An RCT on Support Surfaces for Pressure Ulcer Prevention: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for An RCT on Support Surfaces for Pressure Ulcer Prevention (NCT03351049), a randomized na study. The primary outcome is Pressure Injury.

2. Clinical Question

3. Trial Design

The registry record describes a randomized, double-masked, parallel-group trial, with an enrollment of 457 and a completed status. It lists the study start as 2018-09-04 and primary completion as 2022-07-12. The phase is recorded as NA. The intervention was a device in each group: a reactive support surface with low air loss or a reactive support surface without low air loss.

The primary outcome is the number of participants who had a new pressure injury during study participation. Its timeframe is stated as through study completion, an average of 7 days. The posted outcome groups list 323 participants and 23 events for the low air loss group, and 128 participants and 10 events for the non-low air loss group. Those group figures describe the outcome record; the registry enrollment is reported separately.

Random allocation supports a comparison between assigned groups, while a parallel design means participants were assigned to one group rather than being described as switching between surfaces. The registry record identifies the study as double-masked but does not report who was masked or how masking was implemented. Details of the randomization procedure, allocation concealment, adherence, or deviations from assigned surface use are not reported in the registry record.

The record reports one outcome measure and one analysis. It does not report the trial's original sample-size justification or its prespecified design assumptions. No additional trial procedures should be inferred from the posted design labels. The study summary frames the comparison around people with moisture risk factors in acute care, so the result addresses that stated setting and comparison rather than support surfaces in general.

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: Of 457 enrolled participants, 451 met inclusion criteria and were randomized.

Overall Study

Reasons for not completing this period:

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

6. Primary Endpoint

The registry lists the primary outcome as Pressure Injury, measured over Through study completion, an average of 7 days.

The primary endpoint counts participants who had a new pressure injury during study participation. It is a binary outcome at the participant level: each participant either had a recorded new injury or did not. The registry lists the unit as participants and gives the timeframe as through study completion, an average of 7 days.

This kind of endpoint is used because the study question concerns prevention of new injuries. Counting participants with an event makes the outcome directly interpretable as whether a new injury occurred during the stated observation period. In general, a binary endpoint does not describe how many injuries an affected participant experienced, when an injury occurred, or its severity, unless those details are recorded separately. Those details are not part of the posted primary outcome description.

The posted counts are 23 participants with an event among 323 in the low air loss group, and 10 among 128 in the non-low air loss group. These counts should be read together with their group sizes and the reported statistical estimates. The timeframe is relatively short as described in the registry record; no longer-term primary endpoint is reported. The registry record also does not report additional outcome definitions such as criteria for diagnosing an injury or procedures for adjudicating an event.

7. Results Posted in the Registry

• Reactive, Non-low Air Loss: 7.8% (10/128) versus 7.1% (23/323) on control; risk difference 0.7 percentage points (95% CI -4.7 to 6.1), risk ratio 1.10 (0.54–2.24); unadjusted two-proportion z-test p = 0.799.

8. Statistical Interpretation

9. Statistical Methods Explained

What does the risk difference describe?

The risk difference expresses the difference between the event proportions in the two groups. In the re-calculation, the comparison is non-low air loss relative to low air loss. Its estimate is 0.007. A positive estimate means the observed event proportion was higher in the non-low air loss group, while a negative estimate would mean it was lower. The confidence interval extends from -0.047 to 0.061, so the estimate is imprecise enough to include differences in either direction. General: an absolute difference is often easier to interpret in terms of changes in event frequency than a relative measure, but its meaning depends on the outcome and population.

How do relative risk and odds ratio differ?

The re-calculation also reports a relative risk of 1.097 and an odds ratio of 1.105, each comparing non-low air loss with low air loss. A relative risk compares the event proportions directly. An odds ratio compares event odds. General: when events are uncommon, these relative measures may be numerically similar, but they are not interchangeable and can differ, especially as event frequency changes. Their confidence intervals are 0.537 to 2.24 for the relative risk and 0.511 to 2.393 for the odds ratio. Both intervals include 1, the value associated with no relative difference.

What does the chi-squared test assess?

The registry identifies the sponsor's analysis as chi-squared and reports p = 0.8 for the comparison. General: a chi-squared test evaluates whether the observed distribution of events and non-events across groups is compatible with equal group proportions under the test's assumptions. The reported p-value does not quantify the size of a difference, nor does it establish equivalence. It is interpreted alongside the outcome counts, effect estimates, and uncertainty intervals.

What does a p-value mean?

