February, 2003
© 2000, 2001, 2003 Scott S. Emerson, M.D., Ph.D.
The increased maximal sample size need not mean a
less efficient design when using a stopping rule
·Fixed sample design requires 100 subjects no matter how
effective (or harmful) the treatment is
·O’Brien-Fleming
stopping rule requires fewer subjects on average (worst case: about
88) and the increase in the maximal sample size is only 4%
·Pocock
stopping rule requires even fewer subjects on average over a wide range
of alternatives, but requires a 35% increase in the maximal
sample size
-However,
there is always less than a 25% chance that a trial would
continue to the last analysis