An RCT on Preventing Pressure Ulcers With Wheelchair Seat Cushions: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for An RCT on Preventing Pressure Ulcers With Wheelchair Seat Cushions (NCT00178126), a randomized na study. The primary outcome is Sitting-induced Pressure Ulcers.

2. Clinical Question

3. Trial Design

The record describes a randomized, parallel-group trial with quadruple masking. It lists two device interventions: a skin-protection wheelchair seat cushion and a segmented foam wheelchair seat cushion. The arm descriptions include a seating assessment and wheelchair in both groups, with the cushions distinguishing the assigned seating approaches. The segmented foam group is described as receiving the standard of care in nursing homes; the other group received a cushion meeting the CMS code for a skin-protection wheelchair cushion.

The registry reports enrollment of 232 and marks the study completed. The listed start is 2004-03 and the primary completion is 2008-10. The registry summary calls the proposed study multi-site, but the site count and site-level details are not reported in the registry record. The record labels the phase as NA.

The primary outcome is sitting-induced pressure ulcers, measured as the number of participants with an event, over a 6-month timeframe. The posted group results are 8 participants in the segmented foam group and 1 participant in the skin-protection group. Arm sizes are reported separately as 119 and 113. These are the posted outcome counts and group sizes; the record does not give further detail about follow-up completion or the handling of participants whose outcome status was unavailable.

Randomization and masking describe important features of the design, but the record does not report the randomization procedure, allocation concealment details, or who specifically was masked. It also does not report the trial’s original sample-size assumptions or a full statistical analysis plan. The reported outcome and comparison therefore support a focused reading of the posted primary result, while leaving several design and analysis details unresolved.

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.

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 Sitting-induced Pressure Ulcers, measured over 6 months.

The primary endpoint is sitting-induced pressure ulcers, reported as participants with an event during the 6-month timeframe. The group results count 8 participants in the segmented foam group and 1 participant in the skin-protection group. This endpoint focuses on whether a participant experienced the specified event, rather than how many ulcers an individual developed, how severe they were, how long they lasted, or whether they healed.

A participant-level event outcome is useful for expressing how often an event occurred in each trial group. Comparing the groups can describe the observed difference in event risk, while the confidence interval communicates uncertainty around that estimated difference. A binary outcome also makes the unit of interpretation relatively direct: participants either are or are not counted as having the event under the study’s outcome definition.

The registry does not provide a more detailed operational definition of a sitting-induced ulcer, the assessment schedule, adjudication procedures, or the rules used to classify an event. Those details are not reported in the registry record. Consequently, the posted result can be interpreted as the recorded participant counts for the named endpoint, but the record does not allow a detailed assessment of how the endpoint was measured or verified.

The trial summary also mentions incidence and prevalence as aims, but the posted primary outcome is a count of participants with an event. The result should therefore be read as an event-count comparison over the stated timeframe, not as a reported analysis of prevalence, ulcer severity, costs, or quality of life.

7. Results Posted in the Registry

• Skin Protection Cushion: 0.9% (1/113) versus 6.7% (8/119) on control; risk difference -5.8 percentage points (95% CI -10.7 to -1.0), risk ratio 0.13 (0.02–1.04); unadjusted two-proportion z-test p = 0.021.

8. Statistical Interpretation

9. Statistical Methods Explained

The record presents a binary outcome: participants are counted according to whether they had a sitting-induced pressure ulcer during the stated timeframe. For a binary outcome, the group event count is interpreted in relation to the number of participants in that group. The posted counts are 8 of 119 for segmented foam and 1 of 113 for skin protection. The recalculated comparison reports a risk difference, which is the contrast between the groups’ event proportions on an absolute scale. A negative value for skin protection relative to segmented foam indicates a lower observed proportion in that group.

What does the risk difference convey?

The risk difference describes the absolute separation between the observed group risks. It is often easier to translate into an event-frequency contrast than a ratio because it is expressed on the original risk scale. The estimate is -0.058, and its confidence interval runs from -0.107 to -0.010. A value of zero would represent no absolute difference between groups; the interval does not include zero. This interval is a recalculated, unadjusted estimate based on the posted summary statistics, not a confidence interval reported for the sponsor’s Fisher Exact result.

What do relative measures add?

The record also gives a relative-risk estimate and an odds-ratio estimate. A relative risk compares the event risks as a ratio, while an odds ratio compares event odds as a ratio. Ratios describe relative rather than absolute separation, so they can give a different perspective from a risk difference. The relative-risk estimate is 0.132, with limits 0.017 and 1.036; the odds-ratio estimate is 0.124, with limits 0.015 and 1.007. The intervals reaching to values around the no-difference value illustrate uncertainty in these relative estimates. A ratio should not be substituted for the absolute contrast when the question concerns the absolute difference in event frequency.

