Postoperative Pain Control for Prostatectomy: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Postoperative Pain Control for Prostatectomy (NCT00913068), a randomized na study. The primary outcome is Total Milligrams of Opiates.

2. Clinical Question

The clinical question concerns postoperative opioid use among patients undergoing radical retropubic prostatectomy. The registry record describes a comparison between a transverse abdominal plane (TAP) procedure and standard postoperative pain control. Its stated rationale is to investigate an anesthetic procedure in the context of patient comfort and narcotic use. The posted primary outcome, however, is opioid use in milligrams, not a direct comfort or pain-score measure.

The registry record does not report a posted pain-score result. It therefore allows interpretation of the opioid-use outcome, but not a direct comparison of patient-reported pain or comfort. It also does not describe how measurements at the listed time points were combined into the reported endpoint.

3. Trial Design

The registry describes a completed, randomized, parallel-group trial with triple masking. Its listed enrollment is 110, and the condition is prostate cancer. The study start is recorded as 2009-07 and primary completion as 2011-09. The phase is listed as NA. The sponsor is Nova Scotia Health Authority.

Participants were assigned to a TAP arm or standard postoperative pain control. The TAP arm description states that a staff urologist injected local anesthetic bilaterally into the anterior abdominal wall from inside the abdomen at the end of surgery. The standard arm used the described multimodal postoperative analgesic approach, including local anesthetic around the incision and systemic medications. The record does not explain which individuals were masked, how masking was maintained, or whether the procedures could be distinguished by participants or staff.

The primary outcome is a continuous measure: mean postoperative opioid use in milligrams. The record lists assessment times of 2, 6,12, 24, 48 and 72 hours. It reports outcome data for 48 participants in the TAP arm and 46 in the standard-control arm. The registry record does not explain why these outcome counts differ from the enrollment figure, whether all randomized participants contributed data, or how missing measurements were handled.

The primary-outcome description states that 40 patients per arm were needed for a power of 0.9 with a 20% change in pain scores. This design statement refers to pain scores, while the posted primary endpoint is opioid use. Further details about the original design assumptions, including how that statement was derived, are not reported in the registry record. The trial is marked completed, but the record does not give a detailed account of its conduct or analysis populations.

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.

Before assignment: 16 were excluded before starting the study due to chronic opiate use, or for requiring a secondary general anesthetic

Overall Study

Analysis population for the primary outcome, as defined in the registry: In order to achieve a power of 0.9 with a 20% change in pain scores, 40 patients were needed per arm

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 Total Milligrams of Opiates, measured over 2, 6,12, 24, 48 and 72 hours.

The primary endpoint is total opioid use, expressed as a mean number of milligrams used postoperatively. It is a continuous measure: each participant’s use is represented on a numerical scale, and the arm summaries report a mean and standard deviation. The listed assessment times are 2, 6,12, 24, 48 and 72 hours.

Opioid use is relevant to the stated study rationale, which concerns minimizing narcotic use while addressing postoperative comfort. It captures medication consumption rather than pain directly. A difference in opioid use therefore does not, on its own, describe pain intensity, comfort, satisfaction, or the balance between medication use and symptom relief. No pain-score outcome is posted in the registry record.

The record does not clarify whether the endpoint is cumulative use across all listed times, a particular time-point measure, or another summary of repeated measurements. It describes the endpoint as the mean number of milligrams used postoperatively, but the construction of that summary is not reported in the registry record. This matters because repeated measurements can be analyzed in different ways, and their interpretation depends on the endpoint definition and analysis method.

The mean is useful for comparing average consumption between randomized arms, while the standard deviation describes variation among participants within each arm. Neither summary reveals the distribution in detail. In particular, the registry record does not report whether opioid-use values were skewed or whether a method addressing such features was used. The posted endpoint supports interpretation of the group means and their estimated difference, subject to these reporting limits.

7. Results Posted in the Registry

• TAP Arm: mean 5.15 versus 2.52 on control; difference 2.63 (95% CI 1.11 to 4.15), standardized effect d = 0.69; z-test p = <0.001.

