Value of Mifepristone in Cervical Preparation Prior to Dilation and Evacuation 19-24 Weeks: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Value of Mifepristone in Cervical Preparation Prior to Dilation and Evacuation 19-24 Weeks (NCT01615731), a randomized na study. The primary outcome is Procedure Time.

2. Clinical Question

This is a question about a continuous outcome: procedure time measured in minutes. The posted group means are 10.9 minutes for two sets of dilators and 11.8 minutes for mifepristone plus one set. The estimated mean difference, expressed as the mifepristone-plus-one-set group minus the two-set group, is 0.9 minutes. The confidence interval is compatible with a range of effects, including a lower mean time in the mifepristone group and a higher mean time. It does not establish that either approach is better or worse.

The stated rationale is that multiple dilator sets can require additional painful procedures and time away from work or home. Those considerations motivate the comparison, but they should be distinguished from the primary endpoint. The primary result concerns intraoperative time, not the total burden of preparation, pain, patient experience, or time spent in clinic. Those topics appear among the listed secondary outcomes, but their results are not reported in the registry record.

3. Trial Design

The registry describes a randomized, single-masked, parallel-group trial sponsored by Stanford University. The study status is completed, and enrollment is listed as 50. The start is recorded as 2012-05 and primary completion as 2013-06. The registry lists the phase as NA. The meaning of the masking designation is given only as single; which people were masked and how masking was maintained are not reported in the registry record.

Participants were assigned to one of two cervical-preparation approaches: two sets of osmotic dilators or mifepristone plus one set of dilators. The first arm description says that two sets were inserted one and two days before the procedure. The other arm description specifies one set plus mifepristone. The registry lists misoprostol and intra-amniotic digoxin among the interventions, but the arm descriptions do not explain their use or timing. The complete treatment protocol, including doses, eligibility criteria, randomization procedures, and any co-interventions, is not reported in the registry record.

The primary outcome was procedure time in minutes, defined as the interval from speculum insertion to removal. It is presented as a mean with standard deviation. Outcome counts are 21 for two sets of dilators and 24 for mifepristone plus one set. The registry enrollment figure and outcome counts describe different record entries; the reasons for participants not appearing in the outcome counts are not reported in the registry record. No participant flow details or analysis-population definitions are posted.

The record lists six secondary outcomes, including maximum cervical dilation, estimated blood loss, surgeon-rated ease, patient-perceived pain, overall patient experience, and adverse events. Although the record lists eight outcome measures, no secondary outcome results are described in the posted results addressed. The registry reports zero analyses. The original sample-size rationale, intended statistical methods, handling of missing data, and any interim analyses are not reported in the registry record.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Overall Study

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

6. Primary Endpoint

The registry lists the primary outcome as Procedure Time, measured over Intraoperative Time.

The primary endpoint is procedure time, measured in minutes from speculum insertion to removal during the procedure. It captures the duration of the intraoperative interval defined by the registry. Because time is a continuous measurement, the result is reported as a group mean and standard deviation, with a difference in means used to compare the preparation groups.

Procedure time is relevant to the trial's stated question because cervical preparation might affect the conduct or duration of the subsequent procedure. However, it represents only the interval between the specified start and end points. It does not include the time spent placing dilators, waiting between preparation steps, attending clinic visits, or time away from work or home. The registry's background describes those broader burdens as reasons to study an approach that might avoid a second set of dilators, but they are not included in this primary time measure.

The endpoint also does not directly measure pain, ease of procedure, patient experience, cervical dilation, or adverse events. These are listed as secondary outcomes, but their results are not reported in the registry record. The endpoint therefore supports a focused comparison of intraoperative duration; it cannot by itself describe the overall experience or all consequences of either preparation strategy.

7. Results Posted in the Registry

• Mifepristone Plus One Set of Dilators: mean 11.80 versus 10.90 on control; difference 0.90 (95% CI -2.17 to 3.97), standardized effect d = 0.17; z-test p = 0.566.

