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SUMMARY:Optimising the allocation of participants in a two-stage randomise
 d experiment to estimate selection\, preference and treatment effects - Wa
 lter\, S (McMaster)
DTSTART:20110902T153000Z
DTEND:20110902T160000Z
UID:TALK32622@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:Experimental outcomes may be affected by the choice of treatme
 nt that participants might make (if they were indeed allowed to choose)\, 
 a so-called selection effect\, and by whether they actually receive their 
 preferred treatment\, a so-called preference effect. Selection and prefere
 nce effects can be important (possibly even larger than the usual treatmen
 t effect)\, but they cannot be estimated in conventional randomised experi
 mental designs. \n\nAn alternative approach is the two-stage randomised de
 sign\, in which participants are first randomly divided into two subgroups
 . In one subgroup\, participants are randomly assigned to treatments\, whi
 le in the other\, participants are allowed to choose their own treatment. 
 This approach can yield estimates of the direct treatment effect\, and of 
 the preference and selection effects. The latter two provide insight that 
 goes considerably beyond what is possible in standard randomised experimen
 ts\, notably the usual parallel group design. \n\nIn this presentation\, w
 e will consider the optimal proportion of participants who should be alloc
 ated to the choice subgroup and allowed to determine their own treatment. 
 The precision of the estimated selection\, preference and treatment effect
 s are functions of: the total sample size\; the proportion of participants
  allocated to choose their treatment\; the variances of the response (or o
 utcome)\; the proportions of participants who select each treatment in the
  choice group\; and the selection\, preference and treatment effects thems
 elves. We develop general expressions for the optimum proportion of partic
 ipants in the choice group\, depending on the inverses of these variances\
 , and on which effects are of primary interest. We illustrate the results 
 with trial data comparing alternative clinical management strategies for w
 omen with abnormal results on cervical screening. \n
LOCATION:Seminar Room 1\, Newton Institute
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