This page provides an independent statistical analysis and educational interpretation of publicly reported results. ClinicalTrials.gov provides the official trial registry record.
1. Trial at a Glance
AFFIRM was a randomized, parallel-group, triple-masked phase 3 treatment trial with 1199 participants. The registry identifies overall survival as the single primary endpoint and reports six statistical analyses covering overall survival and five secondary endpoints.
| Feature | AFFIRM |
|---|---|
| Trial | AFFIRM |
| NCT identifier | NCT00974311 |
| Phase | Phase 3 |
| Condition | Castration-Resistant Prostate Cancer |
| Population | Patients with castration-resistant prostate cancer who had previously been treated with docetaxel-based chemotherapy |
| Allocation | Randomized |
| Design model | Parallel |
| Masking | Triple |
| Primary purpose | Treatment |
| Enrollment | 1199 |
| Interventions | Enzalutamide and placebo |
| Primary endpoint | Overall survival |
| Primary endpoint type | Time-to-event |
| Hypothesis type | Superiority |
| Results posted | Yes |
| Statistical analyses posted | 6 |
2. Clinical Question
The primary statistical question was whether enzalutamide produced a different overall-survival experience from placebo in patients with castration-resistant prostate cancer who had previously received docetaxel-based chemotherapy. The registered hypothesis type was superiority.
Population
Patients with castration-resistant prostate cancer who had been previously treated with docetaxel-based chemotherapy.
Intervention
Enzalutamide.
Comparator
Placebo.
Primary question
Does enzalutamide improve overall survival relative to placebo?
3. Trial Design
The registry describes AFFIRM as a randomized, parallel-group, triple-masked phase 3 trial with a primary purpose of treatment. The study enrolled 1199 participants and compared two drug intervention groups.
Enzalutamide
- Drug intervention
- Compared with placebo
- Included in the randomized efficacy comparison
Placebo
- Drug placebo intervention
- Comparator for enzalutamide
- Included in the randomized efficacy comparison
4. Endpoints
The registry identifies one primary endpoint, overall survival, and reports five additional secondary endpoint analyses. Four of the six posted analyses are time-to-event endpoints; two are binary percentage-based endpoints analyzed with the Cochran-Mantel-Haenszel test.
| Role | Endpoint | Time frame | Type | Effect measure |
|---|---|---|---|---|
| Primary | Overall Survival | During study period (up to 101 months) | Time-to-event | Hazard ratio |
| Secondary | Radiographic Progression-free Survival | During DB phase (up to 24 months) | Time-to-event | Hazard ratio |
| Secondary | Time to First Skeletal-related Event | During DB Phase (up to 24 months) | Time-to-event | Hazard ratio |
| Secondary | Percentage of Participants Who Were Responders for Functional Assessment of Cancer Therapy-Prostate (FACT-P) | Baseline up to 24 months | Binary | Risk difference |
| Secondary | Time to Prostate-specific Antigen (PSA) Progression | Baseline and at every study visit from Week 13 while on study drug (up to 24 months) | Time-to-event | Hazard ratio |
| Secondary | Percentage of Participants With Pain Palliation | Baseline up to 24 months | Binary | Risk difference |
Primary endpoint definition
Overall survival was defined as time from randomization to death due to any cause. The duration of overall survival was right-censored for participants who were lost to follow-up since randomization or not known to have died at the data analysis cut-off date. This included participants who were known to have died after the data analysis cut-off date.
Secondary endpoint populations
| Endpoint | Analysis population |
|---|---|
| Radiographic Progression-free Survival | ITT included all participants who were randomized into the study. |
| Time to First Skeletal-related Event | ITT included all participants who were randomized into the study. |
| FACT-P responders | Evaluable intent to treat (ITT) - all participants who were part of the ITT population and had a global FACT-P score at baseline and at least 1 post-baseline assessment. |
| Time to PSA Progression | ITT included all participants who were randomized into the study. |
| Pain Palliation | Evaluable ITT population including participants with metastatic bone disease at baseline and the registry-specified Brief Pain Inventory - Short Form assessment requirements. |
5. Statistical Methodology
Intention-to-treat analysis
The registry defines the primary overall-survival analysis population as the ITT population, including all participants randomized into the study. This is important because the treatment comparison remains anchored to randomized assignment rather than being redefined according to treatment received after randomization.