8. Statistical Interpretation

9. Statistical Methods Explained

Procedure time is a continuous outcome, so the primary summary describes the average duration in each group and the spread of individual values around each average. The mean is the arithmetic average of the recorded times. The standard deviation describes how dispersed individual measurements are around the mean. These summaries answer different questions: the mean describes the group center, while the standard deviation describes variation among participants.

What the mean difference represents

The reported mean difference compares the mifepristone-plus-one-set group with the two-sets-of-dilators group. The difference is 0.9 minutes, with the direction defined as the first named group minus the comparator. A positive estimate means the observed mean was higher in the mifepristone-plus-one-set group. This is a comparison of group averages, not a statement about every participant's procedure time. Individual times may fall above or below either mean.

The difference is an estimate based on the observed participants, not a guaranteed value for a wider population. Random assignment is intended to support a fair comparison of groups, but randomization procedures and the details of the analysis are not reported in the registry record. The posted estimate should therefore be interpreted alongside its uncertainty and the available details of the trial.

How to read the confidence interval and p-value

A confidence interval expresses statistical uncertainty around an estimated effect. In general, a narrower interval indicates greater precision, while a wider interval indicates less precision. The interval from -2.17 to 3.97 minutes includes values below and above no difference. Thus, the data are compatible with a lower average time in the mifepristone-plus-one-set group, little difference, or a higher average time. It does not assign a probability to any one true value, and it does not show that all values in the interval are equally plausible.

The unadjusted p-value is 0.566. In general, a p-value measures how unusual data at least as discrepant as those observed would be under a specified null model. It is not the probability that the null hypothesis is true, nor is it a measure of the size or importance of the effect. The registry record does not report a full statistical analysis plan, so the posted p-value should not be used to infer unreported modeling or adjustment choices.

Why variability matters

The standard deviations are 5.1 and 5.4 minutes. In general, greater variation among individual measurements makes it harder to estimate a group mean precisely unless there are enough observations to offset that variation. A pooled standard deviation of 5.263 is also listed for the comparison. Pooling is a way to summarize within-group variability under assumptions about the groups' variability; its presence does not establish which model or procedure was used in the sponsor's original analysis. The exact analysis method is not reported in the registry record.

Over-dispersion is a general concern when observed variation is greater than a statistical model expects. If a model understates the true variability, standard errors may be too small and confidence intervals too narrow. For a continuous outcome, the relevance of that concern depends on the model and distributional assumptions. The registry record does not report diagnostic checks or a specific model, so over-dispersion cannot be asserted as a feature of these data.

Distinguishing observed outcomes from planning calculations

The displayed group summaries and the mean difference describe the observed outcome counts. The sample-size figures are a separate planning illustration based on the observed effect and stated assumptions, not a description of how the trial was originally designed. Such planning calculations ask how many participants per arm might be needed under specified assumptions to detect a chosen effect with a stated power. They do not retrospectively establish the probability that this completed trial would detect an effect, and they do not change the observed result.

Statistical power, in general, is the chance that a specified analysis would identify an effect at a chosen significance threshold if a particular effect were truly present and the model assumptions held. It depends on the effect being targeted, variability, design, significance level, allocation, and sample size. The original design assumptions, any planned effect size, and any power calculation for this trial are not reported in the registry record. Planning values based on the observed result should not be mistaken for those original design choices.

The registry identifies the trial as randomized and parallel group, but it does not report allocation concealment, the randomization sequence, the precise masked party, or whether the analysis followed assignment as randomized. Those methodological details affect the assessment of bias and cannot be inferred from the design labels alone. Similarly, the registry lists no posted analyses, so model adjustments, protocol deviations, subgroup analyses, and sensitivity analyses are not reported in the registry record.