Why report a p-value?

A p-value measures how compatible the observed data are with a specified null model of no group difference, under the assumptions of the test. It is not the probability that the null hypothesis is true, and it does not measure the size or clinical importance of an effect. The registry reports a Fisher Exact analysis with p = 0.04. Fisher’s exact test is a method for comparing categorical event counts and is particularly applicable to small cell counts. The separate unadjusted recalculation gives p = 0.021. The record does not provide further details explaining the difference between these results.

General statistical explanation: when event counts are sparse, exact methods can be useful because they assess the observed count pattern without relying on a large-sample approximation in the same way as some other tests. The test result still depends on its assumptions and does not address missing data, outcome measurement, or the generalizability of the study. The registry does not report the full rationale for choosing Fisher Exact.

What does randomization contribute?

Random assignment is intended to make the groups comparable, on average, with respect to factors known and unknown at enrollment. That design feature helps support a causal comparison when the trial is conducted and analyzed appropriately. The record identifies the trial as randomized and parallel-group, but does not report the sequence-generation method, allocation concealment, or the exact analysis population. The presence of randomization alone does not show whether every potential bias was prevented.

Masking is listed as quadruple. Masking can reduce the influence of expectations on care, outcome assessment, or reporting, depending on who is masked and how masking is maintained. The registry does not specify which parties were masked, so the designation cannot be unpacked further from this record. The outcome assessment method and whether assessors were independent are also not reported.

How should uncertainty be read?

A confidence interval gives a range of effect estimates compatible with the data and the method used, subject to the method’s assumptions. It should be considered alongside the point estimate. A relatively wide interval indicates less precision than a narrower interval, and intervals may include effects with different practical implications. The confidence level for the recalculated intervals is not reported in the registry record, so it should not be inferred from the posted values. A p-value and interval provide different but related information: the interval gives a range on an effect scale, while the p-value summarizes compatibility with a null model.

General statistical explanation: over-dispersion is a concern when observed variability exceeds what a model assumes. It is particularly relevant to count outcomes modeled with a distribution that expects a particular relationship between its mean and variance. This endpoint is posted as participants with an event, and no over-dispersion parameter or count-model analysis is reported. The binary comparison should not be treated as if an unreported over-dispersion adjustment had been made.

The registry lists one primary outcome and one analysis, with no secondary outcome results listed. It does not report an adjustment for multiple comparisons, a missing-data approach, interim analyses, or the original statistical plan. Those omissions limit how completely the analysis process can be evaluated. They do not change the arithmetic of the posted counts, but they matter when judging how fully the registered analysis can be reconstructed.

10. Confidence Intervals and Precision

The recalculated risk difference for skin protection relative to segmented foam is -0.058, with a confidence interval from -0.107 to -0.010. The point estimate describes the absolute contrast in observed event proportions. The interval indicates uncertainty around that estimate; it does not mean that every value in the interval is equally likely, nor does it guarantee that the true effect lies within it.

The interval does not include zero, the value corresponding to no absolute difference. Its position is consistent with a lower observed event proportion in the skin-protection group in these posted data. Precision should still be considered in light of the small number of events: the group counts are 8 and 1. The point estimate alone can give a more definite impression than the uncertainty warrants, especially when event counts are limited.

The relative-risk interval extends from 0.017 to 1.036, and the odds-ratio interval extends from 0.015 to 1.007. Each range reaches across the ratio value corresponding to no relative difference. Relative and absolute intervals address different effect scales; their different appearance is not a contradiction. The registry does not report the confidence level for the recalculated intervals, so that detail is not reported in the registry record.

These intervals are unadjusted recalculations from the posted summary statistics. The registry’s Fisher Exact analysis reports p = 0.04 but no corresponding estimate or confidence interval. Thus, the precision statements above describe the recalculated measures and should not be presented as interval estimates from the sponsor’s own analysis.

11. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The planning figures are participants per arm, not total events. They are illustrations based on the observed effect and are not the trial’s original design calculations. The registry record does not report the trial’s own sample-size justification or original design assumptions.

The listed assumptions for these illustrations are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The effect to detect is set at the stated fraction of the observed effect. The rows describe planning for 100% of the observed effect, 75% of the observed effect, and 50% of the observed effect. The corresponding figures are 168, 354, and 918 participants per arm for 80% power, and 224, 473, and 1,228 participants per arm for 90% power.