For a randomized superiority comparison, ITT preserves the treatment groups created by randomization and avoids redefining the primary comparison based on subsequent treatment behavior.
Log-rank test
The registry reports the log-rank test for overall survival, radiographic progression-free survival, time to first skeletal-related event, and time to PSA progression. The log-rank test is designed for comparing time-to-event distributions while accounting for the timing of events and right-censoring.
Stratified analysis
The overall-survival, radiographic progression-free survival, time to first skeletal-related event, and FACT-P analyses are described as stratified analyses. For the primary overall-survival analysis, stratification used baseline Eastern Cooperative Oncology Group (ECOG) performance status and mean Brief Pain Inventory - Short Form score (Question #3).
Cox regression and hazard ratio
For overall survival, the reported hazard ratio and 95% confidence interval are from a Cox regression model. The same registry wording identifies Cox regression as the source of the hazard ratio and confidence interval for radiographic progression-free survival, time to first skeletal-related event, and time to PSA progression.
An HR below 1 indicates a lower estimated instantaneous event rate in the enzalutamide group under the fitted time-to-event model. It is not an absolute risk difference and does not state what proportion of individual participants benefit.
Cochran-Mantel-Haenszel test
The registry uses the Cochran-Mantel-Haenszel test for the two binary secondary endpoints: FACT-P responder status and pain palliation. These analyses report a difference in percentages between enzalutamide and placebo as the effect measure, expressed here as a risk difference.
A positive risk difference means that the reported percentage was higher in the enzalutamide group. The confidence interval describes uncertainty around the estimated difference; it is not a prediction interval for individual outcomes.
6. Statistical Methods Explained
Why was an ITT population used for overall survival?
The registry explicitly defines the overall-survival analysis population as all randomized participants. This approach maintains the original randomized comparison. It also means that later treatment changes, discontinuation, or other post-randomization events do not redefine which treatment group a participant belongs to for the primary efficacy analysis.
Why use a log-rank test for overall survival?
Overall survival is a time-to-event endpoint: the outcome is not simply whether death occurred, but the time from randomization to death. The log-rank test compares the survival experience between groups while incorporating event timing and allowing observations to be right-censored.
What does an overall-survival HR of 0.63 mean?
An HR of 0.63 means that the estimated instantaneous rate of death under the fitted Cox model was 63% of the corresponding rate in the placebo group. Equivalently, the estimate corresponds to a 37% lower estimated hazard in the enzalutamide group. This does not mean that 37% of participants avoided death, nor that each participant experienced exactly a 37% reduction in risk.
Why is the confidence interval important?
The 95% confidence interval gives a range of values compatible with the statistical model and observed data under the confidence-interval procedure. For overall survival, the interval is 0.53 to 0.75. Its width provides information about the precision of the estimated hazard ratio that the point estimate alone cannot provide.
Why does the p-value not measure effect size?
A p-value addresses the compatibility of the observed data with the null hypothesis under the specified testing framework. It does not tell us how large the treatment effect is. Effect magnitude is communicated by measures such as the hazard ratio or risk difference, while the confidence interval communicates uncertainty around that estimate.
Why are the binary endpoints analyzed differently from survival endpoints?
FACT-P responder status and pain palliation are reported as binary outcomes, so the registry uses the Cochran-Mantel-Haenszel test and reports differences in percentages. In contrast, overall survival, radiographic progression-free survival, time to first skeletal-related event, and time to PSA progression are time-to-event outcomes and are analyzed using log-rank testing with hazard-ratio estimation from Cox regression.
7. Results
The registry reports one formal primary-endpoint analysis for overall survival and five secondary endpoint analyses. All six analyses compare enzalutamide with placebo and use a superiority hypothesis.