10. Confidence Intervals and Precision

The estimated mean difference in procedure time is 0.9 minutes, with a 95% confidence interval from -2.17 to 3.97 minutes. Since the interval crosses zero, the posted results do not rule out no average difference. The negative and positive ends also allow for differences in either direction. The interval is wide enough that the point estimate alone should not be treated as a precise description of the underlying group difference.

In general, a confidence interval is a range generated by a statistical procedure intended to reflect sampling uncertainty. Its width depends on the variability in the measurements, the amount of information available, and the analysis method. A confidence interval is not a range containing every participant's time, and it is not a guarantee that the true effect lies inside it. It describes uncertainty about the estimated group contrast under the assumptions of the method used.

The group means, 10.9 and 11.8 minutes, are close as point estimates, while the group standard deviations, 5.1 and 5.4 minutes, indicate appreciable individual-level spread relative to those averages. The posted outcome counts are 21 and 24. Together, these summaries help explain why a modest estimated difference may remain uncertain. The registry record does not report the original precision target or any clinically important threshold for procedure time. Without such a threshold, statistical uncertainty cannot be translated into a judgment about whether the interval is sufficiently narrow for a particular practical purpose.

The confidence interval should be considered alongside the study's restricted endpoint and the lack of posted results for secondary outcomes. Precision for mean intraoperative time does not establish precision for total time, pain, or patient experience. Those outcomes are not reported in the registry record.

11. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The sample-size display is a planning illustration, not an account of the original trial design. Its entries are participants per arm for 80% power and participants per arm for 90% power, not total events. The stated planning assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The effect to detect is set to a stated fraction of the observed effect.

The rows describe planning for 100% of the observed effect, planning for 75% of the observed effect, and planning for 50% of the observed effect. For each target, the 80% power column gives the number of participants per arm under the assumptions that would provide an 80% chance of detecting that target effect if it were true. The 90% power column gives the corresponding per-arm planning figure for a 90% chance. These are conditional design calculations, not guarantees about a future study.

Smaller effects are harder to distinguish from random variation, so detecting a smaller effect generally requires more participants. Accordingly, the displayed planning figures increase as the targeted fraction becomes smaller. The figures are based on the observed effect in this trial, which is itself uncertain; they should not be interpreted as a stable or independently established treatment effect.

This is not observed power. It does not measure whether the completed trial was successful, and it cannot explain the result after the fact. The trial's own original design assumptions and sample-size justification are not reported in the registry record. The displayed sample sizes therefore should not be attributed to the investigators' original protocol.

12. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, Δ is the difference in means to detect and σ is the common standard deviation.

Worked calculation for this trial

1. Observed difference Δ = 0.900; pooled standard deviation σ = 5.263.
2. n = 2 × (1.96 + 0.842)² × 5.263² / 0.900² = 536.7, rounded up to 537 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
delta <- 0.9                      # observed difference in means
sigma <- 5.26259287052782                     # pooled standard deviation
n_per_arm <- function(power, frac = 1) {
  ceiling(round(2 * (z_a + qnorm(power))^2 * sigma^2 / (delta * frac)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (t-based, so very slightly larger)
cat("power.t.test:", round(power.t.test(delta = delta, sd = sigma, power = 0.80)$n, 1), "\n")

# Simulation: exact two-sample t-test using simulated means and variances
set.seed(1615731)
n    <- n_per_arm(0.80)
reps <- 20000
m_diff <- rnorm(reps, delta, sigma * sqrt(2 / n))             # difference in sample means
s2     <- sigma^2 * rchisq(reps, 2 * (n - 1)) / (2 * (n - 1))   # pooled sample variance
tstat  <- m_diff / sqrt(s2 * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(tstat) > qt(0.975, 2 * (n - 1))), reps))

Output from running the code

100% of observed effect: n per arm = 537 (80% power), 719 (90% power)
 75% of observed effect: n per arm = 955 (80% power), 1,278 (90% power)
 50% of observed effect: n per arm = 2,147 (80% power), 2,875 (90% power)
power.t.test: 537.7 
Simulated power at n = 537 per arm: 0.803 (20000 replicates)