In plain terms, the 80% power column gives the planned number of participants per arm under those assumptions for a design with an 80% chance of detecting the specified effect if that effect is the true one. The 90% power column applies the same idea at a higher target chance of detection, which generally requires more participants. The sample-size illustrations are not observed power.

The smaller the effect a study aims to detect, the more participants are generally needed under otherwise similar assumptions. That pattern is reflected by the larger planning figures for smaller fractions of the observed effect. These estimates rely on the planning assumptions above; they do not account for assumptions that are not listed, such as a different allocation, missing outcomes, or a different analysis method.

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 = 0.067; treatment proportion p₁ = 0.009; average p̄ = 0.038.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.2705 = 0.5302.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.2674 = 0.2250.
4. n = (0.5302 + 0.2250)² / (-0.0584)² = 167.4, rounded up to 168 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 <- 0.0672268907563025                          # control proportion
p1 <- 0.00884955752212389                          # 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(178126)
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 = 168 (80% power), 224 (90% power)
 75% of observed effect: n per arm = 354 (80% power), 473 (90% power)
 50% of observed effect: n per arm = 918 (80% power), 1,228 (90% power)
power.prop.test: 167.4 
Simulated power at n = 168 per arm: 0.852 (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 registry’s reported analysis compares the two groups using Fisher Exact and gives p = 0.04. It does not provide an effect estimate or confidence interval for that analysis. Separately, the unadjusted recalculation from the posted group counts gives a risk difference of -0.058, with a confidence interval from -0.107 to -0.010, and p = 0.021.

These results should be kept distinct. The reported Fisher Exact result is the sponsor’s stated analysis; the recalculated values are simple, unadjusted estimates based on the posted summary statistics. The registry does not explain the difference between the p-values or provide enough detail to determine whether they use the same test specifications. The record does not mark a discrepancy, but that does not make the two values interchangeable.

The recalculated interval provides an effect-scale description not included with the reported Fisher Exact result. It should not be attributed to the sponsor’s analysis. Similarly, the recalculated p-value should not replace the result reported by the registry. Reading both makes clear what the registry reports directly and what can be estimated from its group summaries, without assuming that unreported analytic details match.

14. Multiplicity, Missing Data and Analysis Populations

The registry lists one primary outcome measure and one analysis. It lists no secondary outcome results. Although a single listed primary outcome reduces the apparent scope for multiplicity within the posted outcome results, the record does not provide a complete analysis plan or say whether other outcomes, subgroup comparisons, or repeated analyses were planned. Adjustment for multiple comparisons is not reported in the registry record.

Missing outcome data can affect a comparison if participants without observed outcomes differ from those with recorded outcomes. The registry does not report how many participants completed follow-up, whether any outcomes were missing, how missing status was handled, or whether sensitivity analyses were conducted. The posted group sizes and event counts do not, on their own, establish that all randomized participants had outcome data.

The analysis population is also not reported in the registry record. It is not stated whether the primary result follows an intention-to-treat approach, includes only participants with observed outcomes, or uses another population definition. The randomized design is recorded, but the analysis approach is a separate detail and should not be assumed from randomization alone.

Interim analyses, stopping rules, and the trial’s complete statistical plan are not reported. The registry gives a Fisher Exact result, but does not describe other analytic decisions or explain how the test was selected. These missing details limit the ability to evaluate selective reporting, departures from a prespecified plan, and the handling of incomplete data. They do not alter the reported counts, but they constrain how completely the result can be reconstructed and assessed.

15. 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 posted serious-adverse-event figures report 0 affected participants among 119 at risk in the segmented foam group and 0 among 113 at risk in the skin-protection group. The other-adverse-event figures also report 0 affected participants in each group, with the same at-risk counts. These are the figures recorded for the adverse-event categories; they should not be read as a general statement about every possible harm or symptom.

The registry does not list serious adverse-event terms. Death counts are not reported in the registry record. The record also does not provide further detail about adverse-event definitions, collection procedures, severity grading, relatedness assessments, or the observation period for safety reporting. The adverse-event figures therefore describe what was posted, not a comprehensive safety evaluation.

Zero affected participants in a posted category does not establish that an event is impossible or that no unrecorded event occurred. It means that the record reports no affected participants for those categories in the stated group summaries. The sample sizes, duration, ascertainment process, and completeness of reporting all matter when interpreting the absence of recorded events.

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

The posted results show fewer participants recorded with sitting-induced pressure ulcers in the skin-protection group than in the segmented foam group over the 6-month timeframe: 1 compared with 8. The unadjusted risk difference is -0.058, with a confidence interval from -0.107 to -0.010. This is an observed group comparison, not a guarantee that the same pattern would occur in another population or setting.

