Short-term Health Effects of Wild Blueberry Juice Consumption: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Short-term Health Effects of Wild Blueberry Juice Consumption (NCT02139878), a randomized na study. The primary outcome is Blood Pressure.

2. Clinical Question

3. Trial Design

The registry describes a randomized, single-masked crossover trial of wild blueberry juice and placebo wild blueberry juice. The recorded condition is pre-diabetes. Enrollment is listed as 20, and the trial status is completed. The record lists the start as 2014-05 and primary completion as 2015-06. Phase is recorded as NA.

Both interventions are described as 240 ml. The primary outcome timeframe is up to 4 weeks. The registry does not report the order in which participants received the interventions, the length of any periods, whether there was a washout, or how the crossover sequence was handled in analysis. It also does not identify who was masked. Those details matter for understanding the conduct and analysis of a crossover trial, so they cannot be inferred from the design labels alone.

The primary posted outcome is blood pressure, specifically systolic blood pressure, expressed in mm Hg. The registry gives an outcome mean and standard error for each posted group. Each group has n = 19. These outcome-group counts should not be treated as an accounting of unique participants in a crossover design: participants may contribute measurements under both interventions, and the record does not explain the relationship between the posted counts and the enrolled population.

The University of Prince Edward Island is listed as sponsor. The brief summary says that researchers wanted to learn about the health effects of wild blueberry juice. That statement gives the broad purpose, but detailed eligibility criteria, recruitment methods, protocol procedures, and the trial’s original sample-size justification are not reported in the registry record.

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: 20 of the 74 participants were randomized. Of those not randomized, 49 did not meet the inclusion criteria and 5 declined to participate.

First Intervention (7 Days)

Washout Period (8 Days)

Second Intervention (7 Days)

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 Blood Pressure, measured over Up to 4 weeks.

The primary endpoint is systolic blood pressure, a continuous measurement reported in mm Hg. The registry labels the summary as a mean with standard error and gives a timeframe of up to 4 weeks. Systolic pressure is the pressure measured as the heart contracts. As a general statistical and clinical concept, it is one component of a blood pressure reading and can be analyzed as a continuous outcome, allowing comparisons of average values between conditions.

The posted means are 116.0 for wild blueberry juice and 120.8 for placebo; the standard error is 2.2 for each. The reported endpoint description is specifically systolic blood pressure, not a broader measure of cardiovascular health. A difference in group means describes the average outcome contrast, but by itself it does not establish why the values differ or whether any difference is meaningful for individual participants.

The registry does not report a rationale explaining why this endpoint was selected, how or when measurements were obtained within the timeframe, or whether multiple readings were combined. Such details could clarify measurement reliability and the planned analysis, but they are not reported in the registry record. The endpoint therefore supports a focused comparison of posted average systolic pressure, with interpretation bounded by the available summary information.

7. Results Posted in the Registry

• Wild Blueberry Juice: mean 116.00 versus 120.80 on control; difference -4.80 (95% CI -10.90 to 1.30), standardized effect d = -0.50; z-test p = 0.123.

8. Secondary Efficacy Endpoints

The registry also posts results for secondary outcomes. For each outcome below, the endpoint type, the test applied to the posted summary statistics, the result and its interpretation are given in turn. Secondary endpoints are usually not adjusted for multiple testing, so they support rather than confirm the primary finding.

Glucose

• Endpoint type: Continuous: a mean value per participant, compared as a difference in means.
• Test: Unadjusted two-sample z-test for the difference in means, using the posted standard errors.
• Result:
  • Wild Blueberry Juice: mean 5.30 versus 5.40 on control; difference -0.10 (95% CI -0.38 to 0.18), standardized effect d = -0.23; z-test p = 0.480.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Wild Blueberry Juice and the control. The interval gives the range of effects that remain compatible with the data.

The registry posts one secondary outcome, glucose, measured in mmol/L over a timeframe of up to 4 weeks. Its inclusion gives a related metabolic measure alongside the primary systolic blood pressure endpoint, but the registry’s results do not establish that the two endpoints represent a single broad health effect. A result on one measure cannot be treated as confirmation of another simply because both were collected in the same trial.

Secondary outcomes are often useful for describing additional questions, but they generally carry less confirmatory weight than a prespecified primary endpoint. Their interpretation depends on whether they were planned in advance, how many outcomes or analyses were considered, and whether any adjustment for multiple comparisons was used. The registry does not report a multiplicity strategy or detailed analysis plan, so the degree to which the glucose finding should be considered confirmatory is unclear.

The secondary outcome should therefore be read as additional, exploratory context rather than as an independent basis for a broad conclusion. The posted record does not report other secondary endpoints, and it does not provide enough detail to assess how the secondary analysis relates to the crossover structure or missing data. A consistent interpretation across endpoints requires caution when only limited analysis information is available.

