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PubMedDate not supplied

Power Doppler imaging and resistance index measurement in the evaluation of acute renal transplant rejection.

PURPOSE: This study was designed to test the hypotheses that power Doppler imaging has a predictive value in the assessment of acute renal allograft rejection and that the information garnered from the combination of resistance index (RI) determination and power Doppler imaging has an even greater predictive value in this assessment. METHODS: Power Doppler images obtained prior to 96 sequential renal allograft biopsies in 92 patients with suspected renal transplant rejection were retrospectively graded for parenchymal vascularity on a scale of 1 to 4: 1, normal, uniform cortical flow; 2, mild peripheral cortical hypoperfusion; 3, vascular pruning in cortex and medulla; and 4, no visible parenchymal flow, with flow seen only in central vessels. Power Doppler grades and RIs for these cases were compared to graft biopsy results. RESULTS: No statistically significant association was observed between parenchymal vascularity as graded by power Doppler imaging, RI, and the presence of acute transplant rejection. CONCLUSIONS: Neither grading of vascularity on power Doppler images, RI measurement, nor the combination of these methods is an accurate means of detecting renal allograft rejection.

Acute Disease↗

PubMedDate not supplied

A note on the power of Fisher's least significant difference procedure.

Fisher's least significant difference (LSD) procedure is a two-step testing procedure for pairwise comparisons of several treatment groups. In the first step of the procedure, a global test is performed for the null hypothesis that the expected means of all treatment groups under study are equal. If this global null hypothesis can be rejected at the pre-specified level of significance, then in the second step of the procedure, one is permitted in principle to perform all pairwise comparisons at the same level of significance (although in practice, not all of them may be of primary interest). Fisher's LSD procedure is known to preserve the experimentwise type I error rate at the nominal level of significance, if (and only if) the number of treatment groups is three. The procedure may therefore be applied to phase III clinical trials comparing two doses of an active treatment against placebo in the confirmatory sense (while in this case, no confirmatory comparison has to be performed between the two active treatment groups). The power properties of this approach are examined in the present paper. It is shown that the power of the first step global test--and therefore the power of the overall procedure--may be relevantly lower than the power of the pairwise comparison between the more-favourable active dose group and placebo. Achieving a certain overall power for this comparison with Fisher's LSD procedure--irrespective of the effect size at the less-favourable dose group--may require slightly larger treatment groups than sizing the study with respect to the simple Bonferroni alpha adjustment. Therefore if Fisher's LSD procedure is used to avoid an alpha adjustment for phase III clinical trials, the potential loss of power due to the first-step global test should be considered at the planning stage.

Analysis of Variance↗

PubMed2003

Power calculations for large multi-arm placebo-controlled studies of dichotomous outcomes.

Several statistical methods can be used to test for differences in placebo-controlled dichotomous multi-arm studies. These tests include linear and quadratic trends, Dunnett's test, Hochberg's test, and the overall chi-square test. We computed the power of these tests under a wide variety of alternatives. For two arms versus a control, the power was computed exactly. For more than two arms, the power was calculated from 100000 simulations per alternative. Against monotonic alternatives, the linear trend test tended to be superior. Against convex alternatives, the quadratic trend test tended to be superior. Against general alternatives, Dunnett's test, Hochberg's test, and the overall chi-square test were superior and had very similar power curves. The Hochberg test was generally less powerful than Dunnett's test but the difference was offset by its increased power to reject multiple hypotheses. These results remained nearly constant over a wide range of sample sizes, average event rates and average effect sizes.

Abciximab↗

PubMed2003

Power comparison of two-sided exact tests for association in 2 x 2 contingency tables using standard, mid p and randomized test versions.

Exact Pearson's chi square, likelihood ratio (LR), and Fisher's tests are obtained from the conditional distribution of its test statistic, given the row and column sums of the contingency table. The power and obtained significance level of the standard, mid p, and randomized versions of these tests are compared for two-sided tests in 2 x 2 tables, using binomial and multinomial sampling. The mid p type I error probabilities seldom exceed the nominal significance level. The mid p and randomized test versions have approximately the same power, and higher power than the standard test version. The power of the Pearson's chi square, LR and Fisher's test differ, and they differ in approximately the same way for standard, mid p and randomized test versions for any given set of parameters. There is no general ranking between the three tests. In many cases, Pearson's chi square and Fisher's tests have almost equal power, and higher power than LR. In a few cases, perhaps characterized by poorly balanced designs, LR performs best. Fisher's test seems to be slightly more robust even if the design is poor.

