Table 2.

In order to have an 80% chance that RSA would not reveal possible failures (excessively migrating stems) in a test sample of well-performing femoral stems (as denoted by the low failure rates in the left column), the RSA sample size would have to be as shown. Columns 2 and 3 show the minimum sample size required when the upper 95% confidence limit of the failure rate is 10% or less (the generally accepted failure rate benchmark for hip prostheses at 10 years). Columns 4 and 5 show the sample size when this confidence limit is raised to 15%. The effect of using a 1-tailed or 2-tailed test for the sample size calculation is also shown

	Required sample size (80% power)
	Upper 95% CL ≤ 10%		Upper 95% CL ≤15%
Failure rate	2-tailed	1-tailed	2-tailed	1-tailed
5%	231	179	75	59
4%	154	116	56	50
3%	100	76	46	40
2%	70	61	35	30
1%	54	46	35	19