This page provides an independent statistical analysis and educational interpretation of publicly reported results. ClinicalTrials.gov provides the official trial registry record. Numerical trial results on this page are restricted to the results posted in the ClinicalTrials.gov record; calculations added by Clinical Biostats are labelled as such.
1. Trial at a Glance
This page covers the registry results for Cannabis Effects on Driving-related Skills of Young Drivers (NCT01592409), a randomized na study. The primary outcome is Psychomotor Impairment (Driving).
| Feature | Registry value |
|---|---|
| ClinicalTrials.gov identifier | NCT01592409 |
| Phase | NA |
| Condition | Psychomotor Impairment |
| Allocation | Randomized |
| Design model | Parallel |
| Masking | Double |
| Enrollment | 99 |
| Arms in the primary outcome | 2 |
| Lead sponsor | Centre for Addiction and Mental Health |
| Results posted | Yes |
2. Clinical Question
Population
The study concerns young drivers. Its brief description says participants aged 19 to 25 years old would be recruited; the registry record does not report the ages or other demographic characteristics of the participants in the posted outcome groups.
Intervention
The active condition was a cigarette containing cannabis with 12.5% active THC. The brief description frames the study as an examination of a single dose and planned testing immediately after administration, 24 hours later, and 48 hours later.
Comparator
The comparison condition was a cannabis cigarette with the active THC removed, described as containing 0% THC. The trial record identifies the arms as Active Cannabis and Placebo.
Primary question
For the posted primary outcome, what was the difference between the groups in mean-speed change from drug-free baseline to 30 minutes after smoking? The measure is change in kilometres per hour in driving-simulator zone or hazard performance. The reported group means and comparison describe this endpoint; they do not by themselves describe every driving-related skill or establish collision risk.
The registry classifies the condition as psychomotor impairment and lists a primary outcome titled “Psychomotor Impairment (Driving).” The endpoint description specifies simulator-measured mean speed and a comparison from baseline to 30 minutes after smoking. Although the brief study description refers to later follow-up testing, no later-time primary results are posted in the registry record. The clinical question addressed by the available result is therefore narrower than the study’s broader stated interest in acute and residual effects.
3. Trial Design
The registry describes a randomized, double-masked, parallel-group study comparing active cannabis with placebo. It lists the study as completed and gives the sponsor as the Centre for Addiction and Mental Health. The listed start date is 2012-07, and primary completion is recorded as 2016-08. The phase is recorded as NA.
The brief description says eligible participants would be randomly assigned to smoke either active cannabis or placebo, after eligibility screening and a practice session. It describes testing on a drug-administration day and follow-up testing 24 hours and 48 hours later, with driving-related skills assessed using a high-fidelity simulator. It also mentions planned assessments of mood, cognition, memory, and complex reaction time. Those details describe the study’s stated plan; the registry record does not post results for those additional measures or later testing times.
The record lists enrollment as 99. Separately, the brief description says that a total of 142 subjects would be randomly assigned. These are different record entries: the former is the listed enrollment, while the latter is the number stated in the description of the planned study. The registry record does not explain the difference, so the entries should not be treated as interchangeable.
The primary outcome results report an Active Cannabis group with 67 participants and a Placebo group with 31 participants. These are the group counts associated with the reported endpoint, not a complete account of attendance, follow-up, exclusions, or analysis populations. Reasons for any differences between the listed enrollment and outcome-group counts are not reported in the registry record.
The design labels indicate random allocation, masking, and a parallel comparison, but details such as the allocation procedure, who was masked, and how adherence was assessed are not reported in the registry record. The record marks the study completed; that status does not supply additional outcome or protocol details.
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: Screened over the telephone n=1024. Assessed for eligibility (n=178). Excluded: Not meeting inclusion criteria n=45, Declined to participate or lost interest n=32, Did not receive IP (did not return after practice session) n=2. Pilots (received active cannabis, not randomized) n=5. Randomized n=94.
Overall Study
| Stage | Active Cannabis | Placebo |
|---|---|---|
| STARTED | 67 | 32 |
| COMPLETED | 67 | 31 |
| NOT COMPLETED | 0 | 1 |
Reasons for not completing this period:
| Reason | Active Cannabis | Placebo |
|---|---|---|
| Withdrawal by Subject | 0 | 1 |
Analysis population for the primary outcome, as defined in the registry: Zone/Hazard performance measure: Mean Speed (Change from drug-free baseline to 30 minutes after smoking cannabis in kilometres per hour (kph)
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.