Secondary outcomes registered for this trial, for reference (only those above were recalculated):

• Glucose

9. Statistical Interpretation

10. Statistical Methods Explained

The registry reports two kinds of statistical information for the primary outcome: summary statistics by posted group and a sponsor-reported ANCOVA p-value. The summary statistics are means and standard errors. The simple re-calculation compares the two means without adjustment, producing a mean difference of -4.8 mm Hg, a 95% confidence interval from -10.9 to 1.298, and an unadjusted p-value of 0.123. These are not the sponsor’s adjusted analysis.

What does the mean difference describe?

A mean difference expresses the contrast between average outcomes in two conditions, in the outcome’s original units. Here, the re-calculated contrast is expressed in mm Hg. A negative sign indicates that the mean for wild blueberry juice is lower than the mean for placebo under the stated comparison order. This is a group-level summary; it does not describe the outcomes of every participant or the variation in individual responses.

What does the standard error describe?

A standard error describes the estimated uncertainty of a sample mean, rather than the spread of individual measurements. The posted standard errors are 2.2 for each primary-outcome mean. A standard error can help quantify how precisely a mean is estimated, but interpretation of the treatment contrast also depends on the relationship between measurements. That is particularly relevant to crossover data, where outcomes from the same participant may be related. The registry’s group summaries do not provide the paired covariance or other information needed to recover that relationship.

How should the confidence interval be read?

A confidence interval is a range of values compatible with the data and the method used to calculate it. In repeated use under the method’s assumptions, intervals constructed this way have a stated coverage property. It is not a range that guarantees where the true effect lies, nor does it assign a probability to each possible value after the data have been observed. The primary interval runs from -10.9 to 1.298 and includes zero. That means the re-calculation does not distinguish a zero mean difference from some nonzero values within the interval at its stated confidence level.

What does the p-value add?

The unadjusted p-value of 0.123 is calculated for the simple mean comparison. The registry reports p = 0.088 for ANCOVA. In general, a p-value summarizes the compatibility of data with a null model under a particular statistical method and its assumptions. It does not measure the probability that a treatment works, the size of an effect, or the practical importance of a result. The different p-values arise from different analyses; the record does not provide enough details to identify which features of the ANCOVA account for the difference.

Why does crossover structure matter?

In a crossover design, participants receive more than one condition, so a comparison may use within-participant information. As a general statistical matter, paired observations are not independent in the same way as measurements from unrelated people. An analysis that accounts for pairing can differ in its estimate of uncertainty from a calculation that treats group summaries without that pairing information. The record labels the design crossover but does not report the treatment sequence, period details, washout information, or enough data to reproduce a paired analysis. The simple re-calculation is therefore an unadjusted comparison of posted summaries, not a reconstruction of the full crossover analysis.

What does ANCOVA usually do?

ANCOVA is a general method for comparing outcomes while adjusting for specified covariates. Depending on the model and design, adjustment can account for baseline measurements or other variables. The registry identifies ANCOVA as the sponsor’s method but does not report the covariates, model specification, adjusted effect estimate, confidence interval, or analysis population. It is not possible to say from the record which adjustments were made or how the crossover structure entered the model.

All calculations and interpretations should remain tied to the stated analysis. The sponsor-reported ANCOVA is the reported trial analysis; the unadjusted calculation is a transparent comparison based on posted means and standard errors. Neither should be described as a complete substitute for the other. Without the underlying participant-level data and fuller reporting, the adjusted estimate and its precision cannot be independently reconstructed.

11. Confidence Intervals and Precision

Precision concerns how narrowly an effect is estimated. The re-calculated primary mean difference is -4.8 mm Hg, with a 95% confidence interval from -10.9 to 1.298. This interval spans zero, and it also spans values on either side of zero. The posted summary statistics therefore leave uncertainty about the direction and size of the average difference under the unadjusted calculation.

The standard errors for the posted primary means are both 2.2. They describe uncertainty in the individual group means, not the full uncertainty of a paired crossover contrast. Since the registry does not report the paired data or the sponsor’s adjusted estimate and confidence interval, the precision of the ANCOVA result cannot be assessed from the record. Its p-value alone does not supply the missing effect estimate or interval.

Confidence intervals are useful because they show a range of estimates compatible with a method and its assumptions, rather than reducing the result to a threshold decision. A broad interval can reflect limited precision; a narrow one indicates greater precision under the same assumptions. Precision and statistical significance are related but not interchangeable: an interval’s position and width both matter. The available primary interval does not establish that the true average difference is exactly zero, nor does it establish a particular nonzero value.

