Primary-Care Based Mindfulness Intervention: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Primary-Care Based Mindfulness Intervention (NCT03922581), a randomized na study. The primary outcome is Number of Participants Retained in Study.

2. Clinical Question

3. Trial Design

The registry describes a completed randomized controlled trial with a parallel-group design and single masking. The identity of the person or group masked is not reported in the registry record. The study enrolled 80 participants. The intervention arm included 44 participants and the wait-list control arm included 36.

Participants assigned to MBCT received the group intervention for 8 weeks. Those assigned to wait-list control received study assessments without active treatment during that period and could take part in MBCT after finishing the study assessments. The registry frames the study as a pilot of a primary-care-based mindfulness intervention for chronically traumatized African Americans screening positive for PTSD and MDD. It also describes psychological and physiological assessment, including evaluation of emotion dysregulation and autonomic function as preliminary mechanisms of action.

The primary completion date was 2023-08-18, and the study status is completed. The start date was 2018-09-14. The phase is listed as NA, which is consistent with a behavioral intervention record rather than a drug-phase designation; no further phase information is reported. Emory University is listed as the sponsor.

The primary endpoint was the number of participants retained in the study. Other registry outcomes include PTSD screening or diagnostic measures, depression measures, mindfulness, and self-compassion. The record lists 14 outcome measures and 8 secondary outcomes. The analytic procedures, including the methods used for secondary outcomes, are not reported in the registry record. The registry also does not identify the masked party, or give additional details about allocation implementation.

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: Study enrollment was suspended in March 2020 due to the Coronavirus 2019 (COVID-19) pandemic. Enrollment reopened in September 2020 with a virtual intervention, rather than an in-person intervention.

Overall Study

Reasons for not completing this period:

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 Number of Participants Retained in Study, measured over Baseline through Follow-up (1 month after the end of the 8-week intervention).

The primary endpoint was the number of participants retained in the study, measured from baseline through follow-up, which was 1 month after the end of the 8-week intervention. The endpoint description says retention rates were assessed using session attendance. The posted arm counts were 22 retained participants among 44 assigned to MBCT and 26 among 36 assigned to wait-list control.

Retention is used in a pilot study to characterize whether participants remain engaged through intervention and follow-up. For a group intervention delivered in primary care, attendance-based retention can inform feasibility of delivering the study procedures and completing follow-up assessments. It is distinct from a symptom outcome: it does not directly measure change in PTSD or depression, and it does not establish whether the intervention was acceptable to every participant. The registry's explanation anticipates at least 75% retention and follow-up in the intervention group, drawing on earlier completion rates described in the endpoint text. The observed retention count in the MBCT arm is the endpoint result posted for this trial; the registry does not report a separate acceptability score as its primary endpoint.

Because this is a count of participants with the retention event, the arm totals matter when interpreting the counts. The unadjusted comparison expresses the difference between the arm-level retention proportions rather than comparing counts alone. The registry does not report a more detailed definition of how attendance qualified a participant as retained.

7. Results Posted in the Registry

• Mindfulness-Based Cognitive Therapy: 50.0% (22/44) versus 72.2% (26/36) on control; risk difference -22.2 percentage points (95% CI -43.0 to -1.4), risk ratio 0.69 (0.48–0.99); unadjusted two-proportion z-test p = 0.044.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is binary at the participant level: a participant was either counted as retained or was not counted as retained over the stated study period. The registry reports arm-level retained counts and denominators, then presents an unadjusted risk difference. A count on its own can be misleading when group sizes differ, so the risk difference compares the proportion with the event in one arm with the proportion in the other. In this record, the comparison is oriented as MBCT relative to wait-list control. Its negative estimate indicates a lower observed retention proportion in the MBCT arm.

What does the risk difference describe?

In general, a risk difference is an absolute contrast between two event proportions. It keeps the comparison on the original probability scale, so it describes how far apart the observed proportions are rather than expressing a relative change. The reported estimate is -0.222. The registry also reports the component proportions as 0.5 for MBCT and 0.722 for wait-list control. The estimate is unadjusted, and the record does not describe any covariate adjustment or stratification for this comparison.

Other measures can express a binary-outcome contrast on relative scales. The registry comparison includes a risk ratio of 0.692, with an interval from 0.484 to 0.991, and an odds ratio of 0.385, with an interval from 0.150 to 0.983. A risk ratio compares event proportions as a ratio, while an odds ratio compares event odds. These measures are not interchangeable with the risk difference: each uses a different scale and has its own interpretation. For this endpoint, the absolute risk difference offers a direct description of the difference in retention proportions, while the relative measures provide alternative summaries of the same binary outcome.

How should the confidence interval be understood?