Population: One participant was excluded from analyses because they admitted to having tried to skew their data. They withdrew from the study.
| Characteristic | Unit | Active Cannabis | Placebo | Total |
|---|---|---|---|---|
| Participants analysed | 67 | 31 | 98 | |
| Age, Continuous | years | 22.22 (SD 1.88) | 21.97 (SD 2.24) | 22.14 (SD 1.995) |
| Sex: Female, Male: Female | Participants | 19 | 9 | 28 |
| Sex: Female, Male: Male | Participants | 48 | 22 | 70 |
| Region of Enrollment: Canada | participants | 67 | 31 | 98 |
6. Primary Endpoint
The registry lists the primary outcome as Psychomotor Impairment (Driving), measured over Approximate: at baseline (30 minutes before smoking), 30 minutes after smoking.
| Element | Registry entry |
|---|---|
| Result type | Mean |
| Measure of spread | Standard Deviation |
| Unit | change in kph |
| Endpoint classification | Continuous Endpoint |
The primary outcome is a change in mean speed, measured in kilometres per hour, during driving-simulator zone or hazard performance. The registry defines the change as the value at drug-free baseline compared with the value 30 minutes after smoking cannabis. The timeframe is described as approximate, with baseline 30 minutes before smoking and follow-up 30 minutes after smoking.
Mean speed is a continuous measure: it can take a range of numerical values rather than placing each participant into a yes-or-no category. The registry reports each arm’s mean change and standard deviation. A positive mean change indicates that the measured mean speed was higher after smoking than at baseline, while a negative mean change indicates a lower value; this is an explanation of the scale’s direction, not a judgment about whether either change is safe or unsafe.
A simulator measure allows driving-related performance to be recorded under programmed scenarios. In general, objective continuous measures can capture differences in performance that a broad event count might not represent. However, the posted result is specific to mean speed in the described simulator measure. It does not directly quantify collisions, real-world driving, or every aspect of driving performance.
The study description refers to testing at later follow-up times and to other cognitive and behavioural assessments. The primary result posted in the registry record is the baseline-to-30-minute mean-speed change. Results for later time points and the other listed assessments are not reported in the registry record.
7. Results Posted in the Registry
- Active Cannabis: mean 3.38 versus -1.54 on control; difference 4.92 (95% CI 1.43 to 8.41), standardized effect d = 0.54; z-test p = 0.006.
Mean difference: Active Cannabis
95% CI: 1.43 to 8.41
Recalculated from the posted summary statistics
8. Statistical Interpretation
For the primary mean-speed change outcome, the Active Cannabis group’s reported mean change was 3.38 kph, with a standard deviation of 9.75 and 67 participants. The Placebo group’s reported mean change was -1.54 kph, with a standard deviation of 7.36 and 31 participants. The registry’s between-group mean difference is 4.92 kph, with a 95% confidence interval from 1.432 to 8.408 and an unadjusted p-value of 0.006.
The positive difference indicates that the reported mean change was higher in the Active Cannabis group than in the Placebo group. Because the endpoint is change from baseline, the comparison is between the groups’ changes, not simply the groups’ post-smoking speeds. The arm-level means show that the active group’s average change was positive and the placebo group’s average change was negative. Those averages do not tell us how every participant changed, nor do they establish an effect on collisions or real-world driving.
The confidence interval expresses uncertainty around the estimated mean difference under the statistical approach used for the reported interval. Its endpoints are both above zero, so the interval is compatible with a positive difference in this outcome. The interval also spans a range of possible effect sizes; it should not be read as identifying the exact difference. The unadjusted p-value is evidence against a zero-difference null model under the test’s assumptions, but the registry does not report a multiplicity adjustment or enough analysis detail to establish how this result was selected or modeled.
The standard deviations describe variation in individual outcome values within each arm, rather than uncertainty in the arm means. They are substantially wider in scale than the mean changes themselves, underscoring that group averages do not describe a uniform response. The registry does not post individual-level results, distributions, or adjusted analyses.
