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PubMed2001

Cerebrovascular reactivity in normal pregnancy and preeclampsia.

OBJECTIVE: To compare cerebrovascular reactivity in normotensive and preeclamptic pregnant women. METHODS: Transcranial Doppler ultrasound was used to measure peak, end-diastolic, and mean velocities in the middle cerebral arteries of 45 normotensive and 36 preeclamptic women in the third trimester. All measurements were done in the left lateral position at baseline, during 5% carbon dioxide (CO2) inhalation, and during an isometric hand-grip test. Blood pressure (BP), heart rate, oxygen (O2) saturation, and end-tidal partial pressure of carbon dioxide (pCO2) were recorded with each Doppler measurement. The mean pulsatility index (PI), resistance index (RI), and cerebral perfusion pressure at each time was compared using two-way repeated measures analysis of variance. Cerebrovascular reactivity, calculated as the percentage change in response to each maneuver, was also compared using analysis of covariance. A post hoc power analysis was performed to evaluate the primary measures of the study (middle cerebral artery PI and RI). Using alpha error of 5%, the statistical power to identify a difference in PI and RI in women with preeclampsia compared with normotensive women was 90% and 67%, respectively. The statistical power to identify a difference in PI and RI in response to the two maneuvers was 69% and 53%, respectively. Statistical significance was set at P <.05. RESULTS: Preeclamptic women had higher baseline cerebral perfusion pressure (90.4 compared with 61.9 mmHg, P <.05) and lower PI (0.64 compared with 0.76, P <.05) and RI (0.46 compared with 0.51, P <.05) than normotensive pregnant women. In normotensive patients, both 5% CO2 inhalation and isometric hand-grip test caused a significant decrease in PI (-9.5% and -6.1%, respectively) and RI (-6.5% and -4.2%, respectively). In contrast, in preeclamptic patients there was no change in any of the middle cerebral artery parameters in response to either maneuver. CONCLUSION: Normotensive pregnant women had normal middle cerebral artery responses to both 5% CO2 inhalation and isometric hand-grip test. Preeclamptic patients had elevated baseline cerebral perfusion pressure and reduced vasodilatory responses to both tests. These findings are consistent with a state of vasoconstriction in preeclamptic women that is unresponsive to stimuli that under normal circumstances result in vasodilation.

Adult↗
PubMed2000

Estimation and detection of event-related fMRI signals with temporally correlated noise: a statistically efficient and unbiased approach.

Recent developments in analysis methods for event-related functional magnetic resonance imaging (fMRI) has enabled a wide range of novel experimental designs. As with selective averaging methods used in event-related potential (ERP) research, these methods allow for the estimation of the average time-locked response to particular event-types, even when these events occur in rapid succession and in an arbitrary sequence. Here we present a flexible framework for obtaining efficient and unbiased estimates of event-related hemodynamic responses, in the presence of realistic temporally correlated (nonwhite) noise. We further present statistical inference methods based upon the estimated responses, using restriction matrices to formulate temporal hypothesis tests about the shape of the evoked responses. The accuracy of the methods is assessed using synthetic noise, actual fMRI noise, and synthetic activation in actual noise. Actual false-positive rates were compared to nominal false-positive rates assuming white noise, as well as local and global noise estimates in the estimation procedure (assuming white noise resulted in inappropriate inference, while both global and local estimates corrected false-positive rates). Furthermore, both local and global noise estimates were found to increase the statistical power of the hypothesis tests, as measured by the receiver operating characteristics (ROC). This approach thus enables appropriate univariate statistical inference with improved statistical power, without requiring a priori assumptions about the shape or timing of the event-related hemodynamic response.

Artifacts↗
PubMed2000

Evaluation of the extent of nonlinearity in reportable range studies.

OBJECTIVES: To extend the polynomial method for evaluating linearity in 2 ways. First, we developed a screen to ascertain whether the data were precise enough to permit a reliable evaluation of linearity and therefore eliminate findings of linearity due to low statistical power. Second, we assessed whether the degree of nonlinearity detected by the polynomial method was clinically relevant using a statistically rigorous method. METHODS: Because we assessed linearity relative to a clinically determined level of importance instead of the default value of zero, we used sampling theory based on the noncentral chi(2) distribution. Using statistical power calculations, we incorporated a screen for imprecision that guarantees that the probability of correctly identifying nonlinear methods is at least 80%. RESULTS: With the described methods, we achieved a sensitivity of at least 80% and a specificity of at least 95%. When the data were too imprecise to achieve a sensitivity of 80%, no determination of linearity was made. This procedure mimics the practice in manual inspection of flagging data that appear imprecise by visual inspection and halting the evaluation. CONCLUSIONS: Formal statistical tests for precision and amount of nonlinearity are advantageous because they allow us to quantify and limit classification errors. By formalizing these various aspects of linearity assessment, we maintain some of the complex features of manual methods while making the linearity assessment feasible to apply to a high volume of assessments and removing the between-analyst variability.

