Contingency Management for Methamphetamine Abstinence and HIV Post-Exposure Prophylaxis in Men Who Have Sex With Men: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Contingency Management for Methamphetamine Abstinence and HIV Post-Exposure Prophylaxis in Men Who Have Sex With Men (NCT00856323), a na phase 2 study. The primary outcome is Self-reported Methamphetamine Use in Previous 30 Days..

2. Clinical Question

3. Trial Design

This completed phase 2 study is registered as a single-group study, with allocation listed as NA and masking listed as none. Those design features mean that the registry record does not describe random assignment or a blinded comparison. Its one intervention group, PEP/CM, combined contingency management for methamphetamine abstinence with the option to initiate Truvada after a qualifying non-occupational HIV exposure.

The behavioral component was an 8-week course of contingency management. Participants could receive vouchers for testing negative for methamphetamine metabolites in periodic urine analyses. The biomedical component was a 28-day course of Truvada, described as 1 pill daily after an unanticipated risk exposure. The registry’s description of qualifying exposure includes receptive or insertive anal sex without a condom with someone who was HIV-positive or whose status was unknown.

The registry reports enrollment of 53 and a status of completed. It gives a primary-completion date of 2010-12 and a study start of 2009-01. The primary-outcome population description says that 53 were enrolled and 2 were withdrawn. It does not explain the timing or reasons for withdrawal, or identify which participants contributed to the endpoint analysis.

This design can describe outcomes observed among participants in the combined-intervention group, if those outcome results are reported. It cannot isolate the contribution of CM from that of PEP, because the components were not assigned or assessed as separate randomized groups. Nor does it provide a concurrent control group against which to distinguish intervention-associated change from changes that might have occurred over time for other reasons.

The registry record does not report a detailed statistical analysis plan, a sample-size justification, or details of planned interim analyses. The study’s stated evaluation aims include methamphetamine use and sexual risk behavior. Secondary outcomes listed in the record concern incident sexually transmitted infections, HIV-related sexual risk behaviors in the previous 30 days, and PEP medication adherence.

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: Reasons for Screen Failure (N = 7): HIV-positive at baseline screening; negative urine screen for methamphetamine§; did not complete baseline assessments; provided incorrect information at phone screening.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: 53 enrolled and 2 were withdrawn.

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 Self-reported Methamphetamine Use in Previous 30 Days., measured over 3-months after baseline.

The primary endpoint is the mean number of days on which participants reported methamphetamine use during the previous 30 days, assessed at 3-months after baseline. It is a continuous measure: each participant’s response is a count of use days over the stated recall period, and the group-level endpoint is described as a mean. The registry specifies standard deviation as the measure of dispersion.

Self-reported days of use provide a way to describe the frequency of methamphetamine use over a defined recall period. A mean gives a group-level summary, while the standard deviation describes how spread out individual responses are around that mean. Together, these statistics can indicate both the average reported frequency and the variability among participants. They do not show the distribution’s full shape or whether responses cluster at particular values.

The endpoint is relevant to the study’s stated goal of evaluating methamphetamine use alongside sexual risk behaviors. It is not the same as a urine-based measure: the intervention used periodic urine analyses to reinforce negative tests, while the primary endpoint is explicitly self-reported. The registry does not report validation procedures, the precise wording of the endpoint question, or how missing self-reports were handled.

Most importantly, the registry record does not report the observed mean or standard deviation for this endpoint. It also does not report a change from baseline, a comparison with a control group, or an inferential result. The outcome definition identifies what was intended to be measured, but without those numerical results it does not establish the observed level of use or the precision of an estimated intervention effect.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

Statistical interpretation starts with the kind of question a design can answer. This registry describes a single-group study. A single-group study can characterize observations among participants receiving an intervention, but it lacks a concurrent untreated or alternative-treatment group for estimating a between-group contrast. General statistical principle: a change over time in one group can be described, but attributing that change to an intervention requires assumptions about what would have happened without it. The registry record does not report a design that permits a randomized comparison.

What does the mean represent?

The primary outcome is a continuous measure summarized by a mean: the number of days of self-reported methamphetamine use in the previous 30 days, assessed at 3-months after baseline. A mean is the arithmetic center of the recorded values, expressed in the endpoint’s units, which are days. It is a useful group summary, but it does not imply that every participant had a value near the mean. The standard deviation, which the registry names as the dispersion measure, describes spread around the mean under the usual interpretation. Neither statistic is reported numerically for this outcome in the registry record.