The record posts one primary outcome and no secondary outcome results. The brief description identifies intended later testing and other domains, but there are no posted results for those measures in the registry record. Thus, the result supports a precise description of the reported primary simulator endpoint only. It does not establish persistence at later follow-up, generalize automatically to other driving skills, or quantify collision risk. The record’s description of a planned sample of 142 and its listed enrollment of 99 are not reconciled in the registry record, and the outcome group counts should be understood as those associated with this endpoint.
9. Statistical Methods Explained
The registry identifies the outcome as continuous and reports a mean and standard deviation for each arm. A continuous outcome is measured on a numerical scale; here, the scale is change in mean speed in kph. For this kind of outcome, the comparison can be expressed as the difference between the groups’ mean changes. The posted estimate is 4.92 kph, with Active Cannabis compared with Placebo.
What does the mean difference represent?
The reported arm means are 3.38 kph for Active Cannabis and -1.54 kph for Placebo. The mean difference describes the contrast between these group-level changes. It is not an individual participant’s change, and it does not mean all participants changed in the same direction or by the same amount. Mean differences are useful because they preserve the units of the outcome, making the estimate interpretable on the original kph scale.
In general, a group mean gives a central summary, while a standard deviation describes how spread out individual observations are around that mean. The posted standard deviations are 9.75 for Active Cannabis and 7.36 for Placebo. A standard deviation is not a confidence interval and is not the same as the standard error of a mean. The record does not provide participant-level distributions, so the extent of skew, outliers, or other distributional features is not reported in the registry record.
How should the confidence interval be read?
The 95% confidence interval for the mean difference is 1.432 to 8.408 kph. In general, a confidence interval gives a range of values compatible with the observed data and the method used to calculate the interval. It represents uncertainty in the estimated group contrast, not the range of individual participants’ changes. Its width matters: a narrow interval indicates greater precision than a wide one, when comparing intervals for the same measure and context. The registry does not state the exact interval method or whether the posted interval was adjusted for any features of the analysis.
What does the p-value add?
The posted p-value is unadjusted and equals 0.006. In general, a p-value describes how unusual data at least as incompatible with a specified null hypothesis would be if that null model and its assumptions were correct. It is not the probability that the null hypothesis is true, nor does it measure the size or practical importance of the effect. Because the value is explicitly unadjusted, it should not be interpreted as accounting for testing multiple outcomes or comparisons. The registry does not report a multiplicity plan.
The record labels the comparison as a mean difference and provides the arm means, spreads, counts, confidence interval, and p-value. It does not report the exact statistical test, covariates, model specification, treatment of missing observations, or the analysis population definition. Those details cannot be inferred from the numerical summary. In general, the choice of a model can matter when observations have unequal variability, unusual distributions, or dependencies, but the record does not report whether or how those issues were assessed.
Why does variation matter?
Variation affects how precisely a group mean and the contrast between groups can be estimated. Larger variability generally makes a fixed difference harder to distinguish from random variation when other aspects of a study are held constant. The posted standard deviations provide a summary of spread, while the confidence interval reflects uncertainty in the comparison. They answer related but different questions.
Over-dispersion is a general concern for outcomes where observed variation exceeds what a chosen statistical model expects. A model that understates variation can produce uncertainty estimates that are too small. This endpoint is continuous, and the registry does not identify an over-dispersion assessment or a model that would require one. The general point is that uncertainty calculations should match the outcome’s distribution and study design; whether that was done cannot be determined from the registry record.
The study is described as randomized, double-masked, and parallel-group. In general, random allocation is intended to reduce systematic differences between groups at assignment, and masking can reduce some forms of expectation-related influence. These design labels do not replace reporting of implementation details or analysis methods. The record does not state who was masked, how randomization was generated or concealed, or how outcome data were handled after assignment.
The result is therefore best read as a posted unadjusted comparison of group mean changes for one primary simulator outcome. It is a numerical summary of the groups, with an estimate and confidence interval, rather than a complete account of the statistical analysis. The record does not report whether the primary analysis adjusted for baseline values, protocol deviations, or other factors.
10. Confidence Intervals and Precision
The mean difference is 4.92 kph, and its 95% confidence interval ranges from 1.432 to 8.408 kph. The interval conveys uncertainty about the size of the group contrast. Its lower and upper endpoints are both positive, so the reported interval does not include zero. This is consistent with the posted unadjusted p-value of 0.006, though the p-value and interval are different summaries and depend on their respective analysis methods.
