Many factors that contribute to a clinic's success rate may have changed, for better or for worse, in the 2 years since these procedures were performed. Personnel may be different. Equipment and training may or may not have been updated. As a result, success rates for 1995 may not reflect those for 1997 or 1998. No reported success rate is absolute.
Every success rate has a margin of error, or range within which it is likely to be correct.
Therefore, a clinic's success rates will vary from year to year even if all determining factors remain the same. The larger the number of cycles that a clinic carries out, the less the rate is likely to vary. Conversely, the smaller the number of cycles, the greater the margin of error and the more variability in success rates from year to year. Thus, rates derived from a small number of cases are almost certain to vary considerably from year to year. For further detail, see the explanation of confidence interval .