A sample size of 93 patients in each group was required to detect a five
point change in the functional status score and 20% change in vaccination
rate (power of 80%, a =0.05). This estimate
was inflated to allow for potential clustering as follows; inflation factor=1+
(n-1),[w1]
where 
(rho) is the intraclass
correlation and n is the mean size of the cluster. On the basis of previous
surveys, 
=0.05 was selected.
The estimate of 93 was inflated by 1.3. Allowing for 15% dropout, we aimed
to recruit 42 doctors and 280 patients.
(n-1),[w1]
where (rho) is the intraclass
correlation and n is the mean size of the cluster. On the basis of previous
surveys, =0.05 was selected.
The estimate of 93 was inflated by 1.3. Allowing for 15% dropout, we aimed
to recruit 42 doctors and 280 patients.
- Donner A, Birkett N, Buck C. Randomization by cluster; size requirements and analysis. Am J Epidemiol 1981;114:906-14.
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