Primary Endpoint: Overall Survival
Hazard ratio for death
95% CI: 0.53–0.75 · P < 0.0001
Time frame: During study period (up to 101 months)
| Primary endpoint | Enzalutamide vs placebo |
|---|---|
| Outcome | Overall Survival |
| Analysis population | ITT included all participants who were randomized into the study. |
| Method | Log-rank test |
| Effect measure | Hazard ratio |
| Estimate | 0.63 |
| 95% CI | 0.53–0.75 |
| P-value | <0.0001 |
| Hypothesis | Superiority |
The estimated overall-survival hazard ratio of 0.63 indicates a lower estimated instantaneous rate of death in the enzalutamide group relative to placebo under the Cox model. Expressed as a relative model-based quantity, 0.63 corresponds to a 37% lower estimated hazard.
The estimate does not mean that 37% of participants survived because of treatment, that 37% of deaths were prevented, or that each participant had exactly a 37% reduction in individual risk. A hazard ratio is a relative time-to-event measure rather than an absolute survival probability.
The 95% confidence interval of 0.53–0.75 describes the statistical uncertainty around the estimated hazard ratio. It is substantially narrower than a very imprecise interval would be, but it still represents uncertainty rather than a guarantee that the true effect lies at any particular point inside the interval.
The P < 0.0001 result indicates strong statistical evidence against the null hypothesis under the reported superiority testing framework. The p-value should not be interpreted as the probability that the treatment effect is exactly 0.63, nor as a measure of clinical effect size.
Because the endpoint is time-to-event and the hazard ratio comes from Cox regression, interpretation also depends on the model's proportional-hazards framework. The registry does not provide sufficient information here to establish how well that assumption held over the full follow-up period.
Secondary Endpoint: Radiographic Progression-free Survival
Hazard ratio for radiographic progression-free survival
95% CI: 0.35–0.47 · P < 0.0001
Time frame: During DB phase (up to 24 months)
The reported HR of 0.40 corresponds to a 60% lower estimated hazard of the radiographic progression-free survival event in the enzalutamide group relative to placebo under the fitted Cox model. The 95% CI of 0.35–0.47 describes uncertainty around this relative estimate.
The result is not a statement that 60% of participants avoided progression or death. It is also not equivalent to a 60-percentage-point improvement in progression-free survival. The time-to-event scale and censoring mechanism remain central to interpretation.
Secondary Endpoint: Time to First Skeletal-related Event
Hazard ratio for first skeletal-related event
95% CI: 0.566–0.835 · P = 0.0001
Time frame: During DB Phase (up to 24 months)
The HR of 0.69 indicates a lower estimated instantaneous rate of the first skeletal-related event in the enzalutamide group relative to placebo under the Cox model. As a simple relative interpretation, this corresponds to a 31% lower estimated hazard.
The 95% CI of 0.566–0.835 quantifies uncertainty around that estimate. The p-value of 0.0001 addresses the statistical test against the superiority null hypothesis; it does not quantify the magnitude or clinical importance of the treatment effect.
Secondary Endpoint: FACT-P Responders
Difference in percentage of participants who were FACT-P responders
95% CI: 18.8–30.9 · P < 0.0001
Time frame: Baseline up to 24 months
The reported risk difference of 24.9 represents a 24.9-percentage-point higher responder percentage in the enzalutamide group than in the placebo group under the reported Cochran-Mantel-Haenszel analysis.
The 95% CI of 18.8–30.9 describes uncertainty around the estimated percentage-point difference. It does not mean that individual participants had probabilities between 18.8% and 30.9%, and it does not describe the distribution of treatment effects among individual patients.
The analysis was performed in the evaluable ITT population, defined as participants in the ITT population who had a global FACT-P score at baseline and at least one post-baseline assessment. That population definition is important because it differs from the all-randomized ITT definition used for the overall-survival analysis.
Secondary Endpoint: Time to PSA Progression
Hazard ratio for PSA progression
95% CI: 0.204–0.303 · P < 0.0001
Time frame: Baseline and at every study visit from Week 13 while on study drug (up to 24 months)
The HR of 0.248 indicates that the estimated instantaneous rate of PSA progression was approximately 24.8% of the corresponding rate in the placebo group under the fitted Cox model. Equivalently, the estimate corresponds to a 75.2% lower estimated hazard.
This does not mean that 75.2% of participants avoided PSA progression, nor does it establish that PSA progression was reduced by exactly 75.2 percentage points. The endpoint remains a time-to-event measure with its own definition and assessment schedule.