Humans↗
PubMed1999

Clinical trials in general surgical journals: are methods better reported?

BACKGROUND: Reports of clinical trials often lack adequate descriptions of their design and analysis. Thus readers cannot properly assess the strength of the findings and are limited in their ability to draw their own conclusions. A review of 6 surgical journals in 1984 revealed that the frequency of reporting 11 basic elements of design and analysis in clinical trials was only 59%. This study attempted to identify areas that still need improvement. METHODS: Eligible studies published from July 1995 through June 1996 included all reports of comparative clinical trials on human subjects that were prospective and had at least 2 treatment arms. A total of 68 articles published in 6 general surgery journals were reviewed. The frequency that the previously identified 11 basic elements of design and analysis were reported was determined. RESULTS: Seventy-four percent of all items were reported accurately (a 15% increase from the previous study), 4% were reported ambiguously, and 23% were not reported; improvement was seen in every journal. The reporting of eligibility criteria and statistical power improved the most. For 3 items, reporting was still not adequate; 32% of reports provided information about statistical power, 40% about the method of randomization, and 49% about whether the person assessing outcomes was blind to the treatment assignment. CONCLUSIONS: Improvements have been made in reporting surgical clinical trials, but in general methodologic questions poorly answered in the 1980s continue to be answered poorly in the 1990s. Editors of surgical journals are urged to provide authors with guidelines on how to report clinical trial design and analysis.

Clinical Protocols↗
PubMed1996

Batch sampling to improve power in a community trial. Experience from the Pawtucket Heart Health Program.

Experiments involving large social units, such as schools, work sites, or whole cities, are commonly limited in statistical power because the number of randomized units is small, leaving few degrees of freedom for residual (between-unit) error. The authors describe a method for increasing residual degrees of freedom in a community experiment without substantially increasing cost or difficulty. In brief, they propose that the experimental units should be divided into random subsamples (batches). Batch sampling can improve statistical power if the community endpoint means are stable over time or if their temporal variation is comparable in period to the batch-sampling schedule. The authors demonstrate the theoretical advantages of the batch system and illustrate its use with data from the Pawtucket Heart Health Program, in which such a design was implemented.

Analysis of Variance↗
PubMed2000

The efficiency of genetic analysis of DNA from aged siblings to detect chromosomal regions implicated in longevity.

Studies of the frequencies of different alleles in young adults and aged individuals have implicated several genes, such as ApoE and ACE, in longevity. However such association studies can easily give rise to spurious results through unsuspected population subdivision, and an approach making use of genetic relationships among relatives is desirable. We have studied the effectiveness of non-parametric genetic analysis to detect different types of loci affecting longevity. The non-parametric method has high statistical power to detect infrequent recessive alleles that are required for, or significantly increase the probability of, survival to advanced age. Statistical power is reduced if a proportion of carriers of the alternative allele is allowed to survive. The method is least effective in detecting alleles that occur at low frequency in young individuals and that subsequently experience high mortality, as is the case for carriers of the epsilon4 allele of ApoE. Genotyping errors will also reduce the value of the NPL statistic in a linear fashion with the error rate and the number of loci genotyped. We have also used the method to analyse genotypes of seven highly polymorphic markers near the ApoE gene in a sample of 188 sibships of nonagenarians and centenarians (n=434) and their children (n=124), however no excess sharing of alleles was detected.

Adult↗
PubMed2002

Meta-analysis of the association between tryptophan hydroxylase and suicidal behavior.

