Therapeutic Stockings to Prevent Foot Ulcers: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Therapeutic Stockings to Prevent Foot Ulcers (NCT01221194), a randomized na study. The primary outcome is Number of Participants Who Developed Foot Ulcer During Study Time Frame.

2. Clinical Question

The registry also describes an objective to evaluate perceived health-related quality of life compared with guideline-directed usual care. However, no secondary outcomes are posted in the registry record, so that objective cannot be assessed from the posted results. The primary question is limited to whether participants developed a foot ulcer during the stated timeframe; it does not, by itself, describe ulcer timing, severity, recurrence, or other aspects of foot health. Those details are not reported in the registry record.

3. Trial Design

This was a randomized, parallel-group trial. The registry lists single masking, but does not report who was masked or how masking was implemented. The two groups were PFC Stockings and Standard Therapy. The PFC Stockings group was assigned shear-reducing stockings to wear. The Standard Therapy group received education, regular foot care, protective shoes and insoles, and continued using the stockings they normally wore.

The registry lists 381 participants as the enrollment and gives the study status as completed. The record lists October 2010 as the study start and February 2016 as primary completion. The condition is diabetes, and the sponsor is University of Texas Southwestern Medical Center. The phase is recorded as not applicable.

The posted primary outcome is the number of participants who developed a foot ulcer during a 30-month timeframe. The record defines ulceration as full thickness loss of epidermis and dermis or involvement of deeper structures. It reports 76 participants with an ulcer in the PFC Stockings group, with a group size of 195, and 82 in the Standard Therapy group, with a group size of 186.

The registry record does not report the trial’s original sample-size assumptions, detailed eligibility criteria, recruitment procedures, allocation-concealment methods, or the identity of those masked. It also does not report a statistical analysis plan or posted analysis results. The comparison estimates discussed in the statistical interpretation are unadjusted recalculations from the posted group summaries, rather than a posted sponsor analysis.

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

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 Number of Participants Who Developed Foot Ulcer During Study Time Frame, measured over 30 months.

The primary endpoint counts participants who developed a foot ulcer during the study timeframe. It is a binary outcome at the participant level: a participant either met the registry’s ulcer definition during the stated period or did not. The definition requires full thickness loss of epidermis and dermis or involvement of deeper structures. The timeframe is 30 months.

A count of participants with an event is useful for comparing how often the specified outcome occurred in each randomized group. Unlike a measure that records only a change in symptoms or a laboratory value, this endpoint concerns whether a clinically defined foot ulcer occurred. The registry outcome does not report the number of ulcers per participant, when an ulcer first appeared, how long an ulcer lasted, or its severity beyond the stated definition. Those features therefore cannot be evaluated from this endpoint.

The posted results give the event count and group size for each arm. The unadjusted recalculation also expresses the comparison as a risk difference and provides relative measures. Such summaries describe the contrast between observed groups; they do not establish why any difference occurred. Interpretation also depends on the uncertainty around the estimates and on whether the posted outcome data capture all participants as intended. Missing-outcome handling and analysis-population definitions are not reported in the registry record.

7. Results Posted in the Registry

• PFC Stockings: 39.0% (76/195) versus 44.1% (82/186) on control; risk difference -5.1 percentage points (95% CI -15.0 to 4.8), risk ratio 0.88 (0.70–1.12); unadjusted two-proportion z-test p = 0.311.

8. Statistical Interpretation

9. Statistical Methods Explained

The main comparison is between the randomized PFC Stockings and Standard Therapy groups. Because the endpoint is whether a participant developed an ulcer, it is binary. The registry reports the number of participants with the event and the group size for each arm. The posted arm summaries are 76 of 195 for PFC Stockings and 82 of 186 for Standard Therapy. The comparison statistics are unadjusted recalculations from these summaries; no adjusted analysis is posted in the registry record.

What does the risk difference describe?

A risk difference compares the event proportions in two groups by subtracting one group’s proportion from the other. It is an absolute measure: the estimate is expressed on the probability scale rather than as a ratio. The recalculated estimate is -0.051 for PFC Stockings relative to Standard Therapy. With the direction used in this comparison, a negative estimate means the observed event proportion was lower in the PFC Stockings arm. The confidence interval from -0.15 to 0.048 includes zero, which represents no absolute difference. It also allows for possible values favoring either group.

What do the relative measures add?