The unadjusted re-calculation reports p = 0.799. General: a p-value describes how unusual data at least as inconsistent with a specified null model would be if that model and the test assumptions held. It is not the probability that the null hypothesis is true, nor the probability that a result occurred by chance. A large p-value can arise when the observed estimate is near the null value, when the data are imprecise, or both. Here, the confidence intervals make the uncertainty in the effect size visible.

Why consider the outcome counts and group sizes together?

The counts are 23 events among 323 participants in the low air loss group and 10 among 128 in the non-low air loss group. General: event counts alone do not give a fair comparison when group sizes differ. Statistical comparisons use both the number of events and the number of participants contributing to each group. In this record, the outcome is binary and the group sizes differ, so estimates based on the underlying group proportions are more informative than comparing the raw event counts by themselves.

What is the role of the confidence interval?

General: a confidence interval is a range of values generated by a method that, over repeated studies under its assumptions, would cover the true effect at its stated confidence level at the specified frequency. It communicates uncertainty around an estimate, rather than a list of effects proven to be plausible or a guarantee that the true effect lies inside the particular interval. The risk-difference interval includes zero, while the relative-measure intervals include 1; those values represent no difference on their respective scales. The intervals are broad enough that the record does not pin down a narrow effect range.

The registry record does not report a detailed statistical analysis plan, model specifications beyond the named chi-squared test, adjustment variables, or a missing-data strategy. The re-calculated comparisons are explicitly unadjusted. No adjustment should be assumed, and the reported estimates should not be described as adjusted for participant characteristics or other covariates.

10. Confidence Intervals and Precision

The confidence intervals describe how precisely the comparison is estimated. For the risk difference, the re-calculated estimate is 0.007, with a 95% confidence interval from -0.047 to 0.061. Because zero lies within that interval, the estimate is compatible with no absolute difference as well as with differences in either direction. The interval is not narrow, so it does not identify a single precise value for the group contrast.

The relative-risk estimate is 1.097, with a confidence interval from 0.537 to 2.24. The odds-ratio estimate is 1.105, with a confidence interval from 0.511 to 2.393. Both intervals include 1, the no-relative-difference value. Their breadth indicates considerable uncertainty about the relative size and direction of the association. The estimates should not be read as demonstrating equivalence simply because their point estimates are close to 1.

General: a confidence interval reflects sampling uncertainty under the method's assumptions; it does not account automatically for every source of uncertainty, such as measurement problems or missing outcomes. The registry record does not report details needed to assess those other sources. The group counts and short stated observation timeframe also define the scope of the estimate: it concerns the recorded new-injury outcome during study participation, an average of 7 days. More precise conclusions about longer follow-up or other outcomes are not supported by this endpoint.

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 sample-size display gives planning illustrations based on the observed effect, rather than reporting the trial's original sample-size plan. These figures are participants per arm for 80% power and participants per arm for 90% power; they are not total event counts. The stated assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The effect to detect is the stated fraction of the observed effect.

At 100% of the observed effect, the planning illustration lists 22,664 participants per arm for 80% power and 30,340 participants per arm for 90% power. At 75% of the observed effect, it lists 39,863 participants per arm for 80% power and 53,365 participants per arm for 90% power. At 50% of the observed effect, it lists 88,724 participants per arm for 80% power and 118,776 participants per arm for 90% power. The pattern illustrates that smaller effects require more participants under the stated assumptions.

In plain terms, the 80% column is the planned group size associated with an 80% chance of detecting the specified effect under the assumptions, if the effect and model assumptions hold. The 90% column describes the larger planning target associated with a 90% chance under those assumptions. These are planning illustrations, not observed power. They are sensitive to the chosen effect size and assumptions and should not be treated as evidence that the trial was designed to detect those effects. The trial's own original design assumptions are not reported in the registry record.

12. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, p₀ and p₁ are the control and treatment proportions, and p̄ is their average.

