Table 2.

P values required to stop at each interim analysis^a.

	O'Brien	Pfam	H-S-D
Analysis	Fleming	Φ=3	γ=-3
1	<.001	<.001	.001
2	<.001	.003	.003
3	.007	.007	.007
4	.02	.01	.01

^aThe power family of spending functions has an adjustable constant Φ > 0; Φ=0 results in equal P values at all interim analyses, increasing Φ gives higher P values for later analyses. The H-S-D family has an adjustable constant γ≠ 0; γ=−4 gives approximately the O'Brien–Fleming spending function, whereas γ=1 results in approximately equal P values at all interim analyses.