Rich,
I would suggest you to take a look in the group sequential methods.
The following references might be useful:
Pocock, S.J. Group sequential methods in the design and analysis of clinical
trials
Biometrika, v. 64, p.191-199 (1977)
O'Brien P.C. & Flemming T.R., A multiple testing procedure for clinical trials
Biometrics v. 35,
p. 549-556 (1979)
Lan K.K.G. & DeMets D.L., Discrete sequential boundaries for clinical trials
Biometrika, v. 70, 659-663
(1983)
Whitehead J., The design and analysis of sequential trials, Wiley, New Yort, 2.
ed (1997)
Regards
Robert
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