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PubMed1998

Prospective individual matching: covariate balance and power in a comparative study.

In phase II to phase IV studies, randomization has gained widespread acceptance as a methodologic tool for the allocation of patients to treatment. However, randomization is not always feasible. At times, the treatment intervention occurs universally throughout one or more units (for example, a hospital unit), while the control therapy is the only intervention provided in other units. Patients may arrive randomly at a unit, based solely on availability of the unit to accept new subjects. Thus, the treatment assignment process is out of the investigator's control and not subject to selection bias. We describe a prospective individual matching procedure through which one can achieve balanced allocation of subjects to treatment groups in this comparative study setting. In this paper, we compare balance of baseline covariates and power for this design, in which the subject is selected at random and assigned to a treatment group, and the traditional randomized block design, in which the treatment is chosen at random and assigned to a subject. We show that the prospective individual matching procedure compares favourably to the traditional randomized blocked design with respect to both baseline covariate comparability and statistical power.

Analysis of Variance↗

PubMed1996

Effect of early hip decompression on the frequency of avascular necrosis in children with fractures of the neck of the femur.

Twenty-eight per cent of 32 children with fractures of the neck of the femur developed avascular necrosis. It occurred in three of the four type I fractures, in five of the 11 type II fractures, in one of the 12 type III fractures and in none of the five type IV fractures. We studied the effect of early hip decompression on the frequency of avascular necrosis. It had no apparent value in type I fractures. To increase the statistical power of our analysis of displaced type II and III fractures we combined our cases with similar cases from other series. Twenty-two (41 per cent) of the 54 combined cases treated without hip decompression developed avascular necrosis while only three (8 per cent) of the 39 combined cases treated with early hip decompression developed avascular necrosis. This difference was statistically significant. This finding supports the use of early hip decompression as a means of reducing the frequency of avascular necrosis in displaced type II and III fractures. Our findings also indicate that the complications of nonunion, malunion and premature closure of the proximal femoral physis are minimized by accurate reduction of displaced fractures, routine use of internal fixation for all fractures and avoidance of permanent physeal damage by internal fixation devices.

Child↗

PubMed2000

Factors critical to the design and execution of epidemiologic studies and description of an innovative technology to follow the progression from normal to cancer tissue.

The results obtained from experimental studies of estrogen carcinogenesis need validation in epidemiologic studies. Such studies present additional challenges, however, because variations in human populations are much greater than those in experimental systems and in animal models. Because epidemiologic studies are often used to evaluate modest differences in risk factors, it is essential to minimize sources of errors and to maximize sensitivity, reproducibility, and specificity. In the first part of this chapter, critical factors in designing and executing epidemiologic studies, as well as the influence of sample collection, processing, and storage on data reliability, are discussed. One of the most important requirements is attaining sufficient statistical power to assess small genetic effects and to evaluate interactions between genetic and environmental factors. The second part of this chapter describes innovative technology, namely, Fourier transform-infrared (FT-IR) spectra of DNA that reveal major structural differences at various stages of the progression from normal to cancer tissue. The structural differences become evident from wavenumber-by-wavenumber statistical comparisons of the mean FT-IR spectra of DNA from normal to cancer tissues. This analysis has allowed distinguishing benign tissues from cancer and metastatic tissues in human breast, prostate, and ovarian cancers. This analysis, which requires less than 1 microg of DNA, is predicted to be used for detecting early cancer-related changes at the level of DNA, rather than at the cellular level.

DNA, Neoplasm↗

PubMed2003

Therapists as fixed versus random effects-some statistical and conceptual issues: a comment on Siemer and Joormann (2003).

The authors disagree with M. Siemer and J. Joormann's assertion that therapist should be a fixed effect in psychotherapy treatment outcome studies. If treatment is properly standardized, therapist effects can be examined in preliminary tests and the therapist term deleted from analyses if such differences approach zero. If therapist effects are anticipated and either cannot be minimized through standardization or are specifically of interest because of the nature of the research question, the study has to be planned with adequate statistical power for including therapist as a random term. Simulation studies conducted by Siemer and Joormann confounded bias due to small sample size and inconsistent estimates.

Analysis of Variance↗

PubMed2002

Statistics review 4: sample size calculations.

The present review introduces the notion of statistical power and the hazard of under-powered studies. The problem of how to calculate an ideal sample size is also discussed within the context of factors that affect power, and specific methods for the calculation of sample size are presented for two common scenarios, along with extensions to the simplest case.

Clinical Trials as Topic↗

PubMed1993

Dehydration symptoms of palliative care cancer patients.