The simulation draws many sets of group means and variances at the planned sample size and counts how often the two-sample t-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

13. Multiplicity, Missing Data and Analysis Populations

The registry lists six secondary outcomes: maximum cervical dilation, adverse events described as estimated blood loss, ease of procedure by a blinded surgeon, pain perceived by the patient, overall patient experience, and adverse events. It also lists eight outcome measures in total. Results for those secondary outcomes are not reported in the registry record. The record reports zero analyses, so there is no posted set of secondary comparisons to interpret.

When a study evaluates multiple outcomes or performs many comparisons, the chance of at least one apparently notable result can increase. In general, multiplicity methods can control that risk by adjusting inference or by distinguishing primary from exploratory analyses. Whether such methods were planned or used for this trial is not reported in the registry record. The listed primary endpoint is procedure time; the registry does not provide enough detail to determine how the investigators handled the family of secondary outcomes.

Missing-data handling and analysis populations are also not described. The registry enrollment entry is 50, while the primary outcome counts are 21 and 24. Participant flow, reasons for unavailable outcome data, exclusions, and the number randomized to each arm are not reported in the registry record. It is therefore not possible to determine from the posted record whether the primary result includes all randomized participants, only participants with measured outcomes, or another defined population.

In general, missing observations can affect the interpretation of a group comparison, particularly if the likelihood of missingness differs between groups or relates to the outcome. No assumptions, imputation methods, sensitivity analyses, or protocol-deviation details are reported in the registry record. These omissions do not establish that the analysis was biased, but they limit assessment of how robust the posted estimate is to unavailable data.

14. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

Most frequent non-serious adverse event terms

The serious adverse event entries report zero affected participants in both groups. The reported at-risk counts are 24 for two sets of dilators and 26 for mifepristone plus one set. These are the denominators associated with the serious adverse event entries; they differ from the outcome counts reported for procedure time. The registry does not explain the distinction between these counts.

Zero recorded serious adverse events means that none are listed as affecting participants in these entries. It does not prove that the risk is absent, and it does not establish that the groups have equivalent safety. With limited numbers of participants, uncommon events may not be observed. The record does not provide event definitions, ascertainment procedures, follow-up duration, or details about seriousness criteria.

The adverse-event record separately lists non-serious events affecting 3 participants in the two-sets group and 2 participants in the mifepristone-plus-one-set group. Deaths are not reported. These entries should be read as registry counts rather than as a complete clinical safety assessment. The listed secondary adverse-event results and further details are not reported in the registry record.

15. What the Result Does, and Does Not, Mean

The primary result is a comparison of mean intraoperative procedure time between two cervical-preparation strategies. The observed means were 10.9 minutes with two sets of dilators and 11.8 minutes with mifepristone plus one set. The estimated mean difference was 0.9 minutes, with a 95% confidence interval from -2.17 to 3.97 minutes and an unadjusted p-value of 0.566. These data do not establish a clear difference in average procedure time, but they also do not establish that the approaches produce identical times.

The result is limited to the endpoint as defined: the period from speculum insertion to removal. The trial rationale also concerns multiple procedures, time in clinic, and time away from work or home. Those broader outcomes cannot be answered by this primary endpoint. Although pain, overall experience, maximum cervical dilation, and other outcomes are listed, their results are not reported in the registry record.

The confidence interval includes values in either direction, so the estimate has meaningful uncertainty. A non-significant p-value is not proof of no effect, and the record does not report an equivalence or noninferiority framework. No threshold for a clinically important difference in procedure time is reported in the registry record, limiting interpretation of how consequential the range of estimates might be.