The relative risk compares the event probability in one group with that in the other. A relative risk of 1 corresponds to equal probabilities. The recalculated estimate is 0.884, and its confidence interval is 0.696 to 1.123. The odds ratio compares event odds rather than event probabilities; an odds ratio of 1 corresponds to equal odds. Its estimate is 0.81, with a confidence interval of 0.538 to 1.218. These two measures summarize the contrast on different scales. Their confidence intervals both include the no-association value, so neither rules out no difference in the observed data.

How should the p-value be understood?

The unadjusted recalculated p-value is 0.311. In general, a p-value describes how unusual data at least as discrepant as those observed would be if a specified null model were true and the assumptions of the test held. It is not the probability that the null model is true, and it does not measure the size or practical importance of an effect. A p-value above a conventional threshold does not prove that groups are the same. Here, it is best read alongside the effect estimates and confidence intervals, rather than as a stand-alone verdict.

The registry lists no analysis results and does not report the statistical method used by the trial investigators. The reported p-value and effect estimates are therefore not a substitute for an unavailable sponsor analysis. The record does not report whether an adjusted analysis, prespecified subgroup analysis, interim analysis, or other procedure was planned or performed. No such details should be inferred from the unadjusted recalculation.

Why does over-dispersion matter in general?

Over-dispersion is a general concern when data vary more than a statistical model expects. It can make standard errors too small if the model does not allow for the additional variation, producing confidence intervals that are too narrow and p-values that are too optimistic. For a participant-level binary outcome, the basic comparison uses event counts and group sizes. The registry record does not report an over-dispersion assessment or a model-based analysis, so there is no basis to say that over-dispersion was present or that a correction was used.

Random allocation supports a comparison between assigned groups, but interpretation still depends on how the trial was carried out and how outcomes were collected. The registry describes single masking without saying who was masked. It also does not report allocation-concealment details, adherence, crossover, or analysis populations. The count-based recalculation cannot resolve those design and conduct questions.

The endpoint is counted over a fixed 30-month timeframe. The registry does not provide participant-level follow-up or event timing, so time-to-event methods cannot be evaluated from the posted summaries. Nor does the record state how participants with incomplete follow-up were handled. These omissions limit assessment of the assumptions behind the comparison and whether the posted group counts represent all randomized participants according to a defined analysis plan.

In general, estimates, intervals, and p-values answer related but distinct questions. The point estimate gives the direction and scale of the observed contrast; the confidence interval conveys its statistical uncertainty; and the p-value summarizes compatibility with a null model under particular assumptions. Considering them together helps avoid treating a single threshold as a complete account of the evidence.

10. Confidence Intervals and Precision

The risk-difference estimate is -0.051, and its 95% confidence interval runs from -0.15 to 0.048. This interval describes a range of effect values that are reasonably compatible with the observed data under the method used to calculate it. In repeated applications of the same procedure under its assumptions, confidence intervals have a defined long-run coverage property; a particular interval is not a probability statement that the true value lies within its endpoints.

For this estimate, the interval includes zero, the value representing no absolute difference. It extends to values below zero and above zero, so the data do not precisely pin down the direction of the underlying difference. The interval is also wide enough to include a range of possible magnitudes. The point estimate alone should not obscure that uncertainty.

The relative risk estimate is 0.884, with a 95% confidence interval from 0.696 to 1.123. The odds ratio estimate is 0.81, with a 95% confidence interval from 0.538 to 1.218. For both relative measures, 1 represents no association, and each interval includes that value. The point estimates are below 1, but the intervals allow values both below and above 1.

Confidence intervals reflect sampling uncertainty under the calculation method; they do not account automatically for every possible source of bias or missing data. The registry does not report missing-outcome handling, analysis populations, or a sponsor statistical analysis. Those omissions are relevant to how fully the precision of the recalculated estimates can be assessed. The intervals should therefore be read as precision summaries for the unadjusted comparisons, not as a complete measure of uncertainty about all aspects of the trial.

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 in participants per arm for this binary, participant-level outcome; it is not a display of total events. Its assumptions are a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation. Each planning scenario uses a stated fraction of the observed effect as the effect to detect. The original design assumptions for the trial are not reported in the registry record, so these figures do not document the trial’s own planning process.