Binomial Distribution↗

PubMed2006

Power to detect clinically relevant carry-over in a series of cross-over studies.

The potential for carry-over effects is an important consideration in the design of any cross-over study, and can cause an investigator to abandon the design altogether. In cross-over studies, carry-over is the lingering effect of a treatment into the subsequent period. Carry-over effects are differences in the extent of the carry-over between the treatments under consideration. It is well known that the test for carry-over effects in individual studies has low power. Empirical evidence of carry-over effects, or the absence of carry-over effects, could be useful for investigators considering the design. Here we develop methods for expressing the power to detect carry-over as a function of the power to detect a clinically relevant treatment effect. Our results suggest that for two-treatment, two-period cross-over studies the power to detect clinically relevant carry-over effects is often less than 15 per cent, and the number of studies needed to differentiate this effect from the type I error rate of 10 per cent is prohibitive. For the three-treatment three-period cross-over design, the power to detect carry-over effects was larger than for the two-period study, but still approached the type I error rate in a number of cases. Unequivocal conclusions about the absence of carry-over effects based on collections of hypothesis tests appear unlikely. Similar findings are presented for bioequivalence studies. For bioequivalence studies, small carry-over effects (e.g. 12.5 per cent of the treatment effect) can seriously inflate the type I error rate, particularly when the power to detect equivalence is high.

Cross-Over Studies↗

PubMed2006

An extended power of cluster detection tests.

Several tests have been proposed to detect spatial disease clustering without prior information on their locations. In order to compare the performance of these tests, most authors employ the usual power, i.e. the rejection probability of the null hypothesis of no clustering due to various reasons. However, the usual power is not always appropriate for evaluating the cluster detection tests since their purpose is to both reject the null hypothesis and identify the cluster areas accurately. In this paper, we propose an extended power of the cluster detection tests, which includes the usual power as a special case. Further, we define the profile of the extended power, which can be expected to play an important role in the evaluation and comparison of several cluster detection tests. The proposed extended power and its profile are demonstrated by two tests--Kulldorff's circular spatial scan statistic and a flexible spatial scan statistic proposed by Tango and Takahashi.

Cluster Analysis↗

PubMed2006

The power of the standard test for the presence of heterogeneity in meta-analysis.

It has been suggested that the standard test for the presence of heterogeneity in meta-analysis has low power. Although this has been investigated using simulation, there is little direct analytical evidence of the validity of this claim. Using an established approximate distribution for the test statistic, a procedure for obtaining the power of the test is described. From this, a simple formula for the power is obtained. Although this applies to a special case, the formula gives an indication of the power of the test more generally. In particular, for a given significance level, the power can be calibrated in terms of the proportion of the studies' variances that is provided by between-study variation. A consideration of this quantity confirms that the test does, in general, have low power. It is suggested that practitioners, who wish to conduct the standard test, use the ideas provided in order to investigate the operating characteristics of the test prior to performing it.

Glycerol↗

PubMed2006

Power and sample size calculations for discrete bounded outcome scores.

We consider power and sample size calculations for randomized trials with a bounded outcome score (BOS) as primary response adjusted for a priori chosen covariates. We define BOS to be a random variable restricted to a finite interval. Typically, a BOS has a J- or U-shaped distribution hindering traditional parametric methods of analysis. When no adjustment for covariates is needed, a non-parametric test could be chosen. However, there is still a problem with calculating the power since the common location-shift alternative does not hold in general for a BOS. In this paper, we consider a parametric approach and assume that the observed BOS is a coarsened version of a true BOS, which has a logit-normal distribution in each treatment group allowing correction for covariates. A two-step procedure is used to calculate the power. Firstly, the power function is defined conditionally on the covariate values. Secondly, the marginal power is obtained by averaging the conditional power with respect to an assumed distribution for the covariates using Monte Carlo integration. A simulation study evaluates the performance of our method which is also applied to the ECASS-1 stroke study.

