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2004 Assisted Reproductive Technology (ART) Report: Appendix A |
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How to
Interpret a Confidence Interval |
Findings from
Validation Visits for 2004 ART Data |
Discrepancy Rates by Data Fields Selected for Validation
How to Interpret a
Confidence Interval
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 2004?
No success rate or statistic is absolute. Suppose a clinic performed 100
cycles among women younger than 35 in 2004 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 2004 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, its live birth rate
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 further information see, important factors to consider when using the tables to assess a clinic.
Findings
from Validation Visits for 2004 ART Data
Clinic site visits for validation of 2004 ART data were
conducted June through August 2006. During each visit, data reported by
the clinic were compared with information recorded in patients’ charts.
Records for 1,379 cycles at 28 clinics were randomly selected for
validation. These selected cycles included 574 cycles that resulted in a
pregnancy and 455 cycles that resulted in a live-birth delivery.
Discrepancy rates are listed on the next page for key data
items that were validated for each of the selected cycles. Review of the
discrepancies indicated that in the majority of cases, the error did not
affect the success rates (included in the national summary table and in the
individual clinic tables). In addition to fully validating data for the
randomly selected 1,379 cycles, during each visit the validation team also
reviewed the documentation for every live birth that had been
reported to CDC. In all, validation indicated that the clinic success rates
presented in this report are valid.
Discrepancy Rates by Data Fields Selected for Validation
|
Data
Field Name |
Discrepancy
Rate*
(Confidence Interval†) |
Comments |
| Patient date of birth |
1.5%
(1.1–1.9) |
Nearly all discrepancies were within 1–2 years and did not result in
a change in categorization of age groups. |
| Diagnosis
of infertility |
19.6%
(14.0–25.3) |
For approximately half of these cases, multiple causes of
infertility were found in the patient’s chart, but only a single
cause was reported. |
| Type of
ART (i.e., fresh vs. frozen; donor vs nondonor) |
<1% |
|
| Use of
ICSI |
3.9%
(1.2–6.7) |
For
approximately one-third of these cases, there was no indication in
the patient’s chart that ICSI was used. |
Number of
embryos
transferred |
5.1%
(1.7–8.5) |
Nearly all discrepancies involved higher-order (>2)
embryo transfers and were only a 1- or 2-embryo
difference. |
| Outcome of
ART treatment (i.e., pregnant vs not pregnant) |
1.4%
(0.5–2.3) |
For approximately half of these cases, there was no information on
pregnancy in the patient’s chart. In seven cases, the information in
the chart indicated there was no pregnancy. |
Number of
fetal hearts on
ultrasound |
3.1%
(1.1–5.1) |
Of those with misreported number of fetal hearts,
nine cases resulted in a change in categorization of
single-versus multiple-fetus pregnancy. |
Pregnancy
outcome
(i.e., miscarriage, stillbirth,
and live birth) |
1.7%
(0.8–2.5) |
In most of these cases, there was no information on pregnancy
outcome in the patient’s chart. |
| Number of
infants born |
<1% |
In most of these cases, there was no information
on the number of infants born in the patient’s chart. In four cases,
a twin delivery was recorded in the patient’s chart and a singleton
delivery was reported. In four cases, a singleton delivery was
recorded in the patient’s chart and a twin delivery was reported. |
| Cycle Cancelation |
5.3%
(0.8–9.7) |
In most of
these cases, the information in the patient’s chart indicated the
cycle was canceled, but the canceled cycle was not reported. |
| 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
findings from smaller practices.
† This table shows a range, called the 95% confidence interval,
which conveys the reliability of the discrepancy rate. For a more
general explanation of confidence intervals.
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Previous ART Reports
Implementation
of the Fertility Clinic Success Rate and Certification Act of 1992
Assisted
Reproductive Technology: Embryo Laboratory
Date last reviewed:
01/14/2007
Content source: Division
of Reproductive Health,
National Center for Chronic Disease
Prevention and Health Promotion
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