For planning at 100% of the observed effect, the display gives 1,458 participants per arm for 80% power and 1,951 participants per arm for 90% power. At 75% of the observed effect, it gives 2,604 participants per arm for 80% power and 3,485 participants per arm for 90% power. At 50% of the observed effect, it gives 5,883 participants per arm for 80% power and 7,875 participants per arm for 90% power.

In plain terms, the 80% power column indicates the planned group size under the stated assumptions for a design with an 80% chance of detecting the specified effect if that effect is real. The 90% power column uses a higher target chance of detection under the same scenario framework. Smaller effects need more participants to detect because they are harder to distinguish from random variation. The accompanying curve expresses the same planning relationship across the effect-size scenarios. These are planning illustrations, not observed power.

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 = 0.441; treatment proportion p₁ = 0.390; average p̄ = 0.415.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.6969 = 1.3659.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.6959 = 0.5857.
4. n = (1.3659 + 0.5857)² / (-0.0511)² = 1,457.7, rounded up to 1,458 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 <- 0.440860215053763                          # control proportion
p1 <- 0.38974358974359                          # 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(1221194)
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 = 1,458 (80% power), 1,951 (90% power)
 75% of observed effect: n per arm = 2,604 (80% power), 3,485 (90% power)
 50% of observed effect: n per arm = 5,883 (80% power), 7,875 (90% power)
power.prop.test: 1457.7 
Simulated power at n = 1,458 per arm: 0.796 (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. Multiplicity, Missing Data and Analysis Populations

Multiplicity arises when several outcomes, comparisons, or analyses are tested. Conducting many tests can increase the chance of at least one apparently notable result arising by chance. The registry record posts one outcome measure and no secondary results. Its brief summary describes a quality-of-life objective, but the record lists no posted secondary outcomes. The number and nature of any additional outcomes or analyses conducted outside the posted results are not reported in the registry record.

No multiplicity adjustment or prespecified hierarchy of outcomes is reported. The unadjusted p-value of 0.311 therefore should not be described as adjusted for multiple testing. At the same time, the record does not establish that multiple testing occurred. The appropriate conclusion is that the information needed to assess multiplicity is not reported in the registry record.

Missing data are also relevant. The record does not describe whether any participants lacked a final ulcer assessment, how incomplete follow-up was handled, or whether an imputation or other missing-data method was used. The posted arm counts and group sizes can be reported as given, but the registry does not state whether they correspond to a defined intention-to-treat, per-protocol, or other analysis population. Analysis populations are not reported in the registry record.

The registry lists randomized allocation, but randomization alone does not resolve questions about missing outcomes or analysis choices. Without these details, readers cannot assess whether the posted counts include all participants as intended or whether exclusions could have affected the comparison. The record also does not post a statistical analysis plan, interim-analysis information, or sponsor analysis results. These are reporting limitations; they do not establish that any particular procedure was or was not used.

14. 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 0% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: non-foot related hospitalization (8.7%), foot related readmission (6.7%).

The registry reports serious adverse events for 30 of 195 participants in the PFC Stockings group and 16 of 186 in the Standard Therapy group. It reports other adverse events for 99 of 195 and 99 of 186, respectively. Deaths are listed as 7 in each group. These are counts and group sizes as recorded; they should not be interpreted as evidence that an event was caused by the assigned intervention.

The registry identifies non-foot related hospitalization at 8.7% and foot related readmission at 6.7% among the top serious adverse event terms. The record does not specify the denominator for these term percentages in the information reported, so they should be read as listed rather than used to infer a rate for either group. The summary does not provide event-level timing, severity beyond the serious classification, relatedness assessments, or details about whether participants experienced more than one event.

Safety counts are descriptive and do not by themselves establish a comparative safety effect. The registry record does not report a formal statistical comparison of adverse events or deaths, nor does it explain how events were collected and classified. Interpretation is therefore limited to the posted counts and listed terms.

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

The posted results describe a randomized comparison of PFC Stockings with Standard Therapy for the participant-level outcome of developing a foot ulcer during 30 months. The registry reports 76 participants with an ulcer in the PFC Stockings group and 82 in the Standard Therapy group. The unadjusted risk-difference estimate is -0.051, with a 95% confidence interval from -0.15 to 0.048. The point estimate favors a lower observed event proportion in the PFC Stockings group, while the interval includes no difference and values on either side.