Worked calculation for this trial

1. Control proportion p₀ = 0.071; treatment proportion p₁ = 0.078; average p̄ = 0.075.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.3717 = 0.7286.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.3717 = 0.3128.
4. n = (0.7286 + 0.3128)² / (0.0069)² = 22,663.7, rounded up to 22,664 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
p0 <- 0.0712074303405573                          # control proportion
p1 <- 0.078125                          # treatment proportion
n_per_arm <- function(power, frac = 1) {
  q1 <- p0 + (p1 - p0) * frac               # smaller effect = fraction of the observed difference
  pbar <- (p0 + q1) / 2
  z_b <- qnorm(power)
  ceiling(round((z_a * sqrt(2 * pbar * (1 - pbar)) + z_b * sqrt(p0 * (1 - p0) + q1 * (1 - q1)))^2 / (q1 - p0)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (unrounded n per arm)
cat("power.prop.test:", round(power.prop.test(p1 = p0, p2 = p1, power = 0.80)$n, 1), "\n")

# Simulation: how often does the z-test reject at the 80%-power sample size?
set.seed(3351049)
n    <- n_per_arm(0.80)
reps <- 20000
x0 <- rbinom(reps, n, p0); x1 <- rbinom(reps, n, p1)
ph <- (x0 + x1) / (2 * n)
z  <- ((x1 - x0) / n) / sqrt(ph * (1 - ph) * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(z) > z_a, na.rm = TRUE), reps))

Output from running the code

100% of observed effect: n per arm = 22,664 (80% power), 30,340 (90% power)
 75% of observed effect: n per arm = 39,863 (80% power), 53,365 (90% power)
 50% of observed effect: n per arm = 88,724 (80% power), 118,776 (90% power)
power.prop.test: 22663.7 
Simulated power at n = 22,664 per arm: 0.797 (20000 replicates)

The simulation draws many pairs of event counts at the planned sample size and counts how often the two-proportion z-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

13. Analyses Reported by the Sponsor

The sponsor-reported analysis is a chi-squared test across the two groups, with p = 0.8. The unadjusted re-calculation reports p = 0.799 and also gives effect estimates and confidence intervals. The p-values are close in their displayed form, but the analyses are reported separately: the registry lists the sponsor's method and result, while the re-calculation supplies an unadjusted comparison from the posted counts.

The sponsor result does not include a reported effect estimate or confidence interval in the registry analysis entry. The re-calculation adds those measures to describe the magnitude and precision of the contrast, but it does not replace or revise the sponsor's reported analysis. The registry record does not report enough detail to establish that every analytic choice was identical between the sponsor analysis and the re-calculation. Both results should be read in the context of the posted counts and the outcome definition.

14. Multiplicity, Missing Data and Analysis Populations

The registry record reports one outcome measure and one analysis. It lists no posted secondary outcomes. This limits the multiplicity question that can be assessed from the record, but it does not establish what outcomes may have been considered or measured outside the posted results. A prespecified multiplicity strategy is not reported in the registry record.

Multiplicity refers generally to the increased chance of obtaining at least one apparently notable result when many outcomes, comparisons, subgroups, or analyses are tested. The number of posted outcomes is limited, but the registry record does not report whether other analyses were planned, whether subgroup comparisons were conducted, or whether any adjustment for multiple testing was specified. No claims about those procedures are warranted.

The registry record does not report how missing outcome data were handled, how many participants had missing primary outcome information, or whether all randomized participants were included in the analysis. It also does not state whether the analysis followed an intention-to-treat population, a per-protocol population, or another analysis population. Those omissions matter because incomplete outcome ascertainment or exclusions can affect estimated group comparisons.

Although the trial is identified as randomized and double-masked, the record does not describe allocation concealment, the masking procedures, or who remained masked. It does not report an interim-analysis plan or stopping rules. The registry record also does not provide a detailed statistical analysis plan, a sample-size justification, or methods for addressing protocol deviations. These items should be treated as not reported in the registry record, rather than inferred from common trial practice.

15. 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 1% reporting threshold.

The serious adverse event entries list 0 affected participants in the reactive low air loss group and 0 in the reactive non-low air loss group. The entries list 323 participants at risk in the low air loss group and 128 at risk in the non-low air loss group. The registry also reports 0 other adverse events in each group and 0 deaths in each group.

These are the figures in the posted adverse-event record. A zero in the affected-participant field means that no affected participants are recorded for that category in the registry entry; it does not by itself establish that no adverse events of any kind occurred, or that the groups have identical safety profiles. The record does not report additional detail about event definitions, ascertainment procedures, severity classification beyond the serious-event category, or follow-up for adverse events. The results should therefore be interpreted as the registry's reported counts, not as a comprehensive estimate of safety.

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

The posted results describe a comparison of reactive support surfaces with and without low air loss among participants in a randomized, double-masked, parallel-group trial. The primary outcome was a new pressure injury during participation, with a timeframe through study completion averaging 7 days. The registry records 23 events among 323 participants in the low air loss group and 10 among 128 in the non-low air loss group.