In general, a confidence interval describes the range of estimates reasonably compatible with the data under the method's assumptions. It reflects sampling uncertainty around an estimate; it is not a range containing the outcomes of individual participants. The reported interval for the risk difference is -0.430 to -0.014. The calculation method and confidence level are not reported in the registry record, so those details should not be assumed. The interval's width is relevant because it indicates that the data do not pin down a single precise value for the arm difference.

What does the p-value add?

In general, a p-value measures how unusual data at least as inconsistent with a specified null model would be if that model and the analysis assumptions held. It does not measure the probability that the null model is true, and it does not quantify the probability that a result occurred by chance in an everyday sense. The posted value is an unadjusted p-value of 0.044. The registry does not specify the analysis test or the prespecified decision threshold for the primary comparison. The significance level of 0.05 appears among assumptions for the separate planning illustration; that planning assumption is not evidence of the trial's own analysis plan.

The record's sample-size illustration is based on fractions of the observed effect and uses a two-sided significance level of 0.05, equal allocation between arms, and a normal approximation. It is an illustration rather than the original sample-size justification. The trial's own design assumptions and sample-size justification are not reported in the registry record. The listed planning values are participants per arm for the binary endpoint, not observed power for this completed trial.

Why can over-dispersion matter?

In general, over-dispersion arises when the observed variability in data exceeds what a chosen statistical model expects. It can make standard errors too small and confidence intervals too narrow if the model does not accommodate the extra variation. This concept is especially relevant to count outcomes modeled with distributional assumptions, but the primary result is a participant-level binary retention outcome and the registry does not state that an over-dispersed count model was used. There is no basis in the record to assert that over-dispersion affected this comparison.

Random allocation is intended, in general, to support a fair comparison by balancing prognostic factors across study arms on average. It does not guarantee balance in a particular pilot sample. The record does not report baseline comparisons or adjusted analyses for the primary endpoint. Single masking is reported, but who was masked is not specified. These omissions limit assessment of how the posted statistical comparison was implemented and interpreted.

10. Confidence Intervals and Precision

The risk difference is estimated as -0.222, with a confidence interval from -0.430 to -0.014. The interval describes uncertainty about the arm-level contrast under the interval method's assumptions. The registry does not report the confidence level or calculation method, so neither should be inferred from the endpoint entry alone.

The interval includes values representing a range of negative differences, rather than concentrating on one exact value. Its upper endpoint is relatively near zero compared with the lower endpoint, while the interval remains below zero. This is a statement about the reported estimate and interval, not proof that the same contrast will be observed in another sample. The interval does not account for uncertainty arising from unreported analysis choices, outcome definitions, missing-data handling, or multiplicity unless those matters were incorporated into the method; the registry does not say whether they were.

The arm counts underlying the comparison were 22 retained participants out of 44 for MBCT and 26 out of 36 for wait-list control. The observed proportions are reported as 0.5 and 0.722, respectively. A confidence interval for a difference is not a prediction interval for future study results, and it does not indicate the range of individual participant experiences. The pilot sample and the incomplete reporting of analysis details call for cautious interpretation of the estimate's precision. The interval and point estimate describe the posted retention comparison, not a clinical outcome effect.

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 figures are participants per arm for the binary retention outcome, not total events. The calculation uses a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. It applies those assumptions to specified fractions of the observed effect. For 100% of the observed effect, the illustration lists 75 participants per arm for 80% power and 100 participants per arm for 90% power. For 75% of the observed effect, it lists 130 participants per arm for 80% power and 173 participants per arm for 90% power. For 50% of the observed effect, it lists 282 participants per arm for 80% power and 377 participants per arm for 90% power.

In plain language, the 80% power column gives the illustrated number of participants per arm for a design with an 80% chance of detecting the specified effect under the calculation assumptions, if that effect and model assumptions hold. The 90% power column gives the corresponding larger planning target for a 90% chance under those assumptions. These are hypothetical design calculations based on the observed effect, not observed power for the completed trial. The trial's own original sample-size assumptions and justification are not reported in the registry record.

The effect fractions describe planning for 100%, 75%, or 50% of the observed effect. Smaller effects require more participants in these calculations, which is why the listed targets increase as the assumed effect becomes smaller. The curve visualizes this planning relationship; it is not an additional analysis of participant outcomes. These figures should be read as illustrations under the stated assumptions, not as a recommendation or as a claim about what the trial was designed to detect.

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, p₀ and p₁ are the control and treatment proportions, and p̄ is their average.

