Financial Incentives to Promote Stimulant Abstinence in a Community-Based Syringe Exchange Program: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Financial Incentives to Promote Stimulant Abstinence in a Community-Based Syringe Exchange Program (NCT05787847), a randomized na study. The primary outcome is Stimulant Free Urine Sample.

2. Clinical Question

The study focused on stimulant abstinence as assessed with urine testing at the center. In the intervention group, a negative point-of-care urine test was the stated basis for the opportunity to earn an incentive. Both groups received health education, so the randomized comparison concerns the addition of incentives to education rather than education versus no intervention.

The posted results report a mean percentage of stimulant-negative tests for each group. They do not report individual participant trajectories, the number of tests contributed by each participant, or how a participant’s repeated tests were handled in the analysis. The summary therefore addresses the average outcome as recorded, not the timing or duration of abstinence for each person. It also does not directly answer questions about other outcomes, such as stimulant use outside the testing occasions or health outcomes, which are not reported as secondary results in the registry record.

3. Trial Design

This was a completed, randomized, parallel-group pilot study, listed as phase not applicable. The registry names the University of Vermont as sponsor and records 27 participants enrolled. The study started on 2024-02-14 and its primary completion date was 2024-07-30. The registry status is completed.

Participants were assigned to one of two behavioral groups. The health-education group received education about the health risks of stimulant use and injection. The other group received the same education and the opportunity to earn financial incentives for stimulant-negative urine tests. The registry describes the study as unmasked. It does not report further details about the randomization procedure, allocation concealment, or whether laboratory or other outcome assessors were masked.

The primary outcome was a continuous measure: the mean percentage of stimulant-negative urine drug tests over 12 weeks. Urine tests were to be conducted at the center using a point-of-care test. The registry reports 13 participants in the health-education group and 14 in the education-plus-incentives group for this outcome.

The record does not report the study’s original sample-size justification, detailed statistical analysis plan, or whether interim analyses were planned or conducted. Nor does it provide the planned duration or schedule of repeated testing beyond the stated 12-week outcome timeframe. These omissions limit assessment of how closely the posted outcome analysis followed the original design and how the pilot’s size was chosen.

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: Inclusion Criteria: * be 18 years or older * report using stimulants in the past 30 days * meet the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) diagnosis for stimulant use disorder at a moderate or severe level * participation in services at syringe exchange program

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 Stimulant Free Urine Sample, measured over 12 weeks.

The primary endpoint, titled “Stimulant Free Urine Sample,” is reported as the mean percentage of stimulant-negative urine drug tests over 12 weeks. A negative point-of-care urine test conducted at the center is the recorded test-based indicator of abstinence. Thus, the outcome summarizes the proportion of tested samples classified as stimulant-negative, averaged as a continuous measure within each group.

Urine testing provides an objective, biologically assessed measure at the time a sample is collected. Repeated test results can describe whether negative tests occurred during the observation period, rather than relying only on participants’ reports of use. The registry’s stated endpoint is a percentage, not a count of participants who remained abstinent throughout the full period. A participant may contribute more than one sample, but the record does not describe the sampling schedule or the number of tests per participant.

Interpreting this endpoint therefore requires attention to both the test results and how they were summarized. The registry reports group means and standard errors, but does not report individual-level data, the total number of urine samples, or details of handling missed tests. The endpoint speaks to the recorded test-based measure; it does not by itself describe all stimulant use between tests or establish effects on other outcomes.

7. Results Posted in the Registry

• Health Education Plus Incentives for Abstinence: mean 23.60 versus 6.10 on control; difference 17.50 (95% CI -3.37 to 38.37), standardized effect d = 0.61; z-test p = 0.100.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is continuous: it is the mean percentage of stimulant-negative urine tests. For a continuous outcome, a group mean describes the average value within that group. The reported values are 6.1 for health education and 23.6 for health education plus incentives. The registry also reports standard errors of 2.7 and 10.3, respectively. A standard error describes the uncertainty in an estimated mean; it is not the spread of individual participant values. The registry does not report standard deviations for the two groups.

What does the mean difference represent?

