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PubMed1987

Design and conduct of antibiotic trials. A report of the Scientific Studies Committee of the Surgical Infection Society.

Several recent publications have identified important methodologic problems in the design and conduct of antimicrobial trials in surgery. Developed by consensus of the members of the Scientific Studies Committee of the Surgical Infection Society, this report provides broad guidelines for the construction of antimicrobial trials. The central issues identified include pretrial definition of study purpose, entry criteria, assignment device, and statistical analysis. These issues are fundamental in designing studies with an acceptable likelihood of finding differences among those antimicrobial regimens at least risk to the study subjects. The importance of stratifying patients on the basis of background condition, disease, and severity of illness is stressed. The inclusion in a study of variables that enhance the statistical power and, therefore, the believability of this study is stressed as an important means of clarifying substantial differences between therapies.

Abdomen↗
PubMed1999

Resampling-based methods for the analysis of multiple endpoints in clinical trials.

We consider multivariate tests for comparing two treatments with multiple endpoints. The test decision is drawn from the simultaneous consideration of the univariate tests for the single endpoints. A general class of these tests, called cut-off tests, can be given, which, however, can lead to highly conservative procedures because the dependencies among the endpoints are not taken into account. In applying resampling-based methods considerable improvements for these tests can be achieved. Resampling-based cut-off tests are proposed which are sensitive against a treatment difference in a single endpoint, in a subgroup of endpoints, or in all endpoints. The results of Monte Carlo simulations demonstrate that a remarkable gain in statistical power as compared to the crude simultaneous consideration can be reached. In particular, for the multivariate one-sided test situation the proposed tests can be recommended. As an example the application of the tests is demonstrated by data from a clinical trial.

Clinical Trials as Topic↗
PubMed2007

Mediational analysis in HIV/AIDS research: estimating multivariate path analytic models in a structural equation modeling framework.

Mediational analyses have been recognized as useful in answering two broad questions that arise in HIV/AIDS research, those of theoretical model testing and of the effectiveness of multicomponent interventions. This article serves as a primer for those wishing to use mediation techniques in their own research, with a specific focus on mediation applied in the context of path analysis within a structural equation modeling (SEM) framework. Mediational analyses and the SEM framework are reviewed at a general level, followed by a discussion of the techniques as applied to complex research designs, such as models with multiple mediators, multilevel or longitudinal data, categorical outcomes, and problematic data (e.g., missing data, nonnormally distributed variables). Issues of statistical power and of testing the significance of the mediated effect are also discussed. Concrete examples that include computer syntax and output are provided to demonstrate the application of these techniques to testing a theoretical model and to the evaluation of a multicomponent intervention.

Data Interpretation, Statistical↗
PubMed1989

How much variation in referral rates among general practitioners is due to chance?

A 20-fold variation in referral rates from general practitioners to hospital outpatient departments has been shown in studies published to date. Most of the hypotheses proposed to account for this variation have not been supported by these studies. A simple model was constructed, which showed that a significant part of the variation may be due to the fairly small numbers of referrals in most studies. Real differences may have been swamped by random variations in the small numbers of referrals. The statistical power of the studies may not have been high enough to determine which factors were significant in describing the variation and how much of the variation was due to differing clinical practice. There remains a substantial part of the variation that cannot be accounted for.

England↗
PubMed1991

Analyzing laboratory marker changes in AIDS clinical trials.

Repeated measurements of laboratory markers of immunologic or disease status, such as CD4 lymphocyte counts and HIV p24 antigen levels, can be important end points in comparative clinical trials. In this report, we consider comparison of treatment groups with respect to such markers, focusing on a distribution-free approach in which each participant's data are characterized by a single summary statistic. The summary statistics examined are (a) the slope of the least-squares regression of the marker, (b) the average of the last r measurements, and (c) the difference between the averages of the last r and the first s measurements. Under various models of marker time trends, these methods are compared with regard to statistical power. It is found that the slope is usually more efficient than the other two types of summaries. Adaptations for missing data are discussed and illustrated in an analysis of CD4 counts from a recent AIDS clinical trial.

Acquired Immunodeficiency Syndrome↗
PubMed2001

Y-chromosome mismatch distributions in Europe.