A cross-sectional survey of inpatient palliative care subjects (n = 52) was performed to determine the severity and distribution of symptoms thought to be associated with dehydration in terminally ill cancer patients and to clarify the association between the severity of these symptoms and commonly used objective measures of dehydration. Each patient rated the severity of seven symptoms using 100-mm visual analogue scales. The symptoms considered were thirst, dry mouth, bad taste, nausea, pleasure in drinking, fatigue, and pain. Associations were sought between these symptoms and predictor variables (fluid intake, plasma osmolality, sodium, and urea) and confounding variables (age, medications, oral disease, and mouth-care regimen). Mean symptom ratings were thirst 53.8 mm, dry mouth 60.0 mm, bad taste 46.6 mm, nausea 24.0 mm, pleasure in drinking 61.6 mm, fatigue 61.8 mm, and pain 33.5 mm. Using multiple-linear regression, no association could be demonstrated between thirst (the principal outcome of interest) and the predictor or confounding variables. Estimates of the study power performed after completion revealed a 76% chance of detecting a 20-mm difference between high and low fluid intake groups. This study provides the first quantitative estimate of the experience of dehydration symptoms in those with advanced cancer. The symptoms appear to be rated moderately severe, but there is no demonstrable association between severity and fluid intake. Further studies with greater statistical power and more accurate hydration assessment would strengthen our understanding of this association.

Cross-Sectional Studies↗

PubMed1999

Event-related functional MRI: implications for cognitive psychology.

Functional magnetic resonance imaging (fMRI) has rapidly emerged as a powerful technique in cognitive neuroscience. We describe and critique a new class of imaging experimental designs called event-related fMRI that exploit the temporal resolution of fMRI by modeling fMRI signal changes associated with behavioral trials as opposed to blocks of behavioral trials. Advantages of this method over block designs include the ability to (a) randomize trial presentations, (b) test for functional correlates of behavioral measures with greater power, (c) directly examine the neural correlates of temporally dissociable components of behavioral trials (e.g., the delay period of a working memory task), and (d) test for differences in the onset time of neural activity evoked by different trial types. Consequently, event-related fMRI has the potential to address a number of cognitive psychology questions with a degree of inferential and statistical power not previously available.

Arousal↗

PubMed1991

Left ventricular regional wall motion assessment by radionuclide ventriculography: a comparison of cine display with Fourier imaging.

Radionuclide ventriculography and contrast ventriculography were performed in two comparable projections on 50 patients with suspected coronary artery disease. The efficacy of conventional cine display and Fourier image analysis of the radionuclide ventriculogram was compared using contrast ventriculography as the gold standard. Of seven different combinations of Fourier images, the combination of left anterior oblique amplitude and phase and left posterior oblique amplitude and phase provided the highest sensitivity (87%), specificity (83%), accuracy (86%), and kappa coefficient (0.64). To increase statistical power, segment data were collapsed to global data in which a heart was considered normal if all segments were normal and abnormal if one or more segments were abnormal. Fourier images had higher sensitivity (Fourier 87%, cine 47%); lower specificity (Fourier 83%, cine 92%), higher accuracy (Fourier 86%, cine 58%), and higher kappa coefficient (Fourier 0.64, cine 0.25), and these differences were statistically significant (p less than 0.01).

Coronary Disease↗

PubMed1990

Pressor tests in clinical pharmacology: response morphology heterogeneity.

Heart rate, blood pressure and venous plasma catecholamine responses to passive upright tilt, immobile erect standing, upright sitting, immersion of hand or foot in cold water, isometric handgrip and delayed auditory feedback were assessed in young healthy volunteers. Each of the tests was characterized by its own typical response morphology. Postural stress caused mainly a rise in heart rate and diastolic blood pressure with little change in systolic blood pressure. The cold pressor test caused a rapid rise in heart rate, systolic and diastolic blood pressure which then was attenuated on further exposure to cold. Isometric handgrip caused the largest pressor responses in linear fashion relative to time. Protracted isometric handgrip (i.e., 5 rather than 3 min) seemed to add psychological stress to the response profile. Delayed auditory feedback caused a less well structured rise in heart rate, systolic and diastolic blood pressure. The postural tests were the most powerful stimuli for venous plasma catecholamines. For all further tests, the catecholamine responses were in general quite small. Therefore, the tests should be considered as complementary and more than one test should be used in order to cover the spectrum of relevant pressor drives. All tests were affected by substantial variability. Sufficiently large samples should be used in order to assure appropriate statistical power and precision when the effects of investigational drugs on these test responses are studied.

Adult↗

PubMed2006

Standard versus adaptive monitoring procedures: A commentary.