Computer Simulation↗

PubMed1989

Effect of unequal censoring on the size and power of the logrank and Wilcoxon types of tests for survival data.

This paper examines the effect of random unequal censoring on the size and power of two-sample logrank and Wilcoxon types of tests for comparing two survival distributions by simulation with small samples from censored exponential distributions. We compared equal-sized samples of n = 8, 16, and 32 with 1000 (size) and 500 (power) simulation trials for 16 combinations of the censoring proportions of 0, 20, 40, and 60 per cent in each of the two samples. For n = 8, the asymptotic normality (AN), Peto-Peto, and the two Wilcoxon-type tests performed at nominal 5 per cent size expectations, but the Mantel test exceeded the 5 per cent size acceptance region in 6 of 16 censoring combinations. For n = 16 and 32, all tests showed proper size, with the Peto-Peto test being most conservative in the presence of unequal censoring. We compared powers of all tests for exponential hazard ratios of 1.4 and 2.0. The Mantal test showed 90 to 95 per cent power efficiency relative to the parametric AN test. Both Wilcoxon tests performed identically and had the lowest relative power of all tests examined but appeared most robust to the differential censoring patterns studied. A modified version of the Peto-Peto test showed power comparable to the Mantel test.

Computer Simulation↗

PubMed1993

Sample size and power for prospective analysis of relative risk.

In a placebo-controlled vaccine efficacy trial or a trial of equivalence of vaccines, one may wish to show that relative risk of disease is less than a specified value R0, not equal to one. This paper compares three methods for estimating relative risk in the binomial setting, based on a logarithmic transformation, likelihood scores, and a Poisson approximation. Exact power and size of test are calculated by enumeration of possible binomial outcomes, and power is approximated from asymptotic formulations. Although the score method is generally preferable, for most studies of practical interest the log and score methods are comparable, and the Poisson method is also appropriate for small risks, up to about 0.05. When true and null relative risks are less than one, unequal allocation of study individuals can increase power, and the asymptotic formula for the log method may substantially underestimate power; in such a study the power approximation for the score method is more reliable, even if the log method is used in analysis. Exact power calculations are helpful in planning studies. The log and Poisson methods, but not the score method, apply readily in the case of unequal follow-up.

Binomial Distribution↗

PubMed2000

Three-dimensional ultrasound and power doppler improve the diagnosis of ovarian lesions.

OBJECTIVE: The aim of this study was to determine whether three-dimensional power Doppler can improve the ability to differentiate benign from malignant ovarian masses. METHODS: Transvaginal color Doppler and three-dimensional power Doppler were performed on 120 patients with ovarian lesions. All patients underwent both ultrasound examinations during the day prior to laparotomy or laparoscopy. Scoring systems combining morphological and Doppler parameters were adopted for two- and three-dimensional ultrasound examinations. RESULTS: In each of 11 ovarian malignancies, preoperative diagnosis by three-dimensional power Doppler was confirmed by histopathology. Transvaginal color Doppler missed 1 case of serous cystadenocarcinoma, while 3 benign lesions were considered false positive. In 1 case of cystadenofibroma both transvaginal color Doppler and three-dimensional power Doppler were falsely positive. Qualitative analysis of the tumor vascularity architecture added to morphological parameters had a sensitivity and specificity of 100 and 99.08%, respectively. CONCLUSIONS: Better results achieved by three-dimensional ultrasound can be explained by improved recognition of the ovarian mass anatomy, characterization of the surface features, detection of the tumor infiltration, and precise depiction of the size and volume. Three-dimensional power Doppler imaging can detect structural abnormalities of malignant tumor vessels, such as arteriovenous shunts, microaneurysms, tumoral lakes, disproportional calibration, coiling, and dichotomous branching. Three-dimensional power Doppler can enhance and facilitate the morphologic and functional evaluation of both benign and malignant ovarian masses.

Adolescent↗

PubMed2001

Three-dimensional power Doppler ultrasound improves the diagnostic accuracy for ovarian cancer prediction.