Worked calculation for this trial

1. Control proportion p₀ = 0.722; treatment proportion p₁ = 0.500; average p̄ = 0.611.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.6894 = 1.3513.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.6713 = 0.5650.
4. n = (1.3513 + 0.5650)² / (-0.2222)² = 74.4, rounded up to 75 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
p0 <- 0.722222222222222                          # control proportion
p1 <- 0.5                          # treatment proportion
n_per_arm <- function(power, frac = 1) {
  q1 <- p0 + (p1 - p0) * frac               # smaller effect = fraction of the observed difference
  pbar <- (p0 + q1) / 2
  z_b <- qnorm(power)
  ceiling(round((z_a * sqrt(2 * pbar * (1 - pbar)) + z_b * sqrt(p0 * (1 - p0) + q1 * (1 - q1)))^2 / (q1 - p0)^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 (unrounded n per arm)
cat("power.prop.test:", round(power.prop.test(p1 = p0, p2 = p1, power = 0.80)$n, 1), "\n")

# Simulation: how often does the z-test reject at the 80%-power sample size?
set.seed(3922581)
n    <- n_per_arm(0.80)
reps <- 20000
x0 <- rbinom(reps, n, p0); x1 <- rbinom(reps, n, p1)
ph <- (x0 + x1) / (2 * n)
z  <- ((x1 - x0) / n) / sqrt(ph * (1 - ph) * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(z) > z_a, na.rm = TRUE), reps))

Output from running the code

100% of observed effect: n per arm = 75 (80% power), 100 (90% power)
 75% of observed effect: n per arm = 130 (80% power), 173 (90% power)
 50% of observed effect: n per arm = 282 (80% power), 377 (90% power)
power.prop.test: 74.4 
Simulated power at n = 75 per arm: 0.811 (20000 replicates)

The simulation draws many pairs of event counts at the planned sample size and counts how often the two-proportion z-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 14 outcome measures, including the primary retention endpoint and secondary outcomes covering PTSD screening and diagnosis, depression symptoms and diagnosis, mindfulness, and self-compassion. Eight secondary outcomes are listed in the record. No analyses are listed. The record does not explain whether all outcome measures were analyzed, which were prespecified for formal testing, or how any multiplicity across outcomes was handled.

In general, testing multiple outcomes can increase the chance that at least one result appears noteworthy even when there is no underlying effect. Methods for addressing this issue may include a prespecified hierarchy or an adjustment to the testing procedure. Neither a multiplicity strategy nor a hierarchy of outcomes is reported in the registry record. The posted primary comparison has an unadjusted p-value, so the record does not establish whether that value was interpreted in a broader family of tests or whether any adjustment was planned.

Missing-data handling is also not reported in the registry record. For a retention endpoint based on attendance, the operational definition of retention and the treatment of participants with incomplete follow-up could affect interpretation. The record does not state whether participants with missing assessments were counted as not retained, excluded, or handled using another method. It also does not report a missing-data sensitivity analysis.

The analysis population is not specified. The record does not say whether the comparison followed an intention-to-treat approach, included only participants with available endpoint data, or used another population definition. The registry gives arm denominators of 44 and 36 for the primary outcome, but does not describe how those denominators were established or whether any randomized participants were excluded from analysis. Details on the analysis population, missing data, and multiplicity are necessary for a fuller assessment of the reported inference.

14. Safety Results

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

Deaths

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 registry reports no serious adverse events in either arm: 0 among 44 participants at risk in the MBCT group and 0 among 36 participants at risk in the wait-list control group. Deaths are reported as 0 in both groups. The record lists 11 other adverse events among 44 participants at risk in the MBCT group and 3 among 36 at risk in the control group.

These figures describe the adverse-event entries posted in the registry; they do not establish that no unrecorded events occurred. No serious adverse event terms are listed. The record does not provide further details about the types, timing, severity, relatedness, or duration of the other adverse events, or about how events were collected. It also does not report a statistical comparison of adverse events. The counts should therefore be read descriptively, with the arm-specific numbers at risk in view, rather than as a complete assessment of comparative safety.

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

The primary result describes retention in a pilot study of an 8-week, primary-care-based MBCT intervention compared with wait-list control. The posted counts show 22 participants retained among 44 in the MBCT arm and 26 among 36 in the control arm. The unadjusted risk difference is -0.222, with a confidence interval from -0.430 to -0.014. These data indicate a lower observed retention proportion in the MBCT arm in this trial, while the interval conveys uncertainty about the size of that difference.

The result does not establish whether MBCT changes PTSD symptoms, depression symptoms, or any other clinical outcome. Those are separate outcomes in the registry, and the record does not post their analyses. It also does not explain the reasons for the retention pattern or show whether attendance reflected satisfaction, accessibility, symptom changes, or other factors. The stated endpoint is an attendance-based retention measure, not a direct measure of acceptability or therapeutic benefit.