These limits apply to the posted summaries and should not be generalized beyond them. The record does not report the details needed to evaluate every source of uncertainty, including the treatment of within-participant dependence and missing observations.

12. 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. They are illustrative calculations based on the observed effect, not the trial’s original design justification. The listed assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, a normal approximation, and an effect to detect set to the stated fraction of the observed effect. The trial’s own original sample-size assumptions are not reported in the registry record.

For each assumed effect size, the 80% power column gives the planned participants per arm for a design that would have an 80% chance of detecting an effect of that size if the assumptions and model were correct. The 90% power column describes the corresponding planning target for a 90% chance. These are prospective design quantities, not summaries of what happened in this completed trial.

The illustration lists 63 participants per arm for planning around 100% of the observed effect, 112 for 75%, and 251 for 50% at 80% power. At 90% power, the corresponding listed values are 84, 150, and 336. The calculations assume a two-arm comparison even though the registry describes the trial as crossover; they do not reproduce the trial’s original design or account for unreported crossover-analysis details.

Smaller effects require more participants to detect under the same planning assumptions. This is not observed power: it does not measure the probability that the completed trial detected an effect after the results were known. These figures are planning illustrations only, and the registry’s original sample-size rationale is not reported.

13. 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 = -4.800; pooled standard deviation σ = 9.590.
2. n = 2 × (1.96 + 0.842)² × 9.590² / -4.800² = 62.7, rounded up to 63 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 <- -4.8                      # observed difference in means
sigma <- 9.58957767578948                     # 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(2139878)
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 = 63 (80% power), 84 (90% power)
 75% of observed effect: n per arm = 112 (80% power), 150 (90% power)
 50% of observed effect: n per arm = 251 (80% power), 336 (90% power)
power.t.test: 63.6 
Simulated power at n = 63 per arm: 0.797 (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.

14. Analyses Reported by the Sponsor

The sponsor-reported analysis is ANCOVA, with p = 0.088. The registry does not report its estimated treatment effect, confidence interval, covariates, or full model specification. The analysis can therefore be identified, but its magnitude and precision cannot be read from the posted result.

The separate unadjusted re-calculation from the group summaries gives a mean difference of -4.8 mm Hg, a 95% confidence interval from -10.9 to 1.298, and p = 0.123. It is a simple comparison of posted summary statistics and does not reproduce the ANCOVA. The two p-values should not be treated as interchangeable estimates or as competing versions of the same fully specified analysis.

The registry record does not provide sufficient detail to explain the difference between the results. In particular, it does not describe the ANCOVA covariates or how the crossover structure was represented. The sponsor’s ANCOVA is the trial’s reported analysis; the re-calculation is supplementary and unadjusted. A fuller comparison would require the analysis details and effect estimate that are not reported in the registry record.

15. Multiplicity, Missing Data and Analysis Populations

Multiplicity arises when a study evaluates more than one outcome, contrast, time point, or analysis. Repeated testing can increase the chance that at least one result appears noteworthy by chance. Statistical plans may address this through a prespecified hierarchy or an adjustment procedure, but the registry does not report a multiplicity strategy. It is therefore not possible to determine whether the primary and secondary results were interpreted within a planned testing framework.

The registry lists blood pressure as the primary outcome and glucose as a secondary outcome, and records two outcome measures and two analyses. Those details do not reveal whether there were additional planned assessments, how outcomes were prioritized, or whether the analyses reported represent the full planned set. The analysis plan and any interim analyses are not reported in the registry record.

Missing-data handling is also not reported. The posted primary and secondary outcome summaries each show n = 19 per group, while enrollment is listed as 20. The record does not explain why these counts differ from the enrollment figure, how missing measurements were handled, or whether all randomized participants were included in the reported analyses. It would be inappropriate to infer attrition reasons or an analysis population from those counts alone.

For a crossover study, interpretation can also depend on whether all randomized participants contributed data from both conditions and how incomplete periods were handled. The registry does not report those details, the treatment sequence, or the handling of period and carryover effects. These reporting gaps limit assessment of the analysis assumptions and the robustness of the estimates; they do not prove that any particular method was or was not used.

16. 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.

The registry reports zero serious adverse events among the 19 participants listed at risk for wild blueberry juice, and zero among the 19 listed at risk for placebo. It also reports zero other adverse events in each of those groups. These are the event counts recorded in the registry and should be read as counts for the reporting period and groups described there.

The deaths field is null for both groups, so deaths are not reported in the registry record. A null entry is not the same as a reported zero. The record also does not provide further details about adverse-event definitions, collection procedures, severity categories beyond the listed serious and other categories, or how exposure during the crossover periods was assigned for safety reporting.

Zero reported events in these groups does not establish that no adverse events occurred outside the registry’s reporting or observation framework, and it does not quantify safety for other populations or use patterns. These figures describe the safety information posted in the record; they do not support medical advice.

