TABLE 2.

Assessment of bias related to study quality items and totals in the randomized controlled trials (RCTs)¹

Source of potential bias (and notes)		No. of RCTs	Combined mean and SE of the residual difference from trend		Statistical significance of the mean residual difference from zero, based on the z test	SD of difference among studies	SE of treatment difference among studies	Among-studies variance
		k	Mean	SE	P > |z|		τ	I 2
			% of the control glycemic response			% of the control glycemic response	% of the control glycemic response
1. Randomization used for allocation	Yes	37	0	1	0.97	9	6	0.70
	No	—	—	—	—	—	—	—
2. Randomization method considered unbiased	Yes	33	0	1	0.98	8	7	0.71
(all with no)	No	4	5	7	0.57	13	10	0.56
(all with no except reference 28)	No	3	1	6	0.85	10	7	0.49
3. Baseline data (blood glucose and insulin) available and similar across treatments	Yes	37	0	1	0.97	9	6	0.70
	No	0	—	—	—	—	—	—
4. Double blinding	Yes	10	0	2	0.94	7	4	0.48
(otherwise single blinding allowing potential for investigator bias)	No	27	1	2	0.97	9	7	0.71
5. Treatments indistinguishable (placebo satisfactory)	Yes	37	0	1	0.98	9	6	0.70
	No	0	—	—	—	—	—	—
6. Dropouts < 20% of participants	Yes	35	0	1	0.84	9	7	0.70
	No	2	−2	3	0.42	4	—	—
7. Adequate explanation of dropouts (including mention of zero dropouts)	Yes	37	0	1	0.98	9	6	0.71
	No	0	—	—	—	—	—	—
8. Crossover study design was used (appropriate to acute responses)	Yes	37	0	1	0.98	9	6	0.71
	No	0	—	—	—	—	—	—
9. Report addresses potential adverse effects	Yes	37	0	1	0.98	9	6	0.71
	No	0	—	—	—	—	—	—
10. English language used in article or abstract	Yes	35	0	2	0.90	10	8	0.74
	No	2	0	6	0.95	9	—	—
11. Funding was independent of a for-profit organization	Yes	0	—	—	—	—	—	—
	No	37	0	1	0.98	9	6	0.71
12. Setting was independent of funding organization (place of study and participants)	Yes	10	0	2	0.98	7	4	0.41
	No	27	0	2	0.87	9	7	0.75
Potentially the highest quality of the 37 RCTs (11/12 possible yeses)	Yes	4	1	3	0.86	6	0	0.00
	No	33	0	2	0.90	9	7	0.72
Quality range	10 or 11	10	0	2	0.86	8	5	0.53
(no. of yeses from a possible of 12)	8 or 9	27	0	2	0.91	9	7	0.74
	6 or 7	0	—	—	—	—	—	—

¹Both individual quality items and the sum of the specified quality items are provided, the former approach being preferred (55). k, number of studies contributing to the combined mean; P > |z|, level of statistical significance based on the z test; τ², among-studies variance; I², among-studies variance expressed as a fraction of the sum of τ² and the within-studies combined SE².