The 95% CI of 0.204–0.303 communicates the precision of the estimated hazard ratio, while the p-value of <0.0001 addresses statistical evidence under the reported superiority test.
Secondary Endpoint: Pain Palliation
Difference in rate of pain palliation
95% CI: 19.4–57.0 · P = 0.0079
Time frame: Baseline up to 24 months
The reported risk difference of 38.2 represents a 38.2-percentage-point higher rate of pain palliation in the enzalutamide group than in the placebo group under the stratified Cochran-Mantel-Haenszel analysis.
The 95% CI of 19.4–57.0 indicates substantial uncertainty in the exact size of the percentage-point difference. The p-value of 0.0079 provides evidence against the null hypothesis of no difference under the reported superiority framework; it does not measure the size of the effect.
The analysis was restricted to an evaluable ITT population involving participants with metastatic bone disease at baseline and the registry-specified Brief Pain Inventory - Short Form assessment requirements. Consequently, this endpoint should not be interpreted as applying to every randomized participant without qualification.
8. Summary of Reported Statistical Results
| Endpoint | Method | Effect | 95% CI | P-value |
|---|---|---|---|---|
| Overall Survival | Log-rank; Cox regression HR | HR 0.63 | 0.53–0.75 | <0.0001 |
| Radiographic Progression-free Survival | Log-rank; Cox regression HR | HR 0.40 | 0.35–0.47 | <0.0001 |
| Time to First Skeletal-related Event | Log-rank; Cox regression HR | HR 0.69 | 0.566–0.835 | 0.0001 |
| FACT-P Responders | Cochran-Mantel-Haenszel | Risk difference 24.9 | 18.8–30.9 | <0.0001 |
| Time to PSA Progression | Log-rank; Cox regression HR | HR 0.248 | 0.204–0.303 | <0.0001 |
| Pain Palliation | Cochran-Mantel-Haenszel | Risk difference 38.2 | 19.4–57.0 | 0.0079 |
Across the six posted statistical analyses, the registry reports effect estimates below 1 for all four time-to-event hazard ratios and positive percentage-point differences for both binary endpoints. These are different effect measures and should not be placed on a common numerical scale: an HR of 0.63 and a risk difference of 24.9 answer different statistical questions.
9. Understanding the Time-to-Event Analyses
Four of the six reported analyses are time-to-event endpoints. Their common statistical structure is important: participants can experience an event at different times, while some participants may be right-censored because their event status is not observed through the relevant analysis time.
Overall survival
Time from randomization to death due to any cause, with right-censoring as specified in the registry definition.
Radiographic progression-free survival
Time-to-event endpoint assessed during the double-blind phase, with a study time frame of up to 24 months.
Skeletal-related event
Time to the first skeletal-related event during the double-blind phase, up to 24 months.
PSA progression
Time to PSA progression assessed from baseline and at every study visit from Week 13 while on study drug, up to 24 months.
Why censoring matters
A time-to-event analysis does not require every participant to experience the event during the study. A participant who has not experienced the event by the time their observable follow-up ends can contribute information up to that point and then be right-censored. For overall survival, the registry explicitly defines circumstances under which observations are right-censored.
Why the hazard ratio is not a median ratio
A hazard ratio compares modeled instantaneous event rates over time. It is not obtained by dividing two median survival times, and it does not directly provide the difference in survival probabilities at a particular month. Those quantities require different calculations and interpretations.
10. Stratification in AFFIRM
The registry identifies stratified analyses for several endpoints. For overall survival, the analysis was stratified by baseline ECOG performance status and mean Brief Pain Inventory - Short Form score (Question #3). The same two factors are specified for the radiographic progression-free survival and time-to-first-skeletal-related-event analyses.
| Analysis | Stratification information in registry |
|---|---|
| Overall Survival | Baseline ECOG performance status and mean Brief Pain Inventory - Short Form score (Question #3) |
| Radiographic Progression-free Survival | Baseline ECOG performance status and mean Brief Pain Inventory - Short Form score (Question #3) |
| Time to First Skeletal-related Event | Baseline ECOG performance status and mean Brief Pain Inventory - Short Form score (Question #3) |
| FACT-P Responders | Baseline ECOG performance status and mean Brief Pain Inventory - Short Form score (Question #3) |
| Pain Palliation | Baseline ECOG performance status (0-1 vs. 2) |
Stratification allows the analysis to account for prespecified baseline factors when comparing treatment groups. It does not mean that treatment effects are estimated separately and independently within every stratum. The reported effect estimate is the treatment comparison produced by the specified stratified method.