The re-calculated risk difference is 0.007, and its confidence interval ranges from -0.047 to 0.061. The relative-risk and odds-ratio estimates are 1.097 and 1.105, respectively, with intervals that include 1. The sponsor-reported chi-squared result is p = 0.8, and the unadjusted re-calculated p-value is 0.799. Together, these statistics describe an uncertain comparison: the point estimates do not settle the direction or size of any difference, and the intervals include values consistent with no difference as well as values on either side.

These findings do not establish that the surfaces are equivalent, nor do they establish a definite difference. A failure to find strong statistical evidence against a no-difference test is not proof of no effect. Conversely, the point estimate alone should not be presented as proof of a difference. The uncertainty intervals and the short stated average observation timeframe constrain interpretation.

The result also does not answer every question raised in the study summary. The posted primary comparison does not report a separate outcome analysis of heat or moisture control performance, or determine when low air loss may be effective. Conclusions should remain limited to the stated group comparison, recorded outcome, and timeframe. The registry does not report enough information to assess missing outcomes, analysis populations, or several design and analysis details that could affect interpretation.

17. Why This Trial Matters Statistically

This trial is statistically informative because it presents a randomized comparison in which the contrast is clearly described: reactive support surfaces with low air loss versus reactive support surfaces without low air loss. That makes the estimand—the group contrast being assessed—more focused than a comparison between broadly different categories of care. The primary outcome is also stated at the participant level as a new pressure injury during participation.

The results illustrate why event counts, effect estimates, and uncertainty should be considered together. The raw counts differ, but the group sizes also differ. The risk difference and relative measures standardize the comparison to the group sizes, while their confidence intervals show how uncertain the estimated contrast remains. A non-significant test result alone cannot establish equivalence or rule out potentially meaningful effects.

The sample-size illustrations also make a general design issue visible: planning requirements rise as the effect size to be detected becomes smaller. Those estimates rely on stated planning assumptions and the observed effect; they are not the trial's original design rationale or a measure of what the trial achieved. In particular, they should not be confused with observed power.

Finally, the registry record shows how the scope of a result depends on outcome definition and follow-up. This comparison addresses new pressure injuries over an average of 7 days, rather than all possible outcomes or longer-term effects. Statistical interpretation therefore requires both attention to uncertainty and restraint about how far to generalize beyond the recorded question.

18. 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 outcome concerns new pressure injuries during participation, with an average timeframe of 7 days. The registry does not report longer-term primary outcome results. The conclusions are therefore limited to the stated observation period and cannot describe outcomes beyond it.

The registry record does not report the diagnostic criteria used to identify a new pressure injury, event adjudication procedures, or details of outcome ascertainment. It also does not report missing outcome counts or how missing data were handled. These omissions make it difficult to assess whether all participants had outcome information or whether ascertainment could have differed between groups.

The record reports an enrollment of 457, while the outcome groups list 323 and 128 participants. It does not explain the relationship between the enrollment figure and the participants included in the posted outcome entries. The analysis population, exclusions, and participant flow are not reported in the registry record, so the group entries should not be interpreted as a complete account of trial disposition.

Although the study is identified as double-masked, who was masked and how masking was maintained are not reported. Details of allocation concealment, adherence to assigned surfaces, protocol deviations, and co-interventions are also not reported. The registry does not provide a detailed statistical analysis plan, original sample-size justification, or information about interim analyses. No specific procedure for these items can be inferred from the study design labels.

Finally, the posted primary results address the randomized low air loss comparison, not all aspects of the broader study aim concerning heat and moisture control performance. No posted secondary outcome results are listed. These record-level limitations constrain assessment of generalizability, robustness, and the full range of outcomes considered.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: An RCT on Support Surfaces for Pressure Ulcer Prevention — NCT03351049.
• Registry record last updated 2023-08-25; status last verified 2023-08.
• Results first posted 2023-08-25.
• 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.

22. Record Summary

The registry record describes a completed randomized, double-masked, parallel-group trial of reactive support surfaces with and without low air loss. Its primary outcome is the number of participants with a new pressure injury during study participation, an average of 7 days. The posted counts are 23 among 323 participants in the low air loss group and 10 among 128 in the non-low air loss group.

The sponsor reports a chi-squared analysis with p = 0.8. The unadjusted re-calculation reports a risk difference of 0.007, with a 95% confidence interval from -0.047 to 0.061, and a p-value of 0.799. The confidence intervals for the relative-risk and odds-ratio estimates also include their no-difference values. The registry does not report a detailed analysis plan, missing-data handling, or original sample-size assumptions, so the findings are best read as an uncertain comparison of the posted outcome counts within the stated timeframe.