80
Case Study:
Impact of Interim Analyses
Required increased maximal sample size in order to maintain power
•Maximal sample size with 4 analyses
–O’Brien-Fleming:       N= 1773 ( 4.3% increase)
–Pocock               :      N= 2340 (37.6% increase)
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•Need to consider
–Average sample size
–Probability of continuing past 1700 subjects
–Conditions under which continue past 1700 subjects