Trials which randomize practices I: how should they be analysed?
BACKGROUND
In some general practice intervention trials, patients must be randomized in practices rather than individually, and this must be taken into account in the analysis.


OBJECTIVE
In this article we aim to show how failure to do this may lead to spurious statistical significance and CIs which are narrower than they should be, and to describe the use of summary measures for each practice as a simple method of analysis.


METHODS
The statistical issues are demonstrated by an example of a trial in general practice.


CONCLUSIONS
The choice of unit of analysis will be most important where there are large numbers of patients recruited from each practice or a high degree of variability between practices.
Kerry
S M
SM
Division of General Practice and Primary Care, St George's Hospital Medical School, London, UK.
Bland
J M
JM
D005194:Family Practice / Q000706:statistics & numerical data*; D006801:Humans; D011897:Random Allocation; D016032:Randomized Controlled Trials as Topic / Q000379:methods* / Q000706:statistics & numerical data*; D013223:Statistics as Topic / Q000379:methods*