11. Confidence Intervals: Reading the AFFIRM Estimates
| Endpoint | Point estimate | 95% CI | What the interval communicates |
|---|---|---|---|
| Overall Survival | HR 0.63 | 0.53–0.75 | Uncertainty around the estimated hazard ratio |
| Radiographic Progression-free Survival | HR 0.40 | 0.35–0.47 | Uncertainty around the estimated hazard ratio |
| Time to First Skeletal-related Event | HR 0.69 | 0.566–0.835 | Uncertainty around the estimated hazard ratio |
| FACT-P Responders | Risk difference 24.9 | 18.8–30.9 | Uncertainty around the percentage-point difference |
| Time to PSA Progression | HR 0.248 | 0.204–0.303 | Uncertainty around the estimated hazard ratio |
| Pain Palliation | Risk difference 38.2 | 19.4–57.0 | Uncertainty around the percentage-point difference |
The confidence intervals also show why different endpoints should not be summarized by p-values alone. The point estimate describes the estimated treatment effect, while the interval gives a measure of precision. For the two binary outcomes, the interval is expressed in percentage points; for the time-to-event outcomes, it is expressed on the hazard-ratio scale.
12. Safety Results
The registry provides serious adverse-event counts by treatment phase and arm. These figures should be presented exactly as reported because the denominators differ across the listed groups.
| Group | Serious adverse events affected | Participants at risk |
|---|---|---|
| Enzalutamide: DB + OLE Phase | 319 | 800 |
| Placebo: DB Phase | 155 | 399 |
| Placebo (DB) / Enzalutamide 160 mg (OLE) | 25 | 50 |
Safety and efficacy also answer different questions. The overall-survival hazard ratio describes a randomized efficacy comparison, whereas serious adverse events describe adverse outcomes within the reported treatment-phase populations. A treatment's statistical efficacy estimate cannot be used to infer its safety profile.
13. Study Completion and Registry Caveat
ClinicalTrials.gov lists the study as COMPLETED. The registry states that the primary objectives concerning overall survival had been met and that all participants who had remained on study treatment had discontinued; therefore, the study was considered completed.
14. What the Primary Hazard Ratio Does — and Does Not — Mean
The primary OS estimate of HR 0.63 means that the estimated instantaneous rate of death was 0.63 times that of placebo under the reported Cox model. In relative terms, this corresponds to a 37% lower estimated hazard.
The HR does not mean that 37% of participants benefited, that 37% of deaths were prevented, or that every participant experienced the same proportional reduction in risk. It also is not an absolute survival probability or a percentage-point difference.
The 95% CI of 0.53–0.75 describes uncertainty around the estimated hazard ratio. A confidence interval should be considered together with the point estimate rather than treated as a range of individual patient outcomes.
The reported P < 0.0001 indicates strong evidence against the null hypothesis under the stated superiority analysis. It does not measure treatment effect size, probability of benefit for an individual patient, or clinical importance.
15. Limitations
- Registry-level reporting: the available trial data provide selected endpoint definitions, methods, estimates, confidence intervals, and p-values, but not every component of a full statistical analysis plan.
- Proportional-hazards interpretation: the reported hazard ratios are derived from Cox regression. A single HR is most straightforward to interpret when the proportional-hazards framework is reasonable over the analyzed period; the ClinicalTrials.gov record does not provide a diagnostic assessment of that assumption.
- Different analysis populations: overall survival and several secondary endpoints use the all-randomized ITT population, whereas FACT-P responder status and pain palliation use evaluable ITT populations with additional assessment requirements.
- Different endpoint time frames: overall survival is reported over a study period of up to 101 months, while several secondary endpoints are restricted to the double-blind phase or to baseline through 24 months. These results therefore should not be treated as though they arose from identical observation windows.
