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Table 4 Cluster and sample size characteristics of the trials included in the review

From: Statistical analysis of publicly funded cluster randomised controlled trials: a review of the National Institute for Health Research Journals Library

Characteristics n % Mean (SD) Median Range IQR
No. of clusters randomised (N = 86)
4–10 2 2 77 (121) 44 7–922 25–74
11–20 11 13     
21–50 40 47     
51–100 21 24     
101–200 5 6     
> 200 7 8     
No. of clusters analysed (N = 86)
0–10 2 2 76 (118) 43 7–864 25–69
11–20 12 14     
21–50 40 47     
51–100 21 24     
101–200 4 5     
> 200 7 8     
No. of subjects recruited (N = 84b)
≤ 300 7 8 15,348 (48315) 1184 141–265,434 597–3653
301–600 14 17     
601–900 11 13     
901–1200 9 11     
1201–1500 9 11     
1501–1800 3 4     
> 1800 29 34     
Not reported 2 2     
No. of subject analysed (N = 84b)
≤ 300 15 18 14,367 (48419) 870 42–264,325 441–2356
301–600 15 18     
601–900 13 15     
901–1200 5 6     
1201–1500 5 6     
1501–1800 2 2     
> 1800 25 30     
Not reported 4 5     
Planned ICC for sample size (N = 86)
0.00–0.02 18 21 0.065 (0.082) 0.05 0.0002–0.5 0.0258–0.0700
> 0.02–0.05 33 38     
> 0.05–0.08 9 11     
> 0.08–0.11 8 9     
> 0.11–0.14 2 2     
> 0.14 6 7     
Not reported 10 12     
Planned design effect (N = 86)
0.00–2.99 47 55 4.5 (8.90) 1.96 1.03–70.5 1.384–4.600
3.00–5.99 12 14     
6.00–8.99 10 12     
9.00–11.99 1 1     
≥ 12 3 3     
Not reported 13 15     
Observed ICC of analysed primary outcome (N = 100)
− 0.02 to 0.02 35 35 0.06 (0.12) 0.02 -0.02 to 0.63 0.0010–0.0600
> 0.02–0.07 9 9     
> 0.07–0.12 3 3     
> 0.12–0.17 6 6     
> 0.17–0.22 2 2     
> 0.22 3 3     
Not reported 42 42     
  1. bTwo trials were excluded because the analysed subjects were measured in person-years. N = total number of trials and/or primary outcomes; n = counts observed in each level of a category; SD = standard deviation. Not reported means that the information of interest was not considered and/or provided in the trial