The mean difference is reported as 17.5, with the incentives group compared against the health-education group. It expresses the difference between the group averages on the percentage-point scale of the endpoint. It is not a relative percentage change. The estimate is an unadjusted comparison based on the posted summary statistics. The registry does not report covariate adjustment or other modeling details for this result.

How should the confidence interval be read?

The reported 95% confidence interval runs from -3.37 to 38.37. In general, a confidence interval represents the range of estimates compatible with the observed data under the assumptions of the statistical method. A wider interval indicates less precision than a narrower interval. Here, the interval includes zero, the value corresponding to no difference between group means. It also includes values in both directions, so the estimate does not pin down the direction or size of the underlying difference precisely. The interval does not mean there is a 95% probability that the true effect lies within these particular limits.

What does the p-value describe?

The reported unadjusted p-value is 0.1. In general, a p-value describes how unusual data at least as incompatible with a specified no-difference model would be if that model and the test assumptions were correct. It is not the probability that the null hypothesis is true, and it does not measure the size or practical importance of a difference. The registry labels the result unadjusted but does not report the exact test, its assumptions, or a complete prespecified analysis plan. The p-value should therefore be read alongside the effect estimate and confidence interval, rather than as a standalone conclusion.

Why does over-dispersion matter?

In general, over-dispersion means that observed variability is greater than a model expects. When that occurs, standard errors can be too small if the model does not account for the extra variability, leading to intervals that are too narrow and tests that overstate precision. The registry does not report a dispersion assessment or a model for repeated urine tests. The reported group means and standard errors remain interpretable as the posted summary, but the record does not permit a check of how within-person variation or repeated measurements were addressed.

What assumptions are not documented?

The registry identifies a randomized parallel design and says that the study was unmasked. It does not report the randomization method, allocation concealment, the statistical test used for the primary comparison, the planned handling of missing urine tests, or analysis populations such as intention-to-treat or per-protocol. It also does not describe adjustment for baseline characteristics or account for repeated samples. These are not details to infer from the means or p-value; they are not reported in the registry record.

The sample-size illustration uses an assumed normal approximation and equal allocation, with a two-sided significance level of 0.05. These are assumptions of that planning illustration, not documented features of the trial’s original design. It uses the observed effect, or stated fractions of it, to illustrate participant counts for target power levels. Such calculations are prospective planning aids. They do not change the uncertainty in the observed result and are not a measure of the trial’s observed power.

10. Confidence Intervals and Precision

The estimated mean difference is 17.5 percentage points, with a 95% confidence interval from -3.37 to 38.37. The interval is broad in relation to the point estimate and crosses zero. This means the posted data do not precisely distinguish a difference favoring either group from no difference. Although the point estimate is higher in the incentives group, plausible values under the reported interval include a small negative difference and a substantially larger positive difference.

Confidence intervals are useful because they display uncertainty about the estimated difference on the outcome’s scale. In general, a confidence interval is constructed by a statistical procedure that would cover the target value at its stated rate over repeated samples, provided its assumptions hold. It is not a guarantee about the true value in this particular study. The record does not give enough detail about the analysis procedure to assess all of those assumptions.

The group sizes for the primary outcome were 13 and 14. The reported standard error was 2.7 in the health-education group and 10.3 in the education-plus-incentives group. The larger standard error in the latter group indicates that its mean is estimated with more uncertainty in the posted summaries. With limited enrollment, estimates can be imprecise, particularly when outcome variability is high. The record does not report individual values or standard deviations that would permit a fuller assessment of the distribution.

The p-value of 0.1 is consistent with the interval’s inclusion of zero, but neither statistic should be treated as a measure of clinical importance. The estimate describes the observed group contrast; its interval communicates that the magnitude remains uncertain. The registry does not report a prespecified clinically meaningful difference against which to judge the estimate.

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, not total event counts. The registry’s planning illustration assumes a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. It uses the observed effect as the target, or uses stated fractions of that observed effect. These are planning assumptions for the illustration; the trial’s own original design assumptions and sample-size justification are not reported in the registry record.

For planning around 100% of the observed effect, the listed requirements are 42 participants per arm for 80% power and 57 participants per arm for 90% power. For planning around 75% of the observed effect, they are 75 participants per arm for 80% power and 100 participants per arm for 90% power. For planning around 50% of the observed effect, they are 168 participants per arm for 80% power and 225 participants per arm for 90% power.