8. Statistical Interpretation

9. Statistical Methods Explained

Continuous outcome summaries. The primary outcome is continuous, and the registry reports a mean and standard deviation for each arm. A mean describes the average value, while a standard deviation describes the spread of participant values around that mean. The arm summaries are 5.15 with standard deviation 4.49 for the TAP arm, and 2.52 with standard deviation 2.91 for standard postoperative pain control. These summaries do not show the full distribution or identify unusually high or low observations.

Mean difference. A mean difference expresses the contrast between the two arm means in the original outcome units. The recalculated unadjusted mean difference is 2.63 mg, with the TAP arm higher. The estimate is based on the posted arm summaries and is not an adjusted analysis. An unadjusted comparison does not account for covariates or other modeling choices. The registry record does not report an adjusted estimate for this endpoint.

Confidence interval and test. The 95% confidence interval for the recalculated difference is 1.107 to 4.153 mg. A confidence interval describes a range of values compatible with the data under the statistical model and its assumptions; it is not a guarantee that the true value lies within that particular interval. The unadjusted p-value is <0.001. A p-value describes how unusual data at least as extreme as those observed would be under a specified null model, assuming the analysis assumptions hold. It does not measure the probability that the null hypothesis is true, the probability that the result occurred by chance, or the clinical importance of the contrast.

Role of the standard deviation. The standard deviations provide context for variability in opioid use. When outcomes vary substantially across participants, the group means may be less precise than when values cluster closely. The registry record does not give distribution plots, medians, ranges, or diagnostic assessments. It is therefore not possible to assess skewness or outliers from the reported summaries alone. In general, pronounced skewness or outliers can influence a mean and methods that rely on distributional assumptions; the record does not state whether these issues were assessed.

Assumptions behind a comparison. A simple unadjusted comparison of means generally relies on assumptions about the observations and the sampling distribution of the estimated difference. The recalculation is based on the posted summaries and does not reproduce an undisclosed sponsor analysis. It should not be confused with a detailed reconstruction of the trial’s planned statistical analysis. The original statistical analysis plan, model specification, handling of repeated observations, and any covariate adjustments are not reported in the registry record.

Repeated time points. The primary outcome timeframe lists measurements at 2, 6,12, 24, 48 and 72 hours. Repeated observations from the same participant are related, rather than independent measurements. General methods for repeated data account for that within-participant relationship when estimating changes over time or an overall contrast. The registry record does not say whether values were assessed separately at each time, combined into a cumulative measure, or analyzed with a repeated-measures model. The single posted comparison cannot resolve that ambiguity.

Over-dispersion in general. For count outcomes, over-dispersion means that variation exceeds what a simple count model expects; it can make standard errors too small if ignored. That concept is not directly the model reported for this continuous endpoint. More broadly, choosing a model that fits the outcome’s scale and variability is important. The registry record does not report a model for the primary continuous outcome, so no claim can be made about whether any particular diagnostic or alternative model was used.

Sponsor-reported test. The registry separately lists a Chi-squared method with p=0.001. A Chi-squared test is generally used for categorical frequency data, but the record does not identify the analyzed variable, the categories, or the test statistic. It also gives no estimate or confidence interval. The method label alone is insufficient to infer what was tested or to treat it as an analysis of mean opioid use. Its result should be described as a separate sponsor-reported analysis, not as confirmation of the recalculated continuous comparison.

Power and detectable effects. Power is the probability that a specified statistical procedure will detect an effect of a stated size under its assumptions. The planning illustrations use the observed effect and listed assumptions, rather than the original trial design. They are not observed power and do not describe what the trial’s actual design was intended to detect. The original design statement in the registry refers to a 20% change in pain scores and 40 patients per arm, but a fuller justification and the relationship to the opioid-use endpoint are not reported.

Interpretation limits. Statistical methods do not replace a clear endpoint definition or complete reporting. The registry does not report missing-data handling, analysis populations, multiplicity control, or the relation of the sponsor’s test to the primary endpoint. Those omissions prevent a comprehensive assessment of the analysis plan. The posted recalculation remains a simple, unadjusted description of the continuous summary statistics, interpreted alongside the registry’s limited reporting.

10. Confidence Intervals and Precision

The recalculated mean difference is 2.63 mg, and its 95% confidence interval extends from 1.107 to 4.153 mg. The interval expresses uncertainty in the estimated contrast under the assumptions used for the recalculation. It is not a range containing every participant’s opioid use, nor does it describe the spread of individual responses. The arm standard deviations, 4.49 and 2.91, describe participant-level variation in each group and answer a different question from the confidence interval.

