Table 1.

This table shows that even a well-performing femoral stem design (as determined from arthroplasty registers) could have a high probability of being designated as a possible future failure by an RSA study. Left column: here, a well-performing femoral stem design is considered to have a population failure rate of up to 5% (aseptic survival rate: 95%). The probabilities of RSA revealing possible failures (excessively migrating stems) in samples of 36 and 54 of such stem designs are shown in columns 2 and 3.

Failure rate	P (≥ 1 in 36)	P (≥ 2 in 54)
5%	84%	76%
4%	77%	64%
3%	67%	48%
2%	52%	29%
1%	30%	10%