In general, power is the chance that a planned study will detect an effect of a specified size at the stated significance level, if that effect is present and the planning assumptions hold. The 80% and 90% columns therefore describe different target chances of detecting the chosen effect under those assumptions. The 90% target requires more participants than the 80% target for each effect scenario. Smaller effects need more participants because they are harder to distinguish from random variation.

These figures are planning illustrations, not observed power. They are based on the observed effect, which is uncertain, so the numbers should not be treated as a guaranteed sample size for a future study or as proof that the pilot was adequately or inadequately sized.

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 = 17.500; pooled standard deviation σ = 28.598.
2. n = 2 × (1.96 + 0.842)² × 28.598² / 17.500² = 41.9, rounded up to 42 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 <- 17.5                      # observed difference in means
sigma <- 28.5976362659574                     # 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(5787847)
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 = 42 (80% power), 57 (90% power)
 75% of observed effect: n per arm = 75 (80% power), 100 (90% power)
 50% of observed effect: n per arm = 168 (80% power), 225 (90% power)
power.t.test: 42.9 
Simulated power at n = 42 per arm: 0.791 (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 reports one primary outcome measure and no posted secondary outcome results. It does not report multiple primary outcomes or a multiplicity-adjustment procedure. When several outcomes or comparisons are tested, the chance of at least one apparently unusual result can increase; statistical plans may address this by defining a primary comparison or adjusting inference. The record does not describe such a plan beyond identifying the primary endpoint and posting one unadjusted comparison. It would be inappropriate to infer whether other comparisons were planned or performed.

The registry does not report how missing urine samples or missed visits were handled. This matters because the primary measure is a percentage of stimulant-negative tests during a 12-week period. If participants contributed different numbers of samples, or if missed samples were treated in a particular way, the resulting group averages could depend on those choices. The record does not state whether missing tests were excluded, counted as positive, imputed, or addressed by another method. No such handling should be assumed.

The analysis population is also not reported in detail. The posted outcome counts are 13 participants in the health-education group and 14 in the education-plus-incentives group, but the registry does not say whether these represent all randomized participants, all participants with outcome data, or a defined analysis set. Although the trial is described as randomized, the record does not state whether the primary analysis followed an intention-to-treat principle or used a per-protocol population. Nor does it report treatment adherence or crossovers.

These omissions do not invalidate the posted summary, but they constrain its interpretation. A reader cannot assess whether missingness differed between groups, whether repeated measures were modeled, or whether the analysis population followed the original randomization. The statistical analysis plan, detailed missing-data approach, and analysis-population definitions are not reported in the registry record.

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.

The registry reports zero serious adverse events and zero deaths in both groups. For the health-education group, 13 participants were listed as at risk for serious adverse events and 13 for other adverse events; the corresponding affected counts were zero. For the health-education-plus-incentives group, 14 were listed as at risk for each category, and the affected counts were zero.

These are the safety figures posted in the registry, not evidence that no adverse events could occur. The record does not list serious adverse event terms, and no serious terms are posted. It reports no affected participants in the listed categories during the recorded reporting period, but does not describe the event-collection procedures, follow-up duration for safety, or how events were defined. The absence of posted events should therefore be interpreted as a feature of the reported data, not as a guarantee of safety or a comparison establishing that the groups have identical risk.

Because the study enrolled a small number of participants, the safety summary is limited in its ability to characterize uncommon events. The registry does not provide additional safety analyses or detailed event narratives.

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

The posted result is a comparison of average percentages of stimulant-negative urine tests over 12 weeks. The mean was 23.6 in the health-education-plus-incentives group and 6.1 in the health-education group. The estimated mean difference was 17.5 percentage points, with a 95% confidence interval from -3.37 to 38.37 and an unadjusted p-value of 0.1. These statistics describe the observed pilot data and their uncertainty.

The point estimate is higher for the group offered incentives, but the confidence interval includes no difference and values in both directions. The findings therefore do not establish that the incentives changed stimulant use. They also do not establish that the groups were equivalent or that an important difference is absent. The uncertainty is substantial, and the posted comparison does not precisely determine the size or direction of a broader underlying effect.

