Therefore, a clinic's success rates will vary from year to year even if all other factors remain the same. The larger the number of cycles that a clinic carries out, the less its rates are 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 intervals .
Some clinics see more than the average number of patients with difficult infertility problems.
Some clinics are willing to offer ART to most potential users, even those who have a low probability of success. Others discourage such patients or encourage them to use donor eggs, which result in higher success rates among older women. Clinics that accept a higher percentage of women who have had multiple previous unsuccessful ART cycles will generally have lower success rates than clinics that do not. Conversely, clinics that offer ART procedures to patients who might have become pregnant with less technologically advanced treatment will have higher success rates. Another related issue is that success rates shown in this report are presented in terms of cycles, not women.