As a pilot trial in a particular primary-care setting and population, the findings should be understood in that context. The record describes chronically traumatized African Americans screening positive for PTSD and MDD in primary care clinics within an urban public hospital. It does not establish that the observed attendance pattern would generalize to other populations, clinics, or delivery settings. The trial's statistical analysis plan, missing-data approach, analysis population, and multiplicity strategy are not reported in the registry record, limiting the extent to which the p-value and interval can be evaluated.

The evidence is best read as a descriptive estimate of retention and its uncertainty in the enrolled sample. It is not a clinical recommendation, proof of effectiveness or lack of effectiveness, or a forecast of retention in a future study. The planning illustration offers hypothetical sample-size scenarios based on the observed effect; those figures do not change what was observed in the completed trial.

16. Why This Trial Matters Statistically

This trial raises a statistical question that is central to early-stage behavioral intervention research: can a randomized study retain participants through intervention and follow-up in a primary-care environment? For a pilot whose stated purpose includes feasibility and acceptability, retention is not merely an administrative detail. It can help characterize whether study visits, intervention sessions, and follow-up procedures are workable in the setting under study.

The contrast also illustrates why denominators and effect scales matter. The posted counts are 22 retained participants among 44 in the intervention arm and 26 among 36 in the control arm. Comparing the counts alone would ignore the different group sizes. The reported risk difference uses the arm-level proportions and gives an absolute measure of the observed contrast. Its confidence interval communicates that the point estimate should not be treated as exact.

The trial's pilot scale and its multiple listed outcomes make transparent analysis reporting important. The registry lists a range of psychological and physiological aims, but does not report the analysis plan, missing-data methods, analysis population, or multiplicity approach. Those details would help distinguish exploratory outcomes from confirmatory tests and clarify how incomplete participation was addressed. The record's planning illustration also emphasizes that sample-size requirements depend on the effect a future study aims to detect; smaller effects require larger planned samples under the listed assumptions.

Statistically, the trial is useful as an example of interpreting an attendance-based endpoint alongside uncertainty and limits in reporting. It documents a randomized comparison in a clinical setting, but the primary outcome answers a feasibility-related question rather than directly resolving the effects on PTSD or depression.

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 primary endpoint is retention measured through session attendance, but the registry does not give a detailed threshold or operational rule for classifying participants as retained. That limits replication of the endpoint and makes it difficult to determine how partial attendance or incomplete follow-up was handled. Missing-data procedures and the analysis population are not reported in the registry record.

The trial enrolled 80 participants, with 44 assigned to MBCT and 36 to wait-list control. The unequal arm sizes and pilot setting should be kept in mind when interpreting the estimate and its uncertainty. The record does not provide baseline characteristics by arm, so the extent of balance in this particular sample cannot be assessed from the posted information. The identity of the party masked in the single-masked design is also not reported.

The registry describes a specialized population: African Americans with chronic trauma exposure who screened positive for PTSD and MDD in primary care clinics within an urban public hospital. This context is important when considering whether the retention results apply in other settings. The record does not report more detailed participant characteristics that might help assess how broadly the findings apply.

The study was described as a pilot, and the primary endpoint was retention rather than a clinical symptom endpoint. Although multiple secondary outcomes are listed, the record reports no analyses. Statistical methods, missing-data handling, multiplicity procedures, and the original sample-size justification are not reported in the registry record. These omissions prevent a complete appraisal of the secondary results and of the inferential framework behind the primary comparison. Adverse-event entries are also limited: types and other details for the non-serious events are not described.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Primary-Care Based Mindfulness Intervention — NCT03922581.
• Registry record last updated 2024-09-19; status last verified 2024-09.
• Results first posted 2024-09-19.
• 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

NCT03922581 is a completed, randomized, single-masked, parallel-group pilot trial sponsored by Emory University. It evaluated an 8-week group MBCT intervention against wait-list control in chronically traumatized African Americans screening positive for PTSD and MDD in primary care. The study enrolled 80 participants.

The primary endpoint was retention from baseline through follow-up, 1 month after the intervention ended, assessed using session attendance. Retention was 22 of 44 participants in the MBCT arm and 26 of 36 in the control arm. The reported unadjusted risk difference was -0.222, with a confidence interval from -0.430 to -0.014 and an unadjusted p-value of 0.044.

The registry lists 14 outcome measures, including 8 secondary outcomes, but no analyses. It does not report the trial's statistical analysis plan, missing-data methods, analysis population, or multiplicity approach. The posted primary result therefore describes an attendance-based retention comparison and should not be taken as a result for PTSD or depression symptoms.