Tryptophan hydroxylase (TPH) has been the candidate gene of focus in many of the association studies of suicidal behavior in recent years. Initial positive findings with respect to an association between the TPH gene and suicidal behavior have been replicated, but not consistently. Typically, individual studies have investigated small samples, and thus they repeatedly had insufficient statistical power to detect a positive association. Meta-analysis is one approach that can be used to achieve greater statistical power and may be helpful in providing a more conclusive understanding. We used meta-analytic techniques to investigate the association between an intron 7 polymorphism in the TPH gene and suicidal behavior. A total of 39 publications were identified and reviewed, 17 of which were selected for inclusion in this study. We performed two meta-analyses. One compared suicide attempters or completers (N = 1,290) with healthy controls (N = 2,295); the other compared suicide attempters (N = 625) with nonattempters (N = 1,475). None of these studies provided evidence for association (odds ratio (OR) = 1.14, 95% confidence interval (CI) = 0.97-1.34 for the former and OR = 0.96, 95% CI = 0.77-1.20 for the latter). The combined results from comparisons within both groups showed no overall association between suicidal behavior and an intron 7 polymorphism of the TPH gene.

Humans↗
PubMed2003

An evaluation of the use of magnetic field maps to undistort echo-planar images.

When a head is placed in an MRI scanner, differences between the magnetic susceptibility of tissue, bone, and air distort the magnetic field. While some of the resulting inhomogeneity can be corrected by the shimming process, much of it cannot, and this causes distortion (sometimes referred to as geometric distortion) of echo-planar images (EPIs). One strategy for the correction of distortion is to acquire a map of the magnetic field achieved in each subject and then to use this to undistort their EPIs after reconstruction (). Here, we present five experiments to evaluate the application of such a strategy on data from a 3-T scanner. We show that after undistortion, the shape of EPIs is more similar to the true shape of the brain, and we investigate the effect of head movement on the efficacy of undistortion. If undistortion was applied first, it was found that less nonlinear warping was required to transform EPIs into a standard space, particularly in the phase-encode direction. We show that if SPM 99 normalization is used to perform a nonlinear warp to standard space, the prior application of undistortion increases the statistical power of group studies with motor and auditory tasks. We show that this increase in power is due to an increase in the overlap of activation of different subjects. Finally, we evaluate where in the brain undistorting EPIs might be expected to have the greatesteffect, in terms both of mislocalization of activationand of a reduction in power. Overall, undistorting EPIs using field maps has proved extremely successful, improving the anatomical localization of activation and increasing statistical power.

Algorithms↗
PubMed1987

The positive approach to negative results in toxicology studies.

Negative results in toxicology studies are often as noteworthy as are results that detect significant toxicological effects. The results of 49.1% of all t tests published in Ecotoxicology and Environmental Safety in 1985 and 1986 were negative. However, despite the importance and prevalence of negative results in toxicology studies, they are frequently misinterpreted. Negative results from statistical tests that have poor statistical power can only be considered to be inconclusive. Because toxicology studies often use small sample sizes, such studies often have poor power to detect small, but biologically significant, effects. Toxicologists may improve the power of their tests by improving experiment design, increasing alpha, increasing sample size, or limiting the analysis to detection of large differences among samples. Selection of both sample size and alpha level should take considerations of statistical power into account.

Research↗
PubMed2001

The OMENS trial: opportunistic evaluation of musculo-skeletal physician care among orthopaedic outpatients unlikely to require surgery.

STUDY OBJECTIVE: For orthopaedic outpatients judged as unlikely to require surgery, how does the cost-effectiveness of treatment from a musculo-skeletal medicine physician compare with that of a conventional orthopaedic surgeon-led service? DESIGN: Partially randomised, pragmatic trial comparing management from a single musculo-skeletal medicine physician with orthopaedic surgeon-led management, with the timing and nature of treatments at the discretion of individual clinicians. Main outcomes were: change in patient reported health (SF-36 and EuroQol), and marginal health service costs. SETTING: Outpatient department of an Edinburgh orthopaedic hospital. SUBJECTS: One thousand three hundred and twenty six people; aged over 18, referred to and later attending the outpatient department with a 'non-surgical' musculo-skeletal condition, who provided baseline information. Eight hundred and twenty nine were randomly allocated to management by either the musculo-skeletal medicine physician (medical group) or a conventional orthopaedic surgeon-led service (surgical group); 497 were assigned to the treatment groups according to their general practitioner's preference. RESULTS: There are aspects of the study design concerning statistical power, potential selection bias and generalisability, which mean that any results must be treated with caution. Although there was no statistically significant difference in health gain between the randomly allocated groups (with similar findings in the non-randomised medical and surgical groups), the patient groups were heterogeneous and the confidence intervals did not rule out clinically important differences. The medical group received more outpatient treatment, 20% vs. 10% (difference 95% CI 5% to 15%) with a higher rate of physiotherapy referral, 45% vs. 30% (15%, 9% to 22%). The surgical group received more inpatient surgical care 6% vs. 17% (-11%, -15% to -7%). Mean healthcare costs in the medical group were 179 Pounds per patient compared with 287 Pounds in the surgical group (-108 Pounds, -25 Pounds to -191 Pounds). CONCLUSIONS: Interpretation should take into account the practical difficulties and circumstances of the study. Although no differences in health gain were found between these two management policies for 'non-surgical' orthopaedic outpatients, the trial was less statistically powerful than planned and the confidence intervals included clinically important differences. Differences in treatment emphasis gave estimated costs savings to the Health Service of 108 Pounds per patient treated by the musculo-skeletal medicine service.

