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  • Poster presentation
  • Open Access

Calculating the design effect for a cluster stepped-wedge trial with varying cluster size; a case study from a trial in type 2 diabetes

  • 1, 2,
  • 1, 2,
  • 1, 2 and
  • 2, 3
Trials201516 (Suppl 2) :P124

https://doi.org/10.1186/1745-6215-16-S2-P124

  • Published:

Keywords

  • Public Health
  • Chronic Disease
  • General Practice
  • Cluster Size
  • Median Practice

Background

Type 2 diabetes (T2DM) is a serious chronic disease which can be improved with education. Only 6.0% of people with T2DM are offered education and only 1.6% attend.

Methods

We have designed a cluster trial with a stepped-wedge design (SWD) to test an intervention to increase the uptake to education. This study will include general practices of varying size. There is no published guidance on powering a cluster SWD with varying cluster size. We present the method used to estimate the design effect (DE) which combines the method for calculating the sample size required for a cluster SWD by Woertman et al. with a method for taking into account varying cluster size in a standard cluster trial by Eldridge et al.

Results

We assume a median practice size of 347, IQR 201-678, which gives a Coefficient of Variation (CV) of 119.25/348=0.34. Using the method of Woertman gives a DE of 1.41; this doesn't take into account variation in cluster size. Using the method in Eldridge with a CV of zero (no variation in cluster size) gave DE of 18.35; replacing CV with 0.34 gave DE of 20.36. Hence variation in cluster size inflates DE by 2.01 (20.36-18.35). The total DE therefore when taking into account the SWD and variation in cluster size was 3.42 (1.41+2.01).

Conclusion

We present a pragmatic way of calculating the DE for cluster SWD with variation in cluster size. Future work should focus on the impact of cluster size variation.

Authors’ Affiliations

(1)
Diabetes Research Centre, University of Leicester, Leicester, UK
(2)
Leicester Clinical Trials Unit, University of Leicester, Leicester, UK
(3)
Department of Health Sciences, University of Leicester, Leicester, UK

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