Precision should not be confused with whether an outcome is important, or with how consistent individual responses were. The standard deviations are 9.75 kph in the Active Cannabis group and 7.36 kph in the Placebo group; these describe spread among outcome values, rather than uncertainty in the mean difference. Individual values could vary considerably around the arm means, and the registry does not post their distribution.
The confidence interval’s range leaves uncertainty about the exact magnitude of the difference. It does not establish which values within the interval are practically important, because the record does not provide a threshold for meaningful change in simulator mean speed. Nor does the interval capture all sources of uncertainty, such as possible bias from missing data, analysis choices, or selective reporting. Those matters are not described in enough detail in the registry record.
The interval pertains only to the posted comparison at the stated primary outcome timeframe. It should not be extended to the planned 24-hour or 48-hour tests, or to mood, cognition, memory, complex reaction time, or collision outcomes. Results for those measures are not reported in the registry record.
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.
| Effect to detect | Participants per arm, 80% power | Participants per arm, 90% power |
|---|---|---|
| 100% of the observed effect | 54 | 72 |
| 75% of the observed effect | 95 | 127 |
| 50% of the observed effect | 214 | 286 |
The planning figures are participants per arm, not total event counts. They are illustrations based on the observed effect and are not the original trial’s design calculations. The listed assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, a normal approximation, and a target effect set to the stated fraction of the observed effect. The original trial’s sample-size assumptions are not reported in the registry record.
For planning for 100% of the observed effect, the illustration lists 54 participants per arm for 80% power and 72 participants per arm for 90% power. For planning for 75% of the observed effect, it lists 95 participants per arm for 80% power and 127 participants per arm for 90% power. For planning for 50% of the observed effect, it lists 214 participants per arm for 80% power and 286 participants per arm for 90% power.
In plain terms, the 80% power column gives the illustrated number of participants per arm for a design that would have an 80% chance of detecting the specified effect under the stated assumptions if that effect were the true effect. The 90% column describes the corresponding target for a higher chance of detection. These probabilities are properties of the planning calculation under its assumptions, not guarantees about a future study. This is not observed power.
The curve should be read as showing the planning relationship between the target effect and the required number of participants per arm: as the target effect becomes smaller, more participants are needed under the stated assumptions. These are planning illustrations tied to the observed estimate, not recommendations and not a report of what the original investigators planned.
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 z1−α/2 = 1.960) and the desired power (z1−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
- Observed difference Δ = 4.920; pooled standard deviation σ = 9.071.
- n = 2 × (1.96 + 0.842)2 × 9.0712 / 4.9202 = 53.4, rounded up to 54 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 <- 4.92 # observed difference in means
sigma <- 9.07102357785493 # 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(1592409)
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 = 54 (80% power), 72 (90% power)
75% of observed effect: n per arm = 95 (80% power), 127 (90% power)
50% of observed effect: n per arm = 214 (80% power), 286 (90% power)
power.t.test: 54.3
Simulated power at n = 54 per arm: 0.802 (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 posts one primary outcome measure and no secondary outcome results. The brief description lists other intended assessments, including later follow-up testing and measures of mood, cognition, memory, and complex reaction time, but results for those are not reported in the registry record. The count of posted analyses is 0, so the record does not provide additional analysis results with which to evaluate consistency across outcomes or time points.
Multiplicity arises when several hypotheses, outcomes, time points, or subgroup comparisons are tested. In general, testing many hypotheses without accounting for that process can increase the chance that at least one result appears statistically notable by chance. The posted p-value is explicitly unadjusted. A multiplicity adjustment or prespecified testing hierarchy is not reported in the registry record, so the p-value should be interpreted as unadjusted rather than as evidence that all potential testing has been accounted for.
The registry identifies the primary endpoint and timeframe, but does not report a statistical analysis plan, the sequence of analyses, interim analyses, subgroup analyses, or which analyses were prespecified. It also does not report the treatment of missing outcome data, reasons for missingness, or whether an intention-to-treat or another analysis population was used. These omissions limit assessment of how closely the posted analysis follows the original plan and how missing observations might affect the estimate.
In general, missingness can affect an analysis if the availability of an outcome is related to the outcome itself or to group assignment. The registry’s outcome-group counts do not explain participant flow, and should not be interpreted as proof that every randomized participant contributed data or that missing data were absent. The registry record does not report the information needed to determine those points.