For a bounded count of days, the distribution may not be well described by a symmetric bell-shaped curve. General statistical principle: distribution shape matters because methods that rely on approximate normality can be less suitable when values are highly skewed, concentrated at boundaries, or otherwise irregular. The registry does not report the observed distribution, so it is not possible to assess those features or say whether a particular model would be appropriate.

What would a confidence interval tell a reader?

In general, a confidence interval expresses the uncertainty around an estimated quantity under the assumptions of the analysis. It is not a range containing a fixed proportion of individual participants’ values, and it does not by itself resolve bias from study design, measurement, or missing data. For a group mean, an interval could describe uncertainty in that mean; for a between-group contrast, it could describe uncertainty in the estimated difference. The registry record does not report a confidence interval or a numerical estimate for the primary endpoint, so no precision statement about that outcome can be made.

A p-value, if reported, would address compatibility between observed data and a specified null model, given the analysis assumptions. It would not measure the size or clinical importance of an effect, and it would not establish causation. The registry lists 0 analyses. No primary-outcome p-value or inferential analysis is reported in the registry record. It is therefore not appropriate to infer a result from the study’s stated aims, design, or sample-size planning assumptions.

Why does the single-group design matter?

The intervention combines CM and access to PEP. Because the registry describes a single PEP/CM group, the effect of the package cannot be separated into effects attributable to CM and effects attributable to PEP. PEP was available after a qualifying exposure, so participants’ actual use of that component could depend on whether such an exposure occurred and whether they initiated the medication. The registry’s listed secondary outcome on PEP adherence is relevant to implementation, but the record does not report adherence results.

In an uncontrolled study, comparison with a participant’s earlier status may be informative descriptively, but it does not supply the counterfactual outcome: what would have happened to the same participants over the same period without the intervention. General statistical principle: without a suitable comparison, secular changes, regression to the mean, changes in circumstances, and measurement effects can be difficult to separate from an intervention-associated change. These are potential interpretive concerns, not established events in this study.

How should planning calculations be understood?

The registry describes a sample-size planning illustration based on the observed effect, rather than the trial’s own design. The stated assumptions are a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation; the target effect is a stated fraction of the observed effect. These assumptions describe a hypothetical planning exercise. They do not describe the single-group study’s allocation, and they do not establish that the original study was designed with those assumptions.

General statistical principle: a planning sample size depends on the target effect, variability, significance threshold, desired power, allocation, and model assumptions. Changing the target effect changes the planning requirement. Such calculations are illustrations of what a future design might require under stated assumptions, not evidence that a completed study had adequate power. Observed power is not a substitute for an effect estimate and its uncertainty. No sample-size justification for the original study is reported in the registry record.

The record also notes that time-to-event planning tables use a total number of events, while other planning tables use participants per arm. Those conventions are not outcome findings. They should be interpreted only within the planning scenario described, rather than as observed enrollment, analyzed sample size, or event counts in this trial.

What remains unknown from the record?

The registry does not report the primary mean, standard deviation, baseline outcome values, an estimate of change, a confidence interval, a p-value, or a model specification. It also does not report diagnostics, transformations, covariate adjustment, or sensitivity analyses. Without these details, readers cannot reconstruct an analysis or judge whether its assumptions were checked. The absence of a reported analysis is not evidence that no analysis was conducted outside the registry; it means that the registry record does not present one.

These method concepts help distinguish outcome description, uncertainty, causal attribution, and planning. In this study, the design information is available, but the primary numerical result and inferential analysis are not. Statistical claims should therefore remain limited to the registered outcome definition and design characteristics.

10. Confidence Intervals for Each Arm

No two-arm comparison is shown for this trial, but each arm's mean can be given with a t-based 95% confidence interval computed from the posted mean, spread and number of participants.