Because the interval lies above zero, the recalculated estimate is consistent with a higher mean in the TAP arm than in the standard-control arm. The width of the interval also indicates that the exact size of the mean difference is uncertain. Readers should consider the interval rather than relying only on the unadjusted p-value of <0.001. A small p-value does not make the estimate exact, and statistical precision does not establish clinical importance.

In general, confidence intervals are affected by the amount of information, variability, and the analysis model. The registry outcome counts are 48 and 46, but the record does not explain the relationship between those counts and the listed enrollment of 110. It also does not report missing-data procedures, the endpoint’s detailed construction across time points, or a model accounting for repeated measurements. These gaps limit assessment of how the reported uncertainty would change under other reasonable analyses.

The interval is a recalculated, unadjusted estimate, not a confidence interval reported by the sponsor. The sponsor analysis lists no interval or effect estimate, so its precision cannot be assessed from the registry entry. The confidence interval should therefore be read as uncertainty around the posted continuous-outcome comparison, not as a full account of all uncertainty about the trial’s conduct, endpoint definition, or generalizability.

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 planning illustrations express the number of participants needed per arm for the stated power targets; they are not total event counts. The table and curve vary the assumed effect from 100% of the observed effect to 75% and 50% of the observed effect. For each effect assumption, the columns describe participants per arm for 80% power and participants per arm for 90% power. The 80% column means the planning calculation targets an 80% chance of detecting the specified effect under its assumptions; the 90% column targets a 90% chance.

The calculation assumes a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The effect to detect is the stated fraction of the observed effect. These are planning illustrations calculated from the observed result, not a report of the original trial’s design process. The registry record separately says that 40 patients per arm were needed for power of 0.9 with a 20% change in pain scores. The original design assumptions beyond that statement are not reported in the registry record.

Smaller effects require more participants to distinguish them from random variation at the same power target. The figures are therefore useful for understanding how a planning target changes when the assumed effect is reduced, but they should not be treated as a recommendation or a description of the trial’s actual statistical power. This is not observed power. Observed results and their uncertainty are described by the estimated difference and confidence interval, not by calculating power after the trial from the observed effect.

The assumptions may not match the trial’s actual endpoint construction, follow-up, analysis method, or allocation details. In particular, the primary outcome is opioid use, whereas the registry’s design statement refers to a change in pain scores. The illustrations should be read as conditional planning scenarios, not as proof that the trial was designed to detect the posted opioid-use difference.

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 = 2.630; pooled standard deviation σ = 3.800.
2. n = 2 × (1.96 + 0.842)² × 3.800² / 2.630² = 32.8, rounded up to 33 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 <- 2.63                      # observed difference in means
sigma <- 3.8001561752804                     # 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(913068)
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 = 33 (80% power), 44 (90% power)
 75% of observed effect: n per arm = 59 (80% power), 79 (90% power)
 50% of observed effect: n per arm = 132 (80% power), 176 (90% power)
power.t.test: 33.8 
Simulated power at n = 33 per arm: 0.795 (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. Analyses Reported by the Sponsor

The registry lists a sponsor-reported Chi-squared analysis with p=0.001. It gives no effect estimate, confidence interval, or description of the outcome and categories tested. The method entry is associated with one group, and its relationship to the continuous primary endpoint is not explained. As a result, the sponsor’s analysis cannot be reconstructed or directly matched to the posted means and standard deviations from the registry information.

The recalculated comparison uses the reported continuous arm summaries: the mean difference is 2.63 mg, with a 95% confidence interval from 1.107 to 4.153 mg and unadjusted p <0.001. This is a simple unadjusted re-calculation, not a substitute for a sponsor analysis with a documented model. The two p-values come from entries whose methods and targets are not shown to be the same. Their closeness does not establish that they test the same hypothesis.

There is no stated discrepancy between the recalculation and sponsor analysis in the registry record. Nonetheless, the sponsor’s own analysis takes precedence when describing what the sponsor reported. Because key details are absent, it is not possible to establish whether the Chi-squared result concerns the primary outcome, a transformed outcome, or another analysis. The recalculation provides a transparent estimate from the posted continuous summaries, while the sponsor entry remains a separate and incompletely specified result.