The record also does not provide a detailed account of protocol deviations, adherence, outcome assessor procedures, or sensitivity analyses. These are reporting gaps, not evidence that any particular problem occurred. Accordingly, the posted result can be described as the reported unadjusted primary comparison, while the robustness of the analysis to missing data, multiple testing, and analysis-population choices cannot be assessed from the registry record.
14. Safety Results
The registry reports serious adverse events as affected participants divided by participants at risk.
| Arm | Participants with serious adverse events | Participants at risk | Percentage |
|---|---|---|---|
| Active Cannabis | 0 | 67 | 0.0% |
| Placebo | 0 | 32 | 0.0% |
Deaths
| Arm | Deaths (participants affected of at risk) |
|---|---|
| Active Cannabis | 0 of 67 |
| Placebo | 0 of 32 |
Other (non-serious) adverse events
Participants with at least one non-serious event at or above the registry's 0% reporting threshold.
| Arm | Participants affected of at risk |
|---|---|
| Active Cannabis | 38 of 67 |
| Placebo | 15 of 32 |
Most frequent non-serious adverse event terms
| Event term | Organ system | Active Cannabis | Placebo |
|---|---|---|---|
| headache | General disorders | 10 of 67 | 2 of 32 |
| tiredness/fatigue | General disorders | 5 of 67 | 3 of 32 |
| dizziness/faintness | General disorders | 5 of 67 | 2 of 32 |
| insomnia | General disorders | 6 of 67 | 0 of 32 |
| low blood pressure | Vascular disorders | 4 of 67 | 1 of 32 |
| muscle/bone/joint pain | Musculoskeletal and connective tissue disorders | 3 of 67 | 2 of 32 |
| somnolence | General disorders | 4 of 67 | 1 of 32 |
| common cold | Respiratory, thoracic and mediastinal disorders | 3 of 67 | 1 of 32 |
| rash | Skin and subcutaneous tissue disorders | 4 of 67 | 0 of 32 |
| anxiety/nervousness | Psychiatric disorders | 1 of 67 | 2 of 32 |
The serious adverse event figures list 67 participants at risk in the Active Cannabis group and 32 in the Placebo group. The registry lists 0 serious adverse events in each group and 0 deaths in each group. The record lists no top serious adverse event terms.
Other adverse events are reported separately: 38 affected participants in the Active Cannabis group and 15 in the Placebo group. These are counts, not rates. The registry gives 67 at risk for the active group and 32 at risk for placebo in this safety listing. The record does not provide event-level descriptions, severity details for these other events, timing, or an assessment of whether they were related to the intervention.
The safety counts should be read as the registry’s posted figures for the listed groups, not as proof that no unrecorded events occurred. Serious adverse events and other adverse events are different categories in the record. The outcome-group counts for the primary endpoint are not identical to the at-risk counts in the safety listing, and the registry does not explain the difference. No inference about comparative safety beyond these reported counts is warranted from the information posted.
15. What the Result Does, and Does Not, Mean
The posted result describes a difference in average change in simulator mean speed between the Active Cannabis and Placebo groups at the primary outcome timeframe. The estimated mean difference is 4.92 kph, with a 95% confidence interval from 1.432 to 8.408. The interval indicates uncertainty around the group-level estimate, and the unadjusted p-value is 0.006. These statistics address the specified continuous simulator measure.
The result does not establish how each participant performed. Group means and standard deviations cannot reveal individual patterns, and the registry does not post the underlying observations. The comparison also does not directly measure collisions, real-world driving, or risk of injury. A simulator-based mean-speed change is not interchangeable with those outcomes.
The brief description presents a broader study objective involving acute and residual effects, including later assessments. The posted primary outcome is tied to the baseline and 30-minute-after-smoking timeframe. Since later-time results and secondary outcomes are not reported in the registry record, the primary result cannot be used to describe what happened at 24 hours or 48 hours or on the other planned assessments.
The record identifies randomization and double masking, but does not give implementation details or a full statistical analysis plan. The posted estimate should therefore be read with the reported design labels and the stated limitations in mind. The unadjusted p-value does not indicate the probability that the finding is true, and statistical evidence about a mean difference does not by itself determine its practical meaning. The registry does not report a threshold for a meaningful change in this outcome.