The trial record identifies randomization and single masking, but does not describe the randomization procedure, the masked party, participant flow, or the analysis population. The primary outcome counts and enrollment entry do not by themselves explain what happened to participants without a posted procedure-time result. These reporting gaps constrain assessment of the result's representativeness and robustness.

Thus, the posted data describe a modest estimated difference with limited precision in the primary endpoint. They do not support conclusions about all aspects of the preparation strategies, uncommon harms, or the broader experience of participants. The interpretation should remain tied to the reported groups, outcome, and available registry entries.

16. Why This Trial Matters Statistically

This trial illustrates why a randomized comparison should be read through both its effect estimate and its uncertainty. The observed means are close, but the confidence interval extends in both directions around no difference. Looking only at the point estimate could make the comparison seem more definite than it is; looking only at the p-value could incorrectly suggest that similarity has been demonstrated. The interval gives a more informative view of the range of effects compatible with the posted result.

It also highlights the distinction between a narrowly defined procedural endpoint and a broader practical question. Procedure time measures only the interval specified in the registry. The stated motivation includes the burden of multiple preparation visits and time in clinic or away from work and home. Those questions require outcome information beyond intraoperative duration. The listed secondary measures could add context, but their results are not reported in the registry record.

Another statistical lesson is that planned sample sizes depend on the effect one wants to detect. The planning illustration based on the observed effect makes clear that targeting a smaller effect requires a larger sample under its stated assumptions. That exercise is distinct from the trial's original design justification, which is not reported. Planning calculations cannot substitute for the actual uncertainty in the observed comparison.

Finally, outcome counts, enrollment, and adverse-event denominators need to be interpreted in their own contexts. Different counts appear for the primary endpoint and safety entries, while participant flow and analysis populations are not described. Clear reporting of these elements is important for evaluating precision, missingness, and the applicability of the estimate.

17. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The registry record leaves several trial-specific details unavailable. Eligibility criteria and participant characteristics are not reported, making it difficult to assess how closely the analyzed participants represent the population described in the trial title. The exact mifepristone dose and timing, full dilator protocol, and the use of listed co-interventions are also not described in the arm information.

Although the study is labeled randomized and single-masked, details of sequence generation, allocation concealment, the masked party, and maintenance of masking are not reported. The record also does not define the analysis population or provide participant flow. Enrollment is listed as 50, while the procedure-time outcome counts are 21 and 24; reasons for the difference are not reported in the registry record.

The primary result has a confidence interval spanning differences in both directions, and no threshold for an important difference is reported. There is no registry-reported equivalence or noninferiority margin. The original sample-size justification, statistical analysis plan, missing-data approach, multiplicity handling, interim analyses, and sensitivity analyses are not reported in the registry record.

The registry lists multiple secondary outcomes but does not report their results. The primary endpoint also captures only intraoperative duration, not total preparation time or the broader burdens motivating the study. Safety entries provide limited counts without event definitions, ascertainment details, or follow-up information. These reporting limitations restrict the conclusions that can be drawn from the posted record; they do not, on their own, establish that any specific flaw occurred.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Value of Mifepristone in Cervical Preparation Prior to Dilation and Evacuation 19-24 Weeks — NCT01615731.
• Registry record last updated 2024-02-13; status last verified 2024-01.
• Results first posted 2015-01-26.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

21. Record Summary

NCT01615731 is a completed randomized, single-masked, parallel-group trial sponsored by Stanford University. The registry lists enrollment of 50, a start of 2012-05, and primary completion in 2013-06. The comparison was two sets of osmotic dilators versus mifepristone plus one set before dilation and evacuation at 19-24 weeks.

The primary endpoint was mean procedure time in minutes, measured from speculum insertion to removal. The posted means were 10.9 and 11.8 minutes, respectively, with a mean difference of 0.9 minutes and a 95% confidence interval from -2.17 to 3.97 minutes. The unadjusted p-value was 0.566. The record lists secondary outcomes, but their results are not reported in the registry record.