Ancient demographic events can be inferred from the distribution of pairwise sequence differences (or mismatches) among individuals. We analyzed a database of 3,677 Y chromosomes typed for 11 biallelic markers in 48 human populations from Europe and the Mediterranean area. Contrary to what is observed in the analysis of mitochondrial polymorphisms, Tajima's test was insignificant for most Y-chromosome samples, and in 47 populations the mismatch distributions had multiple peaks. Taken at face value, these results would suggest either (1) that the size of the male population stayed essentially constant over time, while the female population size increased, or (2) that different selective regimes have shaped mitochondrial and Y-chromosome diversity, leading to an excess of rare alleles only in the mitochondrial genome. An alternative explanation would be that the 11 variable sites of the Y chromosome do not provide sufficient statistical power, so a comparison with mitochondrial data (where more than 200 variable sites are studied in Europe) is impossible at present. To discriminate between these possibilities, we repeatedly analyzed a European mitochondrial database, each time considering only 11 variable sites, and we estimated mismatch distributions in stable and growing populations, generated by simulating coalescent processes. Along with theoretical considerations, these tests suggest that the difference between the mismatch distributions inferred from mitochondrial and Y-chromosome data are not a statistical artifact. Therefore, the observed mismatch distributions appear to reflect different underlying demographic histories and/or selective pressures for maternally and paternally transmitted loci.

Alleles↗
PubMed2004

Asthma genetics 2003.

The use of positional cloning for the identification of complex trait susceptibility genes has gained momentum with the completion of the human genome project. The approach involves the collection of well-phenotyped cohorts (either family-based or case-control designs), the generation of high-density single-nucleotide polymorphism linkage disequilibrium maps, and the application of powerful statistical methods to localize narrow regions of genetic association with disease. In 2003, two novel genes relating to asthma were identified using this approach, PHF11 and DPP10, neither of which had previously been implicated in the pathobiology of either asthma or allergy. In addition, further support for ADAM33 (the first asthma susceptibility gene identified by positional cloning) as an asthma gene was presented, although with mixed results. These discoveries open new avenues for research in asthma and allergy, and highlight the power (and limitations) of positional cloning for the identification of asthma genes, and complex trait genes in general.

ADAM Proteins↗
PubMed1994

Bootstrapping in human genetic linkage.

Linkage analysis of discrete Mendelian traits has made a major contribution to the mapping of the human genome. However, in more complex situations, particularly Mendelian disorders showing locus heterogeneity, linkage results have sometimes been misleading. The bootstrap confidence interval provides an assessment of the goodness of fit of the data to the genetic model and the presence of heterogeneity can inflate the confidence interval indicating that the fit is poor. The loss in statistical power and the potential unreliability of linkage analyses based on small samples or subsamples of pedigree material is explored and illustrated by using simulated data and also real data on 37 families affected by the heterogeneous disorder, tuberous sclerosis.

Chromosome Mapping↗
PubMed2001

[Reflections on some methodological errors in the evaluation of drugs. Ten years experience at the National Marketing Authorization Commission].

A ten year experience at the national french marketing authorization committee has permitted to notice the most commonly methodological errors in the field of clinical research and, particularly, in the dossiers for drug approval: a frequent insufficient sample size resulting in lack of statistical power, an unsatisfactory optimal dosing research, a misuse of the so-called surrogate markers, an erroneous opinion about the meaning of the p value, an abusive claim for equivalence in non significant superiority trials, a misuse of unadjusted multiple comparisons and too much confidence in subgroup analysis results.

Bias↗
PubMed2001

Testing quantitative traits for association and linkage in the presence or absence of parental data.

Zhu and Elston developed a transmission disequilibrium test for quantitative traits by defining a linear transformation to condition out founder information. The method tests the null hypothesis of no linkage or association and can be applied to general pedigree structures. However, this method requires both genotype and phenotype parental information, which may be difficult to obtain. In this paper, we describe parametric and non-parametric methods to relax this requirement when only nuclear families are sampled. We show that neither method is affected by population stratification in the absence of linkage. The statistical power and validity of the tests are investigated by simulation. A simple simulation method to calculate the power of the nonparametric method is also discussed. In practice, the data may have some families with parental phenotype and genotype information available and some without. We briefly discuss how all the data may be analyzed jointly.

Computer Simulation↗
PubMed1999

Behavioral science foundations of the Rorschach test: research and clinical applications.