The registry’s own Fisher Exact analysis reports p = 0.04. The recalculated unadjusted analysis reports p = 0.021. The record does not explain the difference between these values, and the recalculated confidence interval is not part of the reported Fisher Exact result. The result should therefore be understood with a clear distinction between the registry’s analysis and the separate summary-statistic recalculation.

The finding does not establish the size of an effect with certainty. The event counts are limited, and the relative-risk and odds-ratio intervals extend to values around or beyond the ratio value for no difference. Uncertainty is relevant even when a point estimate or one interval suggests a difference. The record does not report the confidence level for the recalculated intervals, nor details about missing outcomes, analysis populations, or endpoint assessment.

The result also does not answer every question raised in the trial summary. It does not report ulcer severity, number of ulcers per participant, healing, healthcare costs, or quality of life. Nor does the adverse-event summary provide a complete assessment of safety. The appropriate interpretation is limited to the posted primary event comparison and the information the registry reports about its design and analysis.

17. Why This Trial Matters Statistically

This trial provides a useful statistical example of how to read a binary clinical outcome when event counts are small. The primary result is a participant-level count, and the comparison can be expressed on both an absolute scale, through a risk difference, and a relative scale, through relative risk or odds ratio. These measures describe different aspects of the same group comparison and should not be treated as interchangeable.

The record also illustrates why point estimates, uncertainty intervals, and p-values should be read together. The recalculated absolute difference has an interval that does not include zero, while the relative-measure intervals extend to values at or beyond the no-difference ratio. The different scales provide different views of uncertainty. The registry’s Fisher Exact p-value and the unadjusted recalculated p-value are both reported, but their difference is not explained. A reader should distinguish the sponsor’s analysis from a recalculation rather than silently substituting one for the other.

Design information matters to interpretation as well. The record identifies randomization, parallel allocation, and quadruple masking, but does not report several implementation and analysis details needed for a full assessment. In particular, the analysis population, missing-data handling, endpoint ascertainment procedures, and original sample-size assumptions are not reported in the registry record.

Finally, the sample-size illustrations demonstrate how planning changes with the effect a study aims to detect: smaller effects require larger participant counts under the stated assumptions. Those calculations are planning scenarios based on the observed effect, not a retrospective measure of the study’s success or failure. The trial is therefore informative both as a posted clinical comparison and as an example of why statistical interpretation depends on scale, precision, and transparent reporting.

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 registry record leaves important clinical and analytic details unspecified. Although the summary describes an intended elderly nursing-home population, the primary outcome’s population field is blank, and detailed eligibility criteria are not included in the posted result. The outcome is named sitting-induced pressure ulcers, but an operational definition, assessment schedule, adjudication process, and severity classification are not reported. These omissions limit interpretation of what qualified as an event and how consistently it was assessed.

The record does not describe the randomization sequence, concealment procedures, or which people were masked under the quadruple-masking designation. It also does not report site count or site-level information, despite the summary describing the proposed trial as multi-site. These details would help readers evaluate trial implementation and the potential for differences in care or outcome assessment.

Several analysis details are absent: the analysis population, missing-outcome handling, interim analyses, multiplicity approach, and complete statistical plan are not reported in the registry record. The sponsor’s Fisher Exact result has no accompanying effect estimate or confidence interval. The confidence level for the recalculated intervals is also not reported. The record does not explain the difference between the sponsor’s p-value and the recalculated p-value.

The safety summary reports zero affected participants in the listed categories but gives no event terms and no death counts. Adverse-event definitions and ascertainment procedures are not reported. These limits mean that the safety figures should be interpreted narrowly as the posted counts, rather than as a complete account of possible harms.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: An RCT on Preventing Pressure Ulcers With Wheelchair Seat Cushions — NCT00178126.
• Registry record last updated 2017-06-02; status last verified 2017-04.
• Results first posted 2017-06-02.
• 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

NCT00178126 is a completed randomized, parallel-group trial with quadruple masking. The registry reports enrollment of 232 and compares a skin-protection wheelchair seat cushion with a segmented foam wheelchair seat cushion. The listed timeframe for the primary outcome is 6 months.

The primary endpoint is sitting-induced pressure ulcers, reported as participants with an event. The posted counts are 8 in the segmented foam group and 1 in the skin-protection group. The registry reports a Fisher Exact result of p = 0.04. An unadjusted recalculation gives a risk difference of -0.058, with a confidence interval from -0.107 to -0.010, and p = 0.021.

The record lists no secondary outcome results. It reports zero affected participants in the serious and other adverse-event categories for both groups; deaths are not reported in the registry record. Missing-data handling, analysis population, and the full statistical plan are not reported.