R code and output

arm <- c("PEP/CM")
n   <- c(51)
m   <- c(1.6)
sp  <- c(2.4)
sd  <- sp      # posted standard deviations
for (i in seq_along(n)) {
  se <- sd[i] / sqrt(n[i]); h <- qt(0.975, n[i] - 1) * se
  cat(sprintf("CARM|%s|%d|%.3f|%.3f|%.3f|%.3f\n", arm[i], n[i], m[i], sd[i], m[i] - h, m[i] + h))
}

CARM  PEP/CM  51  1.600  2.400  0.925  2.275

11. Multiplicity, Missing Data and Analysis Populations

The registry lists 4 outcome measures and identifies secondary outcomes concerning incident sexually transmitted infections, HIV-related sexual risk behaviors in the previous 30 days, and PEP medication adherence. It lists 0 analyses. The primary endpoint is self-reported methamphetamine-use days in the previous 30 days at 3-months after baseline. The registry record does not report a multiplicity strategy, such as a prespecified hierarchy among outcomes or an adjustment for testing several outcomes.

As a general statistical principle, when several outcomes or analyses are examined, the chance of at least one apparently unusual result can increase if each is assessed separately without accounting for the family of questions. This does not show that such a problem occurred in this study; the record does not provide analysis results or a multiplicity plan. Readers cannot determine from the registry which outcomes, if any, were formally tested or how conclusions were selected.

The primary-outcome population description says that 53 were enrolled and 2 were withdrawn. It does not state whether withdrawn participants had endpoint measurements, whether all enrolled participants were included in analysis, or how missing endpoint values were handled. The registry does not report the number with observed primary-outcome data, reasons for missingness, imputation, or sensitivity analyses. Those omissions prevent assessment of possible bias from incomplete follow-up.

The registry also does not define an analysis population, such as an intention-to-treat or per-protocol population. In a single-group study, those labels would not create a control comparison, but they would clarify which participants and observations contributed to a reported estimate. Since no primary estimate or analysis population is reported, the analyzed set and its relationship to enrollment remain unclear. Missing-data and multiplicity conclusions should therefore be treated as unknown, rather than assumed to be handled in a particular way.

12. Safety Results

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

Other (non-serious) adverse events

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

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: depression (1.9%), shortness of breath (1.9%).

The adverse-event information is reported for the PEP/CM group. The registry lists 53 participants at risk for serious adverse events and 2 affected, and 53 at risk for other adverse events and 4 affected. These are the record’s reported group counts. They describe participants recorded as affected, not necessarily the total number of individual event episodes; the registry excerpt does not explain whether a participant could contribute more than one event.

The most frequent serious terms listed are depression at 1.9% and shortness of breath at 1.9%. These terms and percentages should be read as registry safety summaries, not as evidence about cause. The record does not establish whether an event was attributed to CM, Truvada, the underlying circumstances, or another factor. It also does not give event timing, duration, severity details beyond the serious-event classification, or clinical outcomes for these terms.

The death count is not reported in the registry record. A missing value should not be interpreted as proof that no deaths occurred. Likewise, the listed serious and other adverse-event figures do not by themselves permit comparison with another treatment or estimate excess risk, because this is a single-group study and no comparator safety data are reported.

Safety counts are most informative when interpreted alongside definitions, follow-up duration, event-level details, and a suitable comparison. Those details are not reported in the registry record. The figures therefore document the listed safety observations but do not establish that the intervention caused or prevented any event.

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

The study addresses a combined intervention: CM intended to reinforce methamphetamine-negative urine tests and access to Truvada PEP after a qualifying HIV exposure. The registry gives a clear description of the intended primary measure, mean self-reported methamphetamine-use days during the previous 30 days at 3-months after baseline. It also records a completed status and describes a single-group design.

Those details establish what the study set out to evaluate, not the size or direction of an outcome. The primary mean and standard deviation are not reported in the registry record, and no primary analysis result is listed. It is therefore not possible from this record to quantify methamphetamine use at the assessment, describe its variability, or determine whether it changed relative to baseline.

The design also cannot establish how outcomes compare with what would have occurred without the intervention. There is no separate control group in the registry record, and the study combines CM with PEP rather than assigning the components separately. Even a reported change over time would not, by itself, show that the intervention caused that change or reveal which component was responsible.

The secondary outcomes are relevant to the study’s broader aims, including sexual risk behaviors, incident STI infections, and PEP adherence. Their listing indicates intended outcome areas; it does not supply findings. The registry record does not report numerical results for these outcomes or explain how their analyses relate to the primary endpoint.

