Table 1 Anticipated results for different success rates of first PCA procedure

χ ² test for first procedure

P < 0.0001

P = 0.007

P = 0.03

Power of the test

99%

84%

65%

Strength of association

Relative risk

2.2

1.8

1.66

95% confidence interval

1.4 to 3.4

1.2 to 2.9

1.0 to 2.6

Odds ratio

4.3

2.9

2.33

95% confidence interval

2.1 to 9.0

1.4 to 5.9

1.1 to 4.7

Difference between proportions

Fraction of second procedures in the two groups

0.65-0.30

0.55-0.30

0.50-0.30

Difference between fractions

0.35

0.25

0.2

95% confidence interval of difference

0.1 to 0.5

0.1 to 0.4

0.0 to 0.3