- Multiple endpoints: the registry reports one primary endpoint and five secondary endpoint analyses. The ClinicalTrials.gov record does not provide a multiplicity-adjustment scheme for interpreting all six reported p-values as one common confirmatory family.
- Safety denominators: the serious-adverse-event figures are reported for treatment-phase groups with different denominators, so they should not be pooled or directly compared without preserving their population definitions.
- Incomplete detailed baseline information: the ClinicalTrials.gov record does not include a baseline-characteristics table, so no baseline balance assessment is made here.
- No reconstructed survival curves: the registry-reported summary statistics are insufficient to reconstruct valid Kaplan-Meier curves without underlying event and censoring information.
16. Why This Trial Matters Statistically
AFFIRM is a useful teaching case because its registry results connect several core clinical-trial concepts within one randomized superiority analysis: ITT principles, time-to-event endpoints, log-rank testing, Cox regression, hazard ratios, stratified analysis, confidence intervals, p-values, and categorical-data analysis.
| Concept | How it appears in AFFIRM |
|---|---|
| Randomization | The phase 3 study used randomized allocation in a two-arm parallel design. |
| Triple masking | The registry identifies the study as triple-masked. |
| ITT analysis | The primary OS analysis included all participants randomized into the study. |
| Time-to-event endpoints | OS, radiographic progression-free survival, time to first skeletal-related event, and time to PSA progression. |
| Log-rank test | Used for the reported time-to-event comparisons. |
| Cox regression | Used to obtain hazard ratios and 95% confidence intervals for the reported time-to-event analyses. |
| Hazard ratio | Reported as the effect measure for the four time-to-event analyses. |
| Stratified analysis | Several analyses were stratified using baseline ECOG performance status and Brief Pain Inventory information. |
| Cochran-Mantel-Haenszel test | Used for FACT-P responder status and pain palliation. |
| Risk difference | Reported for the two binary secondary endpoints. |
| Confidence intervals | 95% two-sided intervals accompany all six reported effect estimates. |
| Superiority testing | The registry identifies superiority as the hypothesis type for the reported analyses. |
17. Statistical Methods Explained in One View
Kaplan-Meier estimation
Provides a way to estimate survival or event-free probabilities over time while accommodating right-censored observations.
Log-rank test
Compares time-to-event distributions between randomized groups while using information from the timing of observed events.
Cox regression
Provides a model-based estimate of the relative hazard, reported in AFFIRM as the hazard ratio with a 95% confidence interval.
Risk difference
Expresses the absolute difference in outcome percentages between two groups, here reported for FACT-P response and pain palliation.
AFFIRM reports all three kinds of information through different quantities: hazard ratios for relative time-to-event effects, risk differences for binary endpoints, and p-values for hypothesis testing. Keeping these concepts separate prevents common interpretation errors.
18. Statistical Concepts in This Trial
Learn more about the methods used in this trial:
19. Related Statistical Calculators
20. Sources
- ClinicalTrials.gov: AFFIRM, NCT00974311.
- PubMed: PMID 35731340.
- PubMed: PMID 35643841.
- PubMed: PMID 34965947.
- PubMed: PMID 34518652.
- PubMed: PMID 31830211.
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Explore the statistical concepts behind randomized clinical trials, survival analysis, categorical outcomes, effect measures, and confidence intervals.
21. Record Summary
AFFIRM provides a compact example of a randomized phase 3 superiority trial in which the primary endpoint and several secondary endpoints use different statistical structures. Overall survival was analyzed in the all-randomized ITT population using a log-rank comparison, with the reported hazard ratio and 95% confidence interval obtained from Cox regression. The primary estimate was HR 0.63 (95% CI 0.53–0.75; P < 0.0001).
The secondary results extend the statistical story across several endpoint types: radiographic progression-free survival had an HR of 0.40 (95% CI 0.35–0.47; P < 0.0001); time to first skeletal-related event had an HR of 0.69 (95% CI 0.566–0.835; P = 0.0001); FACT-P responder status had a risk difference of 24.9 (95% CI 18.8–30.9; P < 0.0001); time to PSA progression had an HR of 0.248 (95% CI 0.204–0.303; P < 0.0001); and pain palliation had a risk difference of 38.2 (95% CI 19.4–57.0; P = 0.0079).