The relative risk and odds ratio estimates also fall below their respective no-association values, but both confidence intervals include those values. The unadjusted p-value is 0.311. Together, these results indicate that the observed comparison is uncertain; they do not establish a difference, and they do not demonstrate equivalence. The registry record does not post a sponsor analysis or statistical analysis plan, so these recalculated comparisons should not be treated as a substitute for an unavailable trial analysis.

The result does not answer every question raised by the study objectives. Although the registry describes perceived health-related quality of life as an objective, no secondary results are posted. The primary endpoint also does not provide information about time to ulcer, recurrence, ulcer duration, or additional clinical details beyond the stated definition. Safety counts are available, but no formal adverse-event comparison is reported.

Nor does the result resolve questions about missing outcome data, adherence, analysis populations, masking, or other aspects of trial conduct. Those details are not reported in the registry record. The most defensible reading is therefore confined to the posted event counts and the uncertainty shown by the unadjusted estimates: the data point toward a lower observed event proportion in the PFC Stockings arm, but leave substantial uncertainty about the magnitude and direction of the underlying difference.

16. Why This Trial Matters Statistically

This trial is statistically instructive because its primary outcome is a participant-level binary event, making the distinction between absolute and relative comparisons important. The risk difference describes the absolute contrast, while the relative risk and odds ratio express the comparison on relative scales. Reporting these measures together makes clear that the same group summaries can be described in more than one way, and that each measure has its own interpretation.

The estimated risk difference is -0.051, but its confidence interval includes zero. The relative risk and odds ratio intervals likewise include their no-association values. This illustrates why a point estimate cannot be interpreted alone: the confidence interval communicates how much uncertainty remains around the estimate. The unadjusted p-value of 0.311 is another summary, but it does not replace the effect estimate or its interval.

The record also highlights the difference between observed results and planning illustrations. The sample-size figures use fractions of the observed effect under explicit planning assumptions. They are not evidence about the trial’s original design choices or a retrospective measure of its ability to detect an effect. The original design assumptions are not reported in the registry record.

Finally, the registry’s limited reporting of analysis methods, missing-data handling, and analysis populations shows why statistical interpretation depends on more than a p-value. The posted counts permit an unadjusted comparison, but the absence of a posted sponsor analysis and analysis plan constrains assessment of the full analytical approach. This is a reminder to keep conclusions aligned with the outcome and methods that are actually documented.

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.

Several limitations specific to this registry record constrain interpretation. The statistical comparison is an unadjusted recalculation from the posted group summaries; no sponsor analysis results or statistical analysis plan are posted. The record therefore does not establish which primary analysis the investigators used or whether it differed from the recalculation.

The registry does not report missing-outcome handling, follow-up completeness, analysis populations, adherence to assigned stockings, or crossover between groups. Although allocation is listed as randomized, details of allocation concealment are not reported. The record identifies single masking but does not say who was masked or how masking was maintained.

The primary outcome is limited to whether a participant developed an ulcer during 30 months. The registry does not report event timing, ulcer duration, recurrence, or severity beyond the stated definition. It also describes a quality-of-life objective but posts no secondary outcome results. Safety reporting provides group counts and selected serious adverse event terms, but does not include a formal comparative analysis, event-level details, or the stated denominators for the listed term percentages.

The record does not report the trial’s original sample-size assumptions, interim-analysis procedures, multiplicity strategy, or a prespecified statistical plan. Consequently, the planning illustrations based on the observed effect should not be mistaken for documentation of the trial’s design. These reporting gaps do not show that any particular method was omitted; they limit what can be concluded from the registry record.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Therapeutic Stockings to Prevent Foot Ulcers — NCT01221194.
• Registry record last updated 2020-08-26; status last verified 2020-08.
• Results first posted 2020-08-26.
• 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.

21. Record Summary

The registry record describes a completed, randomized, parallel-group study of PFC Stockings versus Standard Therapy among people with diabetes. It lists an enrollment of 381, single masking, and a 30-month primary outcome timeframe. The primary endpoint is the number of participants who developed a foot ulcer, defined as full thickness loss of epidermis and dermis or involvement of deeper structures.

The posted counts are 76 of 195 in the PFC Stockings group and 82 of 186 in the Standard Therapy group. The unadjusted recalculated risk difference is -0.051, with a 95% confidence interval from -0.15 to 0.048 and p = 0.311. The interval includes no difference. The record does not post a sponsor analysis, analysis plan, or secondary outcome results, and does not report missing-data handling or analysis populations.