Never without its critics, the Rorschach Test continues to be widely used in clinical settings. The test continues to be criticized vigorously. Rorschach critics appear to fall into two broad groups: those leveling valid methodological concerns about the test s behavioral science foundations and method critics who appear to deny the validity of the test on strictly a priori or theoretical considerations. Many critics do not appear to be acquainted with the extensive Rorschach research literature. The current paper provides an overview of several domains of applied and laboratory Rorschach behavioral science, including statistical power analysis, interobserver agreement and interrater reliability, Rorschach assessment of thought disorder, and emerging research linking Rorschach variables with diagnostic criteria from the DSM-IV, as a means of educating both adherents and detractors alike concerning the test s scientific track record and applicability to clinical assessment.

Behavioral Sciences↗
PubMed2006

Significant linkage to airway responsiveness on chromosome 12q24 in families of children with asthma in Costa Rica.

Although asthma is a major public health problem in certain Hispanic subgroups in the United States and Latin America, only one genome scan for asthma has included Hispanic individuals. Because of small sample size, that study had limited statistical power to detect linkage to asthma and its intermediate phenotypes in Hispanic participants. To identify genomic regions that contain susceptibility genes for asthma and airway responsiveness in an isolated Hispanic population living in the Central Valley of Costa Rica, we conducted a genome-wide linkage analysis of asthma (n = 638) and airway responsiveness (n = 488) in members of eight large pedigrees of Costa Rican children with asthma. Nonparametric multipoint linkage analysis of asthma was conducted by the NPL-PAIR allele-sharing statistic, and variance component models were used for the multipoint linkage analysis of airway responsiveness as a quantitative phenotype. All linkage analyses were repeated after exclusion of the phenotypic data of former and current smokers. Chromosome 12q showed some evidence of linkage to asthma, particularly in nonsmokers (P < 0.01). Among nonsmokers, there was suggestive evidence of linkage to airway responsiveness on chromosome 12q24.31 (LOD = 2.33 at 146 cM). After genotyping 18 additional short-tandem repeat markers on chromosome 12q, there was significant evidence of linkage to airway responsiveness on chromosome 12q24.31 (LOD = 3.79 at 144 cM), with a relatively narrow 1.5-LOD unit support interval for the observed linkage peak (142-147 cM). Our results suggest that chromosome 12q24.31 contains a locus (or loci) that influence a critical intermediate phenotype of asthma (airway responsiveness) in Costa Ricans.

Adolescent↗
PubMed2000

Cardiac autonomic control mechanisms in power-frequency magnetic fields: a multistudy analysis.

Heart rate variability (HRV), a noninvasive indicator of autonomic control of cardiac activity, is predictive of long-term cardiac morbidity and mortality. Epidemiologic research suggests that occupational exposure to power-frequency magnetic fields may be associated with autonomically mediated cardiac mortality. Results from our laboratory studies of humans exposed to 60-Hz magnetic fields overnight, however, are inconsistent. HRV is altered in some studies but not others. To clarify this, the pooled data from seven studies involving 172 men were analyzed to test specific hypotheses concerning this inconsistency. After analysis, we excluded a) measurement drift or instability over time because HRV was stable under sham-exposed conditions across all studies; b) inadequate statistical power or failure to maintain double-blind controls; c) differences in field intensity (28.3 vs. 127.3 microT) or exposure pattern (intermittent versus continuous) as main effects; or d) the inclusion of individuals sensitive to magnetic field exposure in some studies but not others. Four separate analytic techniques failed to identify a valid subpopulation of sensitive individuals. In some studies, however, hourly blood samples were collected using an indwelling venous catheter. HRV alterations occurred during intermittent exposure in these studies (p < 0.05) but not in similar studies without blood sampling. This result suggests a field interaction with modest arousal or disturbance. Because HRV is tightly coupled to electroencephalographic activity during sleep, these results are physiologically plausible and suggest that HRV alterations during exposure to magnetic fields may occur when accompanied by increases in physiologic arousal, stress, or sleep disturbance.

Adolescent↗
PubMed2005

Methodological aspects of outcomes research.

A critical evaluation of existing scientific evidence of treatment efficacy can be an important part of communicating risk and benefits of treatment options to patients during the course of clinical practice. A checklist of key methodological issues to examine when reading a research study is presented and discussed. Steps in reading a paper include: identifying the research question; identifying the manner in which subjects get enrolled in the study; identifying the treatments and outcomes used; identifying the study design and the comparisons being made; evaluating the study methods for the possibility of bias and uncontrolled confounding; assessing whether the statistical analysis used is appropriate for the study design; assessing whether the study has sufficient statistical power to demonstrate hypotheses being tested. Finally, procedures for grading and evaluating evidence, as used by systematic review groups and international best evidence synthesis consensus groups is briefly described.

