Appendix A: Technical Notes
What is a confidence interval?
Simply speaking, confidence intervals are a useful way to consider margin of error, a statistic often used in voter polls to indicate the range within which a value is likely to be correct (e.g., 30% of the voters favor a particular candidate with a margin of error of plus or minus 3.5%). Similarly, in this report, confidence intervals are used to provide a range that we can be quite confident contains the success rate for a particular clinic during a particular time.
Why do we need to consider confidence intervals if we already know the exact success rates for each clinic in 2008?
No success rate or statistic is absolute. Suppose a clinic performed 100 cycles among women younger than 35 in 2008 and had a success rate of 20% with a confidence interval of 12%–28%. The 20% success rate tells us that the average chance of success for women younger than 35 treated at this clinic in 2008 was 20%. How likely is it that the clinic could repeat this performance? For example, if the same clinic performed another 100 cycles under similar clinical conditions on women with similar characteristics, would the success rate again be 20%? The confidence interval tells us that the success rate would likely fall between 12% and 28%.
Why does the size of the confidence interval vary for different clinics?
The size of the confidence interval gives us a realistic sense of how secure we feel about the success rate. If the clinic had performed only 20 cycles instead of 100 among women younger than 35 and still had a 20% success rate (4 successes out of 20 cycles), the confidence interval would be much larger (between 3% and 37%) because the success or failure of each individual cycle would be more significant. For example, if just one more cycle had resulted in a live birth, the success rate would have been substantially higher—25%, or 5 successes out of 20 cycles. Likewise, if just one more cycle had not been successful, the success rate would have been substantially lower—15%, or 3 out of 20 cycles. Compare this scenario to the original example of the clinic that performed 100 cycles and had a 20% success rate. If just one more cycle had resulted in a live birth, the success rate would have changed only slightly, from 20% to 21%, and if one more cycle had not been successful, the success rate would have fallen to only 19%. Thus, our confidence in a 20% success rate depends on how many cycles were performed.
Why should confidence intervals be considered when success rates from different clinics are being compared?
Confidence intervals should be considered because success rates can be misleading. For example, if Clinic A performs 20 cycles in a year and 8 cycles result in a live birth, its live birth rate would be 40%. If Clinic B performs 600 cycles and 180 result in a live birth, the percentage of cycles that resulted in a live birth would be 30%. We might be tempted to say that Clinic A has a better success rate than Clinic B. However, because Clinic A performed few cycles, its success rate would have a wide 95% confidence interval of 18.5%–61.5%. On the other hand, because Clinic B performed a large number of cycles, its success rate would have a relatively narrow confidence interval of 26.2%–33.8%. Thus, Clinic A could have a rate as low as 18.5% and Clinic B could have a rate as high as 33.8% if each clinic repeated its treatment with similar patients under similar clinical conditions. Moreover, Clinic B’s rate is much more likely to be reliable because the size of its confidence interval is much smaller than Clinic A’s.
Even though one clinic’s success rate may appear higher than another’s based on the confidence intervals, these confidence intervals are only one indication that the success rate may be better. Other factors also must be considered when comparing rates from two clinics. For example, some clinics see more than the average number of patients with difficult infertility problems, whereas others discourage patients with a low probability of success. For more information see important factors to consider when using the tables to assess a clinic.
Site visits for validation of 2008 ART data were conducted April through June 2010 for 35 randomly selected clinics. During each visit, data reported by the clinic were compared with information documented in medical records. In total, 1,660 cycles were randomly selected for full validation. These included 662 cycles that resulted in a pregnancy of which 537 resulted in a live-birth delivery. For reporting year 2008, validation was planned for a total of 1,609 cycles that resulted in a multiple-fetus pregnancy. Of these, 72 (4%) were inadvertently omitted from the validation. Statistical analysis of the missing information indicates that omitting a small number of records from the process did not materially effect the estimates of the discrepancy rates. In addition, the validation team also reviewed the documentation for 332 embryo banking cycles.
Discrepancy rates are listed below for key data items that were validated for each of the selected cycles. In all, validation indicated that data discrepancy rates were low (<5%; except for the data field for “Diagnosis of infertility” — see Comments for explanation). These discrepancies in a limited number of individual clinic summary tables had a minimal (e.g., by one or two decimal points) affect on statistics other than percentages of live births.
|Data Field Name||Discrepancy Rate* (Confidence Interval†)||Comments|
|Patient date of birth||2.0%
|Slightly less than one-third of the discrepancies resulted in changes to the clinic table age group categorization (Age of Woman), which mostly differed by one or two age categories.|
|Diagnosis of infertility||12.4%
|For approximately half the discrepancies a single wrong diagnosis is reported, mainly ‘other’ or ‘unexplained’ instead of a specific cause. For nearly 40% of the discrepancies, multiple causes of infertility were found in the medical record, but only a single cause was reported by the clinic.|
|Type of ART
(i.e., fresh vs. frozen, donor vs. nondonor)
|Use of ICSI||<1%|
|Number of embryos/oocytes transferred||<1%|
|Outcome of ART treatment
(i.e., pregnant vs. not
|Number of fetal hearts on ultrasound||1.6%
|Of the discrepancies, six cases resulted in a change in categorization of single fetus pregnancy to multiple-fetus pregnancy.|
(i.e., miscarriage, stillbirth,
and live birth)
|For slightly more than half of the discrepancies, there was no information on pregnancy outcome in the medical record.|
|Number of infants born||<1%|
Notes: ART = assisted reproductive technology; ICSI = intracytoplasmic sperm injection.
* Discrepancy rates estimate the proportion of all treatment cycles with differences for a particular data item. The discrepancy-rate calculations weight the data from validated cycles to reflect the overall number of cycles performed at each clinic. Thus, findings from larger clinical practices were weighted more heavily than those from smaller practices.
† This table shows a range, called the 95% confidence interval, that conveys the reliability of the discrepancy rate. For more information, see explanation of confidence intervals.