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

The registry records a completed randomized, single-masked crossover trial comparing wild blueberry juice with placebo in people with pre-diabetes. The primary outcome was systolic blood pressure over up to 4 weeks. The posted group means differ, and the simple unadjusted re-calculation estimates a mean difference of -4.8 mm Hg. Its 95% confidence interval extends from -10.9 to 1.298 and includes zero, reflecting uncertainty about the size and direction of the average difference under that calculation.

The sponsor’s reported ANCOVA result is p = 0.088, but the registry does not provide the adjusted effect estimate or its confidence interval. The sponsor result and the unadjusted re-calculation come from different methods, and the available record does not explain their difference. This means the posted information supports a description of the results, but not a complete assessment of the sponsor’s estimated effect or its precision.

The result does not demonstrate that wild blueberry juice changes blood pressure, and it does not demonstrate that there is no effect. Statistical uncertainty is not proof of equivalence or absence of an effect. Nor do the posted means establish effects for an individual participant, a longer period, or a different population.

The glucose endpoint is secondary, and the registry does not report a multiplicity plan. Missing-data handling, the analysis population, and important details of the crossover analysis are not reported. The safety record lists zero serious and zero other adverse events in each group, while deaths are not reported in the registry record. These limitations should be kept in view when interpreting the posted findings. They are a record of this trial’s reported outcomes, not medical advice or evidence sufficient to make a general claim about health effects.

18. Why This Trial Matters Statistically

This trial illustrates why a mean difference, an uncertainty interval, and the analysis method should be considered together. The posted primary means show a numerical contrast, while the unadjusted confidence interval includes zero and covers a range of values on both sides. Reporting only the group means would conceal the uncertainty; reporting only a p-value would conceal the estimated scale and range of the contrast.

It also demonstrates the importance of matching a re-calculation to the study design. This record identifies a crossover design and an ANCOVA analysis, but provides only group-level means and standard errors for the primary outcome. Without the paired participant data or fuller model details, a simple unadjusted calculation cannot reproduce the sponsor’s analysis. The two results answer related but not identical statistical questions.

Finally, transparent reporting of the analysis plan, missing-data handling, multiplicity, and effect estimates would make the findings easier to assess. The registry identifies a primary and a secondary outcome, but does not report several details needed to evaluate how the analyses were planned and executed. The lesson is methodological: interpretation depends not only on the numeric result, but also on whether its uncertainty and analytical context are clearly reported.

19. 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.

Several limitations are specific to what this registry record reports. Although the trial is labeled randomized, single-masked, and crossover, the record does not identify who was masked or provide the treatment sequence, period structure, washout details, or approach to possible period and carryover effects. These details are important for assessing a crossover comparison and cannot be inferred from the design label.

The primary outcome summaries give means and standard errors, but not participant-level or paired data. The sponsor’s ANCOVA is reported with a p-value only; its adjusted estimate, confidence interval, covariates, and model specification are not reported. As a result, the sponsor analysis cannot be independently evaluated in terms of effect size and precision from the record.

Enrollment is listed as 20, while the outcome summaries report n = 19 for each group. The record does not explain this relationship or report missing-data handling, reasons for unavailable measurements, or the analysis population. The original sample-size justification is also not reported. The planning illustration is based on the observed effect and is not evidence about the trial’s original design assumptions.

The record names one secondary outcome, glucose, but does not report a multiplicity strategy or detailed analysis plan. The timing and procedures for outcome measurement are not fully described. These reporting limitations constrain interpretation of the posted results; they do not establish that a particular procedure was absent from the trial.

20. Related Tutorials

21. Related Statistical Calculators

22. Sources

• ClinicalTrials.gov: Short-term Health Effects of Wild Blueberry Juice Consumption — NCT02139878.
• Registry record last updated 2017-03-28; status last verified 2016-12.
• Results first posted 2017-02-17.
• 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.

23. Record Summary

The registry record describes a completed randomized, single-masked crossover trial sponsored by the University of Prince Edward Island. It lists pre-diabetes as the condition, enrollment of 20, and dates of 2014-05 for start and 2015-06 for primary completion. The interventions were wild blueberry juice and placebo wild blueberry juice, each described as 240 ml.

The primary endpoint was mean systolic blood pressure in mm Hg over up to 4 weeks. The sponsor-reported analysis was ANCOVA with p = 0.088. The simple unadjusted re-calculation gives a mean difference of -4.8 mm Hg, with a 95% confidence interval from -10.9 to 1.298 and p = 0.123. Glucose was listed as a secondary outcome. The registry records zero serious and zero other adverse events in each group; deaths are not reported in the registry record.