The measure is based on urine tests, not a complete record of every episode of stimulant use. It represents the average percentage of stimulant-negative tests as reported, rather than the proportion of participants who were abstinent for the entire observation period. The record does not describe how many samples each participant contributed or how missed tests were handled. It also does not report results for other clinical or behavioral outcomes.

The study was a randomized pilot with 27 participants enrolled. Its size and the limited information in the registry record make the result best understood as preliminary comparative evidence. It may help frame questions for future research, but the posted data alone do not settle the effectiveness of incentives or support conclusions beyond this measured outcome, study population, and period. No medical advice or treatment recommendation follows from these results.

16. Why This Trial Matters Statistically

This study raises a statistical question about how to assess an intervention when the endpoint is built from repeated biological tests. The outcome is a mean percentage of stimulant-negative urine samples over 12 weeks, rather than a single measurement or a simple participant-level event. That distinction affects what the average represents and makes information about sample schedules, repeated observations, and missing tests important to interpretation.

The comparison also illustrates why effect estimates, uncertainty intervals, and p-values should be read together. The point estimate is 17.5 percentage points in favor of the incentives group, but the 95% confidence interval ranges from -3.37 to 38.37 and the unadjusted p-value is 0.1. The result is therefore not just a question of whether a threshold was crossed: the interval shows a wide range of plausible differences, and the estimate’s precision is limited.

Small pilot studies can be valuable for studying feasibility and generating estimates, but estimates from limited enrollment are often uncertain. Planning calculations based on an observed effect are especially sensitive to uncertainty in that estimate. The displayed planning figures illustrate how target power and the assumed effect size affect the number of participants per arm; they do not establish the trial’s original design rationale or its observed power.

Finally, the registry record illustrates the importance of complete reporting. Details on missing data, analysis populations, repeated-measure methods, and the prespecified statistical plan would help readers assess the robustness and scope of the result. Their absence does not make the posted statistics uninterpretable, but it limits the conclusions that can be drawn from them.

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 posted comparison comes from a pilot with 27 participants enrolled and small outcome groups of 13 and 14. The confidence interval is wide, so the estimated difference is imprecise. The higher standard error in the incentives group, 10.3 compared with 2.7 in the education group, also signals greater uncertainty around that group’s mean.

The endpoint is an average percentage of negative urine tests, but the registry does not report the number or schedule of tests per participant, the distribution of individual outcomes, or how repeated measurements were analyzed. It also does not report how missed tests or other missing data were handled. Without these details, it is difficult to judge how the summary relates to participant-level patterns or whether missingness may have affected the comparison.

The record describes randomization and no masking, but does not detail the randomization process, allocation concealment, adherence, or analysis population. The exact statistical test and a complete prespecified analysis plan are not reported. The posted comparison is unadjusted, and no secondary outcome results are reported. These factors limit assessment of potential bias, robustness, and the breadth of the findings.

The registry does not report the original sample-size justification, so the planning illustration should not be mistaken for the trial’s own design assumptions. Safety reporting lists no affected participants, but does not provide event definitions, collection procedures, or further detail on follow-up. These limitations are specific to the information recorded and do not establish that unreported procedures or events did or did not occur.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Financial Incentives to Promote Stimulant Abstinence in a Community-Based Syringe Exchange Program — NCT05787847.
• Registry record last updated 2025-09-17; status last verified 2025-09.
• Results first posted 2025-09-17.
• 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

The registry record describes NCT05787847, “Financial Incentives to Promote Stimulant Abstinence in a Community-Based Syringe Exchange Program,” sponsored by the University of Vermont. It is a completed randomized, unmasked, parallel-group pilot study listed as phase not applicable, with 27 participants enrolled.

Participants were assigned to health education or health education plus financial incentives for stimulant-negative urine tests. The primary outcome was the mean percentage of stimulant-negative urine tests over 12 weeks. The posted means were 6.1 for health education and 23.6 for education plus incentives; the unadjusted mean difference was 17.5, with a 95% confidence interval from -3.37 to 38.37 and p = 0.1.

The record reports zero serious adverse events, zero other adverse events, and zero deaths in each group. It does not report secondary outcome results, the original sample-size justification, detailed missing-data procedures, or the full statistical analysis plan.