Re: {MEDSTATS} Planning an individually randomised trial with natural clusters

[John Whittington]

Siobhan, for my two pence worth, I would invoke the fact that, at heart, I
am generally conservative and keen on making as few assumptions about
results/outcome at design stage. I presume that the only real advantage of
making the 'adjustment' you mention would be the reduction in sample size
estimate that this would facilitate. However, the adequacy of the sample
size thus estimated would be dependent upon your assumptions about ICC in
your data proving to be correct; if not, you could end up with an
under-powered trial. For that reason, I would personally be inclined to
design with a sample size that would be adequate even in the absence of
appreciable ICCs.

Kind Regards,
John

At 06:51 13/10/2009 -0700, Siobhan Creanor wrote:
>Dear All,
>
>We are in the process of designing a trial to compare two groups of
>patients seeing either their GP or practice nurse. We are currently
>looking at recruiting around 10 practices. The primary outcome
>measure is how enabled the patient felt after his/her consultation.
>The PI wants to individually randomise patients to intervention or
>control. This is not therefore a designed cluster RCT, however, from
>the very scant literature, there have been reasonably high ICCs
>reported (up to 0.15 at a health professional level) for this outcome,
>when used in a cluster design.
>
>What is the consensus about adjusting for the 'natural' clustering at
>the design stage? Clearly there are several potential levels of
>clustering, but at the moment we are primarily interest in the
>practice level (i.e. the "centre"). Some authors advocate reducing
>the sample size according to the magnitude of the centre effect,
>which, with an ICC of 0.1 or 0.2, could be quite a substantial
>reduction. For some reason, I don't feel all that comfortable with
>this!
>
>There seems to have been quite a bit of discussion recently about
>'therapist' effects in trials but I'm struggling to come up with
>something comparable to our proposal.
>
>Any thoughts gratefully received!
>
>Kind regards,
>Siobhan

John

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