14. Multiplicity, Missing Data and Analysis Populations

The registry lists one outcome measure and one analysis, and no secondary outcomes are posted. The primary-outcome timeframe nevertheless names multiple postoperative assessment times: 2, 6,12, 24, 48 and 72 hours. The record does not clarify whether these are repeated measurements contributing to one combined endpoint, separate comparisons, or measurement occasions summarized in another way. It also does not report a multiplicity plan or state whether any adjustment for multiple tests was used.

Multiplicity matters when researchers conduct several statistical tests, because repeated testing can increase the chance of at least one apparently unusual result. General approaches include specifying one primary comparison in advance or adjusting inference across a family of tests. Which approach is appropriate depends on the analysis plan and endpoint structure. The registry does not report enough detail to determine what family of tests, if any, was considered or how the listed times were handled.

Missing-data procedures are not reported in the registry record. The posted outcome counts are 48 for the TAP arm and 46 for standard postoperative pain control, while listed enrollment is 110. The record does not explain this difference, state whether these counts represent all randomized participants with available outcome data, or describe imputation, exclusions, or sensitivity analyses. Without that information, the impact of missing observations on the estimated comparison cannot be assessed.

The analysis population is also not described. The registry does not say whether the primary analysis included participants as randomized, only those with observed outcomes, or another population. Nor does it report protocol deviations or an analysis plan defining inclusion criteria. These omissions limit assessment of whether the posted result follows the intended randomized comparison and how robust it is to alternative analysis choices.

The record contains a sponsor-reported Chi-squared test but does not explain its target or its relation to the primary outcome. This limits assessment of whether it contributes to multiplicity or represents an additional analysis. The absence of detail does not show that a particular procedure was or was not used; it means these matters are not reported in the registry record.

15. 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 0.5% reporting threshold.

The adverse-event figures list zero serious adverse events in the TAP arm among 56 participants at risk, and zero in the standard postoperative pain-control arm among 54 participants at risk. The record also lists zero other adverse events among 56 and 54 participants at risk, respectively. Death counts are not reported, and no top serious-event terms are listed.

These figures describe what the registry reports for the stated at-risk groups. They should not be confused with the primary-outcome counts of 48 and 46; the record does not explain why the populations differ. Zero reported events in these entries does not establish that no adverse events occurred outside the recorded categories or reporting period. The registry record does not provide further detail about event definitions, ascertainment, timing, severity grading, or completeness of safety follow-up.

In general, adverse-event counts are descriptive and need context about who was monitored, for how long, and how events were collected. The record’s figures do not support a detailed comparison of safety risks, and they do not establish that either approach is free of risk. They should be interpreted narrowly as the serious and other event counts displayed for the listed at-risk groups.

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

The posted results show higher mean opioid use in the TAP arm than in the standard postoperative pain-control arm: 5.15 mg compared with 2.52 mg. The recalculated unadjusted mean difference is 2.63 mg, with a 95% confidence interval of 1.107 to 4.153 mg and p <0.001. These numbers describe the analyzed continuous outcome as reported in the registry, with the limitations of an unadjusted re-calculation.

The result does not directly report pain intensity, comfort, satisfaction, or quality of recovery. Although the study rationale mentions patient comfort and minimizing narcotics, the posted primary endpoint is opioid consumption. A difference in consumption cannot, by itself, establish a difference in pain or explain medication use. The record does not post a pain-score result or explain how the multiple time points were combined in the endpoint.

The sponsor-reported Chi-squared p=0.001 is distinct from the recalculated comparison. The registry does not identify its target or provide an effect estimate and confidence interval. It therefore cannot be interpreted as a directly equivalent test of the mean opioid-use difference. The sponsor’s own analysis takes precedence as the sponsor-reported result, but its sparse description limits what can be concluded.

The outcome counts and enrollment figure differ, and the registry does not report how missing data or analysis populations were handled. It is not possible to establish from the posted record whether all randomized participants contributed to the primary analysis or how the difference in counts arose. The record also does not give enough detail about the analysis across time points to evaluate alternative statistical approaches.