Accordingly, the record supports a description of the intervention, population, endpoint, and design, but it does not support a conclusion that the intervention reduced methamphetamine use, changed sexual risk behavior, or affected PEP adherence. Nor does the absence of reported results establish that the intervention had no effect. Both positive and null-effect claims require outcome estimates and uncertainty, interpreted in light of the single-group design and the missing analysis details.

14. Why This Trial Matters Statistically

This trial is statistically instructive because its intervention and research question involve more than one component, while its registered design is single-group. The combination of CM and PEP makes the program clinically and behaviorally integrated, but also means that a group-level outcome cannot isolate the effect of either component. This distinction between evaluating a package and identifying component-specific effects is central to interpreting intervention studies.

The primary endpoint is a self-reported frequency measure with a defined recall period. Such an endpoint is accessible and directly connected to the stated study aim, but it is distinct from the periodic urine analyses used to reinforce abstinence. Those measures represent different forms of information: a self-report about days of use and a biological test result. The registry record does not report how they relate empirically in this study.

The record also illustrates why outcome definitions and numerical results must be considered separately. It specifies the mean and standard deviation for the primary measure and the assessment timeframe, yet does not report the actual values or an analysis. Knowing the measurement plan clarifies what the study intended to assess, but does not establish the observed outcome or its precision.

Finally, the presence of planning assumptions should not be confused with the trial’s design or findings. The registry characterizes a sample-size exercise as a planning illustration, with assumptions that include equal allocation between two arms, even though the registered study is single-group. Recognizing that distinction helps prevent hypothetical planning quantities from being read as observed evidence or as proof of adequate power.

These features make careful separation of description, estimation, causal inference, and planning especially important. The registry supports discussion of those statistical issues, while leaving the primary effect and its uncertainty unreported.

15. 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 limitation for interpreting efficacy is that the registry record does not report the primary endpoint’s numerical result. Although the outcome is defined as a mean with standard deviation, neither statistic is given. There is no reported estimate of change, confidence interval, or inferential result from which to assess direction, magnitude, or precision.

The single-group design is an additional limitation. Allocation is listed as NA, masking as none, and the model as single-group. No concurrent comparator is described. Consequently, the record does not support a randomized contrast or a causal estimate against an alternative. Because CM and access to PEP were combined, the design also cannot separate component-specific effects.

The endpoint is self-reported, whereas the behavioral intervention used periodic urine analyses to reinforce negative methamphetamine tests. The registry does not report validation details for the self-report, the exact assessment instrument, or an analysis relating self-report to urine results. Recall and reporting effects are general measurement concerns; the record does not establish that they affected this trial.

The primary-outcome population description notes that 53 were enrolled and 2 were withdrawn, but the registry does not report how withdrawals were handled, how many contributed endpoint data, or how missing values were addressed. It also does not report baseline endpoint values, an analysis population, covariate adjustment, model diagnostics, or sensitivity analyses.

Finally, the listed secondary outcomes broaden the evaluation, but the record reports 0 analyses and no multiplicity strategy. The adverse-event summary provides limited counts and selected terms without a reported comparator or detailed event-level context. These limitations are specific to the information and design described in the registry; they do not establish what may have been reported elsewhere.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Contingency Management for Methamphetamine Abstinence and HIV Post-Exposure Prophylaxis in Men Who Have Sex With Men — NCT00856323.
• Registry record last updated 2016-09-20; status last verified 2016-08.
• Results first posted 2012-11-27.
• 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.

19. Record Summary

The registry identifies NCT00856323 as a completed phase 2, single-group study of contingency management and Truvada PEP among HIV-negative men who have sex with men who use methamphetamine. The study started in 2009-01, with primary completion in 2010-12, and enrollment listed as 53. Allocation is NA and masking is none.

The primary endpoint is mean self-reported methamphetamine-use days during the previous 30 days, assessed at 3-months after baseline, with standard deviation specified as the dispersion measure. The registry record notes that 2 participants were withdrawn, but does not describe how they were handled in analysis. The numerical primary result and inferential analysis are not reported.

Secondary outcomes concern incident STI infections, HIV-related sexual risk behaviors, and PEP adherence. The safety record lists serious and other adverse-event counts for PEP/CM, selected serious terms, and no reported death count. Overall, the registry describes the intervention and planned outcome measures but does not provide the primary outcome estimates needed to judge their magnitude or precision.