In the standard approach to designing definitive clinical trials, the primary endpoint and test statistic to be used for the primary analysis are specified before trial initiation. The false positive error rate for the null hypothesis and statistical power to detect the targeted size of treatment effect are also specified. Standard monitoring procedures, such as the group sequential guidelines, enable interim monitoring while maintaining the integrity of this approach. In contrast, adaptive monitoring procedures seek to provide flexibility to modify these pre-specified design features during the course of the trial. However, these procedures have several undesirable properties, including lesser statistical efficiency, reduced interpretability of primary outcome results, basing design changes on unreliable interim estimates of efficacy, risks to the integrity and credibility of the trial, loss of flexibility to use emerging results from external sources to alter key design features, and overemphasis of the importance of statistical significance relative to clinical significance.

Clinical Trials Data Monitoring Committees↗

PubMed2003

Transmission/disequilibrium test based on haplotype sharing for tightly linked markers.

Studies using haplotypes of multiple tightly linked markers are more informative than those using a single marker. However, studies based on multimarker haplotypes have some difficulties. First, if we consider each haplotype as an allele and use the conventional single-marker transmission/disequilibrium test (TDT), then the rapid increase in the degrees of freedom with an increasing number of markers means that the statistical power of the conventional tests will be low. Second, the parental haplotypes cannot always be unambiguously reconstructed. In the present article, we propose a haplotype-sharing TDT (HS-TDT) for linkage or association between a disease-susceptibility locus and a chromosome region in which several tightly linked markers have been typed. This method is applicable to both quantitative traits and qualitative traits. It is applicable to any size of nuclear family, with or without ambiguous phase information, and it is applicable to any number of alleles at each of the markers. The degrees of freedom (in a broad sense) of the test increase linearly as the number of markers considered increases but do not increase as the number of alleles at the markers increases. Our simulation results show that the HS-TDT has the correct type I error rate in structured populations and that, in most cases, the power of HS-TDT is higher than the power of the existing single-marker TDTs and haplotype-based TDTs.

Computer Simulation↗

PubMed2006

Rosetta error model for gene expression analysis.

MOTIVATION: In microarray gene expression studies, the number of replicated microarrays is usually small because of cost and sample availability, resulting in unreliable variance estimation and thus unreliable statistical hypothesis tests. The unreliable variance estimation is further complicated by the fact that the technology-specific variance is intrinsically intensity-dependent. RESULTS: The Rosetta error model captures the variance-intensity relationship for various types of microarray technologies, such as single-color arrays and two-color arrays. This error model conservatively estimates intensity error and uses this value to stabilize the variance estimation. We present two commonly used error models: the intensity error-model for single-color microarrays and the ratio error model for two-color microarrays or ratios built from two single-color arrays. We present examples to demonstrate the strength of our error models in improving statistical power of microarray data analysis, particularly, in increasing expression detection sensitivity and specificity when the number of replicates is limited.

Algorithms↗

PubMed2000

Dying to go on holiday.

OBJECTIVE: The Christmas period road toll is a highly publicized 'summary' of the number of road deaths that occur each year during New Zealand's summer holiday period. Our aims were to identify significant changes in the Christmas road toll during 1968-98, highlight the ability to detect changes and determine if focusing on the changes in the Christmas road toll provided a useful insight into trends in fatal crashes. METHODS: Regression modelling of the number of fatalities was used to examine trends in the Christmas road toll in New Zealand over time. A power analysis was used to investigate the ability to detect significant reductions in the Christmas road toll. RESULTS: The Christmas road toll has not decreased significantly in the past 30 years and has not improved significantly in contrast to the rest of the year. The number of deaths in any given year can be expected to vary 'naturally' between 10 and 39. The statistical power to detect a hypothetical 19% reduction in the Christmas road toll is minimal. CONCLUSIONS: Stochastic fluctuations in the Christmas road toll make it extremely difficult to identify underlying trends, even if substantial. Little insight can be gained from yearly comparisons of road tolls considered over short periods. More focus should be placed on road tolls that are calculated over longer periods, e.g. the previous six months. IMPLICATIONS: Natural fluctuations arising in small counts will make it difficult to highlight real improvements in road tolls.

Accidents, Traffic↗

PubMed1985

Critical evaluation of thioridazine bioequivalence.

The phenothiazines have exhibited a history of problems associated with the bioequivalence of solid oral dosage forms. The more recent availability of chemically equivalent forms of thioridazine has raised new and interesting questions about the appropriateness of generic product interchange, even among brands that have been designated "therapeutically equivalent" by the Food and Drug Administration. The scrutiny that has accompanied the consideration of thioridazine products for inclusion into various state generic substitution formularies has offered an opportunity to examine issues involving bioequivalency in considerable detail. Specific bioequivalency concerns relate to: correct analysis of drug in biological fluids; the importance of evaluating active metabolites: single-dose vs. multiple-dose crossover studies; appropriate statistical power analysis; the "70/70" rule, and comparison of product variabilities. Examples of problems are cited to illustrate that significant questions still remain about the appropriate factors that should be used to establish bioequivalency.