Adult↗
PubMed2005

Diffusion-controlled electron transfer processes and power-law statistics of fluorescence intermittency of nanoparticles.

A mechanism involving diffusion-controlled electron transfer processes in Debye and non-Debye dielectric media is proposed to elucidate the power-law distribution for the lifetime of a blinking quantum dot. This model leads to two complementary regimes of power law with a sum of the exponents equal to 2, and to a specific value for the exponent in terms of a distribution of the diffusion correlation times. It also links the exponential bending tail with energetic and kinetic parameters.

Journal Article↗
PubMed2004

Power for genetic association studies with random allele frequencies and genotype distributions.

One of the first and most important steps in planning a genetic association study is the accurate estimation of the statistical power under a proposed study design and sample size. In association studies for candidate genes or in fine-mapping applications, allele and genotype frequencies are often assumed to be known when, in fact, they are unknown (i.e., random variables from some distribution). For example, if we consider a diallelic marker with allele frequencies of 0.5 and 0.5 and Hardy-Weinberg proportions, the three genotype frequencies are often assumed to be 0.25, 0.50, and 0.25, and the statistical power is calculated. Unfortunately, ignoring this source of variation can inflate the estimated power of the study. In the present article, we propose averaging the estimates of power over the distribution of the genotype frequencies to calculate the true estimate of power for a fixed allele frequency. For the usual situation, in which allele frequencies in a population are not known, we propose placing a prior distribution on the allele frequency, taking advantage of any available genotype information. This Bayesian approach provides a more accurate estimate of power. We present examples for quantitative and qualitative traits in cohort studies of unrelated individuals and results from an extensive series of examples that show that ignoring the uncertainty in allele frequencies can inflate the estimated power of the study. We also present the results from case-control studies and show that standard methods may also overestimate power. As discussed in this article, the approach of fixing allele frequencies even if they are not known is the common approach to power calculations. We show that ignoring the sources of variation in allele frequencies tends to result in overestimates of power and, consequently, in studies that are underpowered. Software in C is available at http://www.ambrosius.net/Power/.

Alleles↗
PubMed1995

A comparison of three statistics for detecting differences in digitized dental radiographs: a simulation study.

OBJECTIVES: Because of methodology-induced structural differences in dental radiographs, determination of change has always depended upon expert interpretation. However, new methods should be able to considerably reduce structured error in digitized subtracted images. Once true change in density is obscured only by random variation in pixel density, statistical methods may be brought to bear on the problem of detecting change. The most appropriate statistic is not obvious, however, since density change can be quantified with respect to both magnitude and dimensional extent. Whereas mean density loss is often intuitively defined as the average density of those pixels losing density (to preclude gaining pixels from offsetting losing pixels), the extent of change may be defined in a variety of ways. In this study, extent was defined as either (a) the total number of pixels losing density, or (b) the size of the largest cluster of losing pixels. The object was to evaluate the comparative statistical power of three possible statistics (based on mean density, number of losing pixels, and size of largest losing cluster) for detecting change. METHODS: In a series of simulations of comparative clinical trials, density was reduced in the centre of 1600-pixel square regions of interest by either one or 10 grey-scale units, and t-tests, based on the three statistics, were then compared for their ability to detect differences. RESULTS: Each of the three statistics was shown to exhibit superior relative power under particular conditions of loss magnitude, loss distribution, and pixel threshold for change. CONCLUSION: Selection of the appropriate statistic for identifying change between radiographs will require further information about the anticipated distribution of density changes for the different disease processes under investigation.