Finally, the record’s listed enrollment is 99, whereas the brief description states that 142 subjects would be assigned. The record does not reconcile these entries. This discrepancy in the record limits clarity about the planned versus listed enrollment, but it does not change the numerical description of the reported endpoint groups and comparison.
16. Why This Trial Matters Statistically
This study illustrates how a clinical trial result can be narrower than its stated research question. The brief description includes acute and residual effects, multiple follow-up times, and several domains of performance. The registry posts a result for one primary continuous measure at the baseline-to-30-minute comparison. Keeping the reported endpoint and timeframe distinct from the broader study aims is essential when interpreting trial evidence.
The outcome is expressed as a change from baseline, and the comparison is a difference between groups’ mean changes. That distinction matters: a post-smoking value alone would not describe the same estimand as a change score. Reporting both arm means and a between-group estimate makes the direction and scale of the contrast clearer, while the confidence interval shows uncertainty around its magnitude.
The record also demonstrates why a p-value should not be used as a stand-in for effect size or precision. The mean difference is expressed in kph, the interval provides a range of compatible values under the method, and the unadjusted p-value addresses a null-model question. These answer related but separate statistical questions. Without a reported threshold for meaningful change, statistical evidence cannot settle practical importance.
Planning illustrations add a different perspective. They show how required sample size depends on the effect a study aims to detect and on explicit assumptions. Smaller target effects require more participants per arm under the stated planning framework. Those illustrations are not the original trial’s design justification and are not observed power.
Finally, the missing analysis details matter. Without reported handling of missing data, multiplicity, or analysis populations, readers cannot fully assess how robust the estimate is to analytic choices. Careful interpretation means recognizing both what the reported statistics establish about this endpoint and what the record leaves unresolved.
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 result is limited to simulator mean-speed change from drug-free baseline to approximately 30 minutes after smoking. Although the brief description includes follow-up at 24 hours and 48 hours, the registry does not post those results. It also does not post results for the other planned assessments of mood, cognition, memory, or complex reaction time. Residual effects and performance in those domains therefore cannot be evaluated from the posted outcome.
The registry does not report actual participant characteristics beyond the brief description’s intended age range of 19 to 25 years. It does not give demographic composition, cannabis-use history, eligibility details, adherence, or how closely participants followed the assigned procedure. Generalizability to populations beyond the studied participants cannot be assessed from the posted record.
Although the design is labeled randomized, double-masked, and parallel, details of random sequence generation, allocation concealment, masking procedures, and outcome assessment are not reported in the registry record. Nor are the statistical test, model specification, baseline adjustment, analysis population, missing-data methods, or sensitivity analyses described. These gaps constrain assessment of possible bias and analytic robustness.
The registry lists enrollment as 99, while its brief description states that 142 subjects would be assigned; the reason for this difference is not reported. The outcome and safety listings also use different group counts, without an explanation of participant flow. No individual-level data or distributional displays are posted in the record, limiting assessment of variability, outliers, and response patterns.
Finally, the endpoint is a simulator measure and is not a direct measure of collision incidence or real-world driving. The result should not be generalized to outcomes that were not posted or to settings not described in the registry record.
18. Related Tutorials
19. Related Statistical Calculators
20. Sources
- ClinicalTrials.gov: Cannabis Effects on Driving-related Skills of Young Drivers — NCT01592409.
- Registry record last updated 2019-02-11; status last verified 2018-09.
- Results first posted 2019-02-11.
- 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.
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Use this trial's endpoint as a starting point for deeper study of effect measures, confidence intervals, and sample-size planning.
21. Record Summary
The registry record describes a completed randomized, double-masked, parallel-group comparison of active cannabis and placebo, sponsored by the Centre for Addiction and Mental Health. The listed enrollment is 99. The brief description states a planned assignment of 142 subjects aged 19 to 25 years old, and the registry does not explain the difference between those entries.
The posted primary outcome is change in simulator mean speed from drug-free baseline to 30 minutes after smoking. The reported mean difference for Active Cannabis versus Placebo is 4.92 kph, with a 95% confidence interval from 1.432 to 8.408 and an unadjusted p-value of 0.006. No secondary outcome results or analyses are posted.
The registry lists 0 serious adverse events and 0 deaths in each safety group. Other adverse events are listed for 38 participants in the active group and 15 in the placebo group. Details on analysis methods, missing data, multiplicity, and participant flow are not reported in the registry record.