OBJECTIVE: The goal of this study was to determine if three-dimensional power Doppler ultrasound improves the specificity for ovarian cancer detection as compared with two-dimensional ultrasound. METHODS: Seventy-one women with a known complex pelvic mass were referred for a preoperative ultrasound evaluation with both two-dimensional and three-dimensional gray-scale ultrasonography. The 3D studies were performed with the Kretz Voluson 530D using a mechanized transvaginal probe. Surface rendering and power Doppler imaging were performed by the same gynecologic sonologist, and reassigned to one of four echo patterns: cystic, multicystic, complex, or solid. Sonographic criteria used for diagnosing ovarian cancer were based on a system that included morphological characteristics, histological prediction, and power Doppler imaging. RESULTS: Seventy-one women underwent surgical exploration: 14 (19.7%) had ovarian cancer (2 FIGO stage I, 2 stage II, 7 stage III, and 3 metastatic colon) and 2 had uterine cancer. Two-dimensional gray-scale ultrasound identified 40 masses as suspicious for cancer, including all 14 malignancies, yielding a sensitivity, specificity, and positive predictive value of 100, 54, and 35%, respectively. However, evaluation with 3D power Doppler identified only 28 cases as suspicious (including all 14 cancers), resulting in a sensitivity, specificity, and positive predictive value of 100, 75, and 50%, respectively. CONCLUSIONS: Three-dimensional power Doppler imaging better defines the morphological and vascular characteristics of ovarian lesions. All malignancies were correctly identified by both 2D and 3D imaging; however, the specificity significantly improved with the addition of 3D power Doppler. This improved diagnostic accuracy may promote improved patient care by separating complex benign masses from ovarian cancer, therefore facilitating appropriate physician referral.

Adult↗

PubMed1983

Words of power: personal empowerment and healing.

This paper examines instances of ritual use of words in a diverse selection of alternative healing groups in a modern society. These words are distinguished by their users' belief that they are endowed with a power, an effectiveness, separate from and in addition to their literal meaning. Three specific features of ritual language contribute to its effectiveness: (1) its function as an objectification of power, (2) its transformative functions--especially its metaphoric and metonymic usages, and (3) its performative aspects. This paper argues that one of the key factors in healing illness is mobilizing resources of power, especially enhancing the ill person's sense of personal empowerment. Ritual language use in alternative healing is one of the foremost elements in this empowerment, because it both represents and objectifies power. Within a belief system in which they are significant, words of power indeed have the power to effect healing.

Humans↗

PubMed1990

Modelling human power and endurance.

A generalised three component hydraulic model has been proposed to represent the human bioenergetic processes relating internal energy stores to performance during exercise, and into recovery. Further development of the model allows testable predictions to be made. In particular in this paper I examine certain hypotheses of chemical fuel shortage as a subgroup of the potential causes of fatigue, and their implications for maximal power and for endurance. The assumption that the limitation to sustainable power is direct proportionality to the glycogen store remaining, appears the most feasible. Based on this assumption, equations for the decline in maximum attainable power over time, the endurance at fixed workrates and the endurance at incremental tests (as a function of the increment slope) are obtained. Using published data for fit males, the maximum exertable power declines after about 6 s at 972 W to very low levels after about 2 min. For constant powers selected between 208 and 927 W, endurance declines from ad infinitum to only 6 s. Endurance at VO2max is predicted to be about 9 min. For incremental exercise tests of slope ranging from 30 W/min to 60 W/min, endurance lessens from 14 to 9 min. In these tests the anaerobic threshold is reached in times between 6 and 3 min. Although the power at termination of a test increases with incremental slope, terminal oxygen consumption is effectively constant. Almost all these model predictions are observed to correspond well with published experimental findings. These results suggest that the model can be used to represent an adequate overview of the operation of the human bioenergetic system.

Adenosine Triphosphate↗

PubMed1993

Effect of one- and two-leg training on arm and two-leg maximum aerobic power.

