.02*
	50% versus 10% discount on healthy foods	Adjusted for statistically significant characteristics only (= none)	Fruit (g)	544	−193	1,280	0.33*
			Vegetables (g)	1,108	350	1866	<0.01*
		Fully adjusted	Fruit (g)	410	−361	1,182	0.52*
		Fully adjusted	Vegetables (g)	1,017	226	1,807	0.02*
	Special offer label versus combined label	Adjusted for statistically significant characteristics only (= none)	Fruit (g)	61	−676	797	0.94*
			Vegetables (g)	−436	−1,193	322	0.52*
		Fully adjusted	Fruit (g)	74	−704	853	0.78*
		Fully adjusted	Vegetables (g)	−479	−1,286	328	0.50*
	Healthy choice label versus combined label	Adjusted for statistically significant characteristics only (= none)	Fruit (g)	−76	−858	705	0.94*
			Vegetables (g)	−217	−1,021	587	0.52*
		Fully adjusted	Fruit (g)	−200	−1,014	613	0.78*
		Fully adjusted	Vegetables (g)	−282	−1,138	574	0.50*

*Overall p-value comparing the three groups.
CI = confidence interval; F&V = fruit and vegetables.

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