Biotransformation↗

PubMedDate not supplied

Factors influencing the effectiveness of mailed health surveys.

The authors investigated sources of bias in health surveys by examining responses to their 1989 questionnaire mailed to 1,255 Massachusetts men who were eligible for dental care provided by the Department of Veterans Affairs. After a maximum of three mailings and one telephone call to nonrespondents, a total of 1,049 veterans had responded out of 1,228 finally determined to be eligible, a response rate of 85 percent. The investigators found that small differences in univariate estimates would have occurred had the field phase been terminated after the first mailing, which had a response rate of 61 percent. To evaluate multivariate distributions, they duplicated their previously published logistic regression model for sources of dental care, using only those who responded to the first and second mailings. Although model fits would have been substantively the same had the field phase been terminated after the first or the second mailings, analysis of parameter estimates and their statistical significances suggested bias that would have led to different substantive conclusions, in some instances. Another potential source of bias in surveys was found to be item omission. Fifty-eight percent of respondents answered all 46 survey questions, and 90 percent answered at least 91 percent of the questions. Fewer questions were answered by those whose responses were received last, but trends regarding missing data by age or education were not statistically significant. Although the survey using this methodology met all objectives, subject nonresponses, the ineligibility of potential respondents, item nonresponses, and skewed distributions of outcome variables combined to reduce the statistical power to detect differences among groups or to alter the analysis of the differences. These factors need to be planned for by investigators undertaking similar surveys.

Analysis of Variance↗

PubMed1997

Aldose reductase inhibitors: the end of an era or the need for different trial designs?

Despite numerous attempts over 16 years, the results of aldose reductase inhibitor (ARI) trials for the treatment of diabetic neuropathy have not proven efficacy. This paper reviews each of the ARI trials, examines confounding factors, and proposes a future course. The confounding factors considered are pharmacokinetics (ARI penetration of human nerve), length of trial (in terms of the natural history of diabetic neuropathy), trial endpoints (reversibility or slowing of progression), reproducibility of clinical measurements (in terms of power calculations), standardization and quality control of endpoints, and clinically meaningful differences in endpoints. We conclude that ARIs are most likely to have a beneficial effect in the management of diabetic distal symmetrical polyneuropathy and autonomic neuropathy but that the clinical role of ARIs is to slow the progression of diabetic neuropathy rather than to reverse it. Future trials should be designed with adequate statistical power, with consideration of the variability of the endpoint measurements for long enough duration, and with rigorous quality control to definitively confirm the utility of ARIs in the treatment of diabetic distal symmetrical polyneuropathy and autonomic neuropathy.

Aldehyde Reductase↗

PubMed2000

Statistical issues in tumor marker studies.

CONTEXT: Inferences from tumor marker studies are complicated by a variety of statistical concerns, which can make proper interpretation of results difficult. This article focuses on important issues that should be addressed when designing, conducting, and analyzing tumor marker studies. OBJECTIVE: To highlight the importance of considering statistical significance, risk ratios, statistical power, reproducibility, multiple testing, confirmatory studies, and missing data in the design of marker studies used for prognosis. RESULTS: Suggestions are provided for more effectively conducting marker studies. These include more careful attention to adequacy of the number of subjects for a marker study and improved documentation and standardization of assay methods. The importance of complete reporting of study results and description of the statistical analysis methods used is also emphasized. CONCLUSION: Cooperation among clinicians, laboratory scientists, and statisticians will be required to conduct statistically sound tumor marker studies and to facilitate prioritizing markers for new and confirmatory studies in an environment of limited patient and specimen resources.

Biomarkers, Tumor↗

PubMed1995

[Studies and recommendations on the design ov ROC analyses in nuclear medicine].

ROC analysis is the method of choice for an objective assessment of diagnostic tests; however, it requires large sample sizes. We investigated the influence of study design and data analysis on sampling requirements. A sample size of 123 for the patient as well as the control group, is required to prove a difference in sensitivity of 75% vs 90% (specificity 90%, statistical power 0.8). Analysis with the binormal bivariate ROC model allows > 35% reduction in sample size. If the patient group is increased the control group can be smaller, and vice versa; here the correlated ROC model also allows substantial decreases in sample size. If both diagnostic tests are performed in the same patient and control group and evaluated with the correlated ROC model, an objective, statistically sound assessment of diagnostic performance is possible with relatively small samples.

Analysis of Variance↗