The purpose of this study was to examine the effect of one- and two-leg training on arm and two-leg maximum aerobic power. Seven subjects cycle-trained both legs simultaneously for 30 min.day-1, 4 days.week-1 for 4 weeks. Nine subjects cycle-trained each leg 15 min.day-1, 4 days.week-1 for 4 weeks. Both groups trained at a heart rate equal to that measured at 75% of their two-leg maximum aerobic power. Thus, during each training session the groups performed 30 min of work at the same heart rate intensity. Five subjects served as a non-training control group. Arm and leg maximum oxygen uptake tests were conducted before and after training. Only two-leg training induced significant gains in arm aerobic power (P < 0.0003), whereas both modes of training resulted in significant increases in two-leg aerobic power (P < 0.0008). The data demonstrate that improvements in arm aerobic power were dependent on the quantity of leg muscle mass involved in the training, whereas gains in two-leg aerobic power occurred regardless of whether the legs were trained separately or simultaneously.

Adult↗

PubMed1996

The prediction of power and efficiency during near-maximal rowing.

The relationship between power and gross efficiency during near-maximal rowing, and physiological measures of strength, power, aerobic and anaerobic capacities and United State Rowing Association (USRA) performance tests (independent variables) was investigated among collegiate male rowers. Criterion measures of rowing power and gross efficiency were measured in a moving-water rowing tank, using an oar instrumented with strain gauges to assess force and a potentiometer to assess oar position. Bivariate correlation analysis (n = 28) indicated no relationship between the independent variables and rowing gross efficiency (P > 0.05). Rowing power [mean (SD) 483.4 (34.75) W] was significantly related to inboard leg extension strength (IL strength, r = 0.63), outboard leg extension strength (r = 0.45), combined leg extension strength (r = 0.45), and time to complete the USRA 2000-m simulated rowing race (r = -0.52; P <0.05). Stepwise regression using resampling cross-validation of 15 random samples (21 subjects per sample selected from a total group of 28 intercollegiate oarsmen) indicated that predictors of rowing power were IL strength and blood lactate following a peak oxygen uptake rowing test with significant multiple correlations of R 0.61 to 0.86 (P <0.05). The standard error of estimate (SEM) ranged from 18.1 to 29.9 W, or 5.3 (0.77) percent of the criterion value. Cross-validation with a hold-out group (seven subjects per sample) was performed for each equation and correlations ranged from R = 0.14 to 0.97 (SEM = 8.0 to 38.9 W). In conclusion, data from the present study suggest that to increase rowing power, training should emphasize leg strength and anaerobic training to decrease the level of lactate accumulated during rowing.

Adult↗

PubMed1980

Anaerobic power output of young obese men: comparison with non-obese men and the role of excess fat.

Anaerobic power output was measured by the staircase climb test in 14 obese, 16 lean, and 21 ordinary men aged from 18--22 years. Fat storage rate (%fat) was estimated by densitometry. The obese group ranked highest with an average power output of 1,012 W. This value was significantly higher than those of the other two groups, 890 W for lean subjects and 855 W for ordinary subjects. The power output per kilogram of lean body mass of the obese group was the highest also. However, the vertical velocity was the lowest although the difference among the three average values was not statistically significant. To investigate the effect of excess fat, eight non-obese subjects engaged in an added-weight experiment. The value obtained was almost the same as for the obese group. The added weights made the vertical velocity decrease but the power output increase. Consequently, it was obvious that the excess fat of an obese man played a role only as an inert mass in the power output measurement. A significantly higher power output of the obese group might be due to more excess fat, and obesity itself was an advantage in the staircase climb test.

Adipose Tissue↗

PubMed1985

Linear increase in optimal pedal rate with increased power output in cycle ergometry.

This experiment was designed to estimate the optimum pedal rates at various power outputs on the cycle ergometer. Five trained bicycle racers performed five progressive maximal tests on the ergometer. Each rode at pedal rates of 40, 60, 80, 100, and 120 rev X min-1. Oxygen uptake and heart rate were determined from each test and plotted against pedal rate for power outputs of 100, 150, 200, 250, and 300 W. Both VO2 and heart rate differed significantly among pedal rates at equivalent power outputs, the variation following a parabolic curve. The low point in the curve was taken as the optimal pedal rate; i.e., the pedal rate which elicited the lowest heart rate or VO2 for a given power output. When the optimum was plotted against power output the variation was linear. These results indicate that an optimum pedal rate exists in this group of cyclists. This optimum pedal rate increases with power output, and when our study is compared to studies in which elite racers, or non-racers were used, the optimum seems to increase with the skill of the rider.

Adult↗