Back Pain↗
PubMed2001

International collaboration provides convincing linkage replication in complex disease through analysis of a large pooled data set: Crohn disease and chromosome 16.

Numerous familial, non-Mendelian (i.e., complex) diseases have been screened by linkage analysis for regions harboring susceptibility genes. Except for rare, high-penetrance syndromes showing Mendelian inheritance, such as BRCA1 and BRCA2, most attempts have failed to produce replicable linkage findings. For example, in multiple sclerosis and other complex diseases, there have been many reports of significant linkage, followed by numerous failures to replicate. In inflammatory bowel disease (IBD), linkage to two regions has elsewhere been reported at genomewide significance levels: the pericentromeric region on chromosome 16 (IBD1) and chromosome 12q (IBD2). As with other complex diseases, the subsequent support for these localizations has been variable. In this article, we report the results of an international collaborative effort to investigate these putative localization by pooling of data sets that do not individually provide convincing evidence for linkage to these regions. Our results, generated by the genotyping and analysis of 12 microsatellite markers in 613 families, provide unequivocal replication of linkage for a common human disease: a Crohn disease susceptibility locus on chromosome 16 (maximum LOD score 5.79). Despite failure to replicate the previous evidence for linkage on chromosome 12, the results described herein indicate the need to further investigate the potential role of this locus in susceptibility to ulcerative colitis. This report provides a convincing example of the collaborative approach necessary to obtain the sample numbers required to achieve statistical power in studies of complex human traits.

Chromosome Mapping↗
PubMed2001

The power of the transmission disequilibrium test (TDT) with both case-parent and control-parent trios.

The transmission disequilibrium test (TDT) customarily uses affected children and their parents (often case-parent trios, TDTD). Control-parent trios are necessary to guard against spurious significant results due to segregation distortion but are not generally utilized in the identification of disease susceptibility loci (DSL). Controls are often easy to recruit and the TDT can easily be extended to include control-parent trios into the analyses with unrelated case-parent trios. We present an extension of the TDT (TDTDC) that incorporates unrelated cases and controls and their parents into a single analysis. We develop a simple and accurate analytical method for computing the statistical power of various TDT (e.g. the TDTD, TDTDC, TDTDC and TDTC that employ control-parent trios only) under any genetic model. We investigated the power of these TDT, and particularly compared the relative power of the TDTD and TDTDC. We found that the TDTDC is almost always more powerful than the TDTC and TDTD. The relative power of the TDTDC and TDTD depends largely upon a number of parameters identified in the study. This study provides a basis for efficient use of control-parent trios in DSL identification.

Case-Control Studies↗
PubMed1997

Differences in muscle contractile characteristics among bodybuilders, endurance trainers and control subjects.

The purpose of this investigation was to compare the myosin heavy chain (MHC) isoform expression of the triceps brachii muscle and isoinertial, isometric and isokinetic strength indices in competitive bodybuilders (CB, n = 5), recreational resistance trainers (RT, n = 5), endurance-trained rowers (ER, n = 5) and control (C, n = 5) subjects. Muscle tissue samples were analysed for MHC isoform content using 6% sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The CB possessed significantly smaller (P < 0.05) percentage of MHC type IIb proteins [12.92 (SD 7.08)%] than RT [30.08 (SD 6.58)%] ER [31.20 (SD 2.74)%] and C [38.22 (SD 2.95)%] groups (i.e. CB < RT approximately ER < C). While the content of MHC type IIa isoforms did not differ significantly between the two resistance-trained groups [CB = 55.76 (SD 5.38)%; RT = 45.72 (SD 7.8)%], CB presented significantly more type IIa MHC isoforms than ER [42.84 (SD 2.98)%] and C [34.72 (SD 1.57)%] subjects (i.e. CB approximately RT > ER approximately C). The MHC type I protein content did not differ significantly among RT [24.20 (SD 4.89)%] ER [25.38 (SD 1.67)%] and C [27.06 (SD 1.81)%] groups. The CB [31.32 (SD 2.67)%] presented significantly more type I MHC isoforms only in comparison with RT. However, when changes in the percentage of MHC type I isoforms were converted to effect sizes (ES), it appeared that low statistical power rather than the absence of an effