Open Access Highly Accessed Correction

Correction: study protocol: improving patient choice in treating Low back pain (IMPACT - LBP): a randomised controlled trial of a decision support package for use in physical therapy

Shilpa Patel1*, Sally Brown2, Tim Friede3, Frances Griffiths4, Joanne Lord5, Anne Ngunjiri1, Jill Thistlethwaite6, Colin Tysall2, Mark Woolvine7 and Martin Underwood1

Author Affiliations

1 University of Warwick, Clinical Trials Unit, Warwick Medical School, Gibbet Hill Road, Coventry CV4 7AL, UK

2 Universities/User Teaching and Research Action Partnership (UNTRAP), Institute of Health School of Health and Social Studies, University of Warwick, Coventry CV4 7AL, UK

3 Department of Medical Statistics, University Medical Centre Göttingen, Humboldtallee 32, Göttingen D-37073, Germany

4 University of Warwick, Warwick Medical School, Gibbet Hill Road, Coventry CV4 7AL, UK

5 Brunel University, Health Economics Research Group, London, UK

6 University of Warwick, Institute of Clinical Education, Warwick Medical School, Gibbet Hill Road, Coventry CV4 7AL, UK

7 NHS Coventry Community Physiotherapy, Coventry & Warwickshire Hospital, Stoney Stanton Road, Coventry CV1 4FH, UK

For all author emails, please log on.

BMC Musculoskeletal Disorders 2013, 14:158  doi:10.1186/1471-2474-14-158

Published: 3 May 2013

First paragraph (this article has no abstract)

After publication of this protocol a change in study design was needed [1]. Due to changes in the service configuration in the host physiotherapy department individual randomisation as originally planned could not be implemented. It was necessary to change to cluster randomisation with the unit of randomisation being the treating physiotherapist. Potential participants are given outpatient appointments by booking staff unaware of the physiotherapist’s randomisation. Trial recruitment is also done blind to physiotherapist allocation. In this manner we have ensured allocation concealment prior to participants joining the study. Cluster randomised trials need to inflate their sample size to account for clustering. Typically primary care trials use an intra-cluster correlation coefficient (ICC) of 0.05 in this calculation [2]. Our past experience is that clustering effects by therapist in trials of this nature may be very small [3]. To account for this we developed a provisional revised sample size using an ICC of 0.05 and did an interim analysis of pooled data, just for ICC of the primary outcome, after the first 40 participants had completed the three month follow-up questionnaire. The ICC was close to zero, suggesting that using an ICC of 0.05 was too conservative. We therefore assumed an ICC of 0.01 to estimate the design effect due to clustering. Based on an average cluster size of nine this results in a revised final sample size of 158.