More broadly, this is a trial-specific result. It does not establish what would happen in other patient groups, surgical settings, or analgesic protocols. The results do not support medical advice or a broad claim that one approach is preferable. They show a group-level difference in the posted opioid-use endpoint, with uncertainty and important gaps in endpoint, analysis, and safety reporting.

17. Why This Trial Matters Statistically

This trial illustrates why the outcome being measured must be kept distinct from the clinical motivation for a study. The stated rationale includes comfort and narcotic use, but the posted primary endpoint measures mean opioid consumption. That endpoint addresses medication use, not pain intensity itself. Readers should avoid treating a result on one measure as if it directly answered questions about another.

It also demonstrates the value of reporting an effect estimate and confidence interval alongside a p-value. The recalculated difference of 2.63 mg is accompanied by a 95% confidence interval from 1.107 to 4.153 mg. The interval conveys uncertainty about the magnitude of the group contrast, while the p-value addresses compatibility with a specified null model. Neither statistic supplies a clinical threshold or explains whether the difference matters to patients.

The separate sponsor analysis highlights a reporting challenge. A method label and p-value without an identified outcome, effect estimate, or confidence interval do not make the analysis easy to interpret or compare with a continuous endpoint. This case underscores the importance of a clear account of the analysis target and how it relates to the primary question.

Finally, the record leaves key questions about participant counts, repeated time points, missing data, analysis populations, and multiplicity unanswered. These details are not merely administrative: they affect how readers understand the precision and robustness of an estimated effect. The trial therefore offers an example of how statistical interpretation depends on both numerical results and transparent reporting of the endpoint and analysis.

18. 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 does not describe how the primary endpoint was constructed across the listed times of 2, 6,12, 24, 48 and 72 hours. It is unclear whether opioid use was combined across times, analyzed separately, or represented by another summary. Without that definition, the estimand behind the posted means is not fully clear.

The registry lists enrollment of 110, while the primary-outcome arm counts are 48 and 46. The record does not explain the difference, provide a participant-flow account, or state how missing observations were handled. It also does not identify the analysis population. These gaps restrict assessment of attrition and whether the posted comparison preserves the randomized groups.

The trial design statement refers to 40 patients per arm for power of 0.9 with a 20% change in pain scores. The posted endpoint is opioid use, and the record does not explain the relationship between the design target and that endpoint. The original sample-size justification and detailed assumptions are not reported in the registry record.

The sponsor-reported Chi-squared analysis has no specified outcome, estimate, or interval. Its relationship to the continuous primary endpoint cannot be determined. The recalculated comparison is unadjusted, and the record does not report an adjusted analysis, a model for repeated observations, distributional diagnostics, or sensitivity analyses.

The adverse-event listing reports zero serious events in each at-risk group, but the record does not describe safety-event definitions, ascertainment, or follow-up completeness. It does not report death counts. These limitations mean the safety entries are descriptive rather than a comprehensive assessment.

Finally, the record does not report secondary outcomes or a multiplicity strategy. It also does not describe which individuals were masked or how masking was maintained, despite listing triple masking. These absences prevent a full evaluation of conduct and analysis, and should not be interpreted as evidence that a particular procedure was or was not undertaken.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: Postoperative Pain Control for Prostatectomy — NCT00913068.
• Registry record last updated 2021-10-04; status last verified 2021-09.
• Results first posted 2021-10-04.
• 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.

22. Record Summary

The registry record describes a completed, randomized, parallel-group, triple-masked trial of a TAP procedure versus standard postoperative pain control for patients undergoing radical retropubic prostatectomy. Listed enrollment is 110. The primary endpoint is mean postoperative opioid use in milligrams, with assessment times of 2, 6,12, 24, 48 and 72 hours.

The posted outcome means are 5.15 mg for the TAP arm and 2.52 mg for standard control. The recalculated unadjusted mean difference is 2.63 mg, with a 95% confidence interval of 1.107 to 4.153 mg and p <0.001. The registry also lists a sponsor-reported Chi-squared analysis with p=0.001, but does not identify its target or provide an estimate and interval.

Important details are not reported in the registry record, including endpoint construction across time, missing-data handling, analysis population, and the relationship between the sponsor analysis and the primary endpoint. The safety listing reports zero serious adverse events in the stated at-risk groups, with limited additional safety detail.