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 .
Some clinics see more than the average number of patients with difficult infertility problems.
Others discourage such patients or encourage them to use donor eggs, which have higher success rates among women older than 35. Clinics that accept a higher percentage of women with multiple previous unsuccessful attempts will generally have lower success rates than clinics that do not. Alternatively, some clinics may raise their success rates by offering ART procedures to patients who might have become pregnant with less technologically advanced treatment. In an unstimulated cycle, the woman ovulates naturally rather than through the daily injections required by stimulated cycles. Unstimulated cycles are less expensive because they eliminate the cost of the injectable drugs.