Absorptiometry, Photon↗
PubMed1991

Statistical conclusion validity. Multiple inferences in rehabilitation research.

The problem of multiple statistical inferences and Type I error rates in rehabilitation research is examined. The Bonferroni method is the most commonly advocated procedure to control Type I error in clinical research. The traditional Bonferroni method is often overly conservative and results in a loss of statistical power when more than a small number of comparisons are evaluated. Adjustments to the Bonferroni method designed to control or reduce the incidence of Type I errors and improve the statistical conclusion validity of rehabilitation research are presented. The adjusted or sharpened Bonferroni methods allow the researcher to control the incidence of Type I errors while maintaining statistical power. Adjustments to the Bonferroni method are simple to compute and applicable to a wide variety of statistical tests. The use of appropriate multiple comparison procedures will reduce the number of Type I errors and improve the statistical conclusion validity of rehabilitation research studies.

Bias↗
PubMed2005

Verification of inefficacy of the glucose method in detecting aspiration associated with tube feedings.

While some authors believe that testing for glucose in suctioned tracheal secretions can be used to detect aspiration of glucose-containing formula, others disagree. Previous evaluative studies of the glucose method's efficacy have lacked adequate statistical power and a gold standard for aspiration. In this animal study, a gold standard for aspiration was used and possessed sufficient statistical power to address the glucose method's sensitivity and specificity. As such, the results from the study provide the clinician with useful data to decide if the glucose method is appropriate for use in clinical settings.

Analysis of Variance↗
PubMedDate not supplied

Using multiple imputation for analysis of incomplete data in clinical research.

BACKGROUND: Sample loss and missing data are inevitable in multivariate and longitudinal research. Ad hoc approaches such as analysis of incomplete data or substituting the group mean for missing data, while common, may unnecessarily reduce statistical power and threaten study validity. Multiple imputation for missing data is a newly accessible, methodologically rigorous approach to dealing with the problem of missing data. APPROACH: To (a) discuss the problem of missing data in clinical research, and (b) describe the technique of multiple imputation. A case of analysis of multivariate psychosocial data is presented to illustrate the practice of multiple imputation. RESULTS: The advantages of multiple imputation are it (a) results in unbiased estimates, providing more validity than ad hoc approaches to missing data; (b) uses all available data, preserving sample size and statistical power; (c) may be used with standard statistical software; and, (d) results are readily interpreted. DISCUSSION: Accessible, user-friendly computer programs are available to perform multiple imputation for missing data making ad hoc approaches to missing data obsolete.

Data Interpretation, Statistical↗
PubMed2007

The Hanford Thyroid Disease Study: an alternative view of the findings.

The Hanford Thyroid Disease Study (HTDS) is one of the largest and most complex epidemiologic studies of the relation between environmental exposures to I and thyroid disease. The study detected no dose-response relation using a 0.05 level for statistical significance. The results for thyroid cancer appear inconsistent with those from other studies of populations with similar exposures, and either reflect inadequate statistical power, bias, or unique relations between exposure and disease risk. In this paper, we explore these possibilities, and present evidence that the HTDS statistical power was inadequate due to complex uncertainties associated with the mathematical models and assumptions used to reconstruct individual doses. We conclude that, at the very least, the confidence intervals reported by the HTDS for thyroid cancer and other thyroid diseases are too narrow because they fail to reflect key uncertainties in the measurement-error structure. We recommend that the HTDS results be interpreted as inconclusive rather than as evidence for little or no disease risk from Hanford exposures.

Cohort Studies↗
PubMed2002

Power and sample size for DNA microarray studies.

A microarray study aims at having a high probability of declaring genes to be differentially expressed if they are truly expressed, while keeping the probability of making false declarations of expression acceptably low. Thus, in formal terms, well-designed microarray studies will have high power while controlling type I error risk. Achieving this objective is the purpose of this paper. Here, we discuss conceptual issues and present computational methods for statistical power and sample size in microarray studies, taking account of the multiple testing that is generic to these studies. The discussion encompasses choices of experimental design and replication for a study. Practical examples are used to demonstrate the methods. The examples show forcefully that replication of a microarray experiment can yield large increases in statistical power. The paper refers to cDNA arrays in the discussion and illustrations but the proposed methodology is equally applicable to expression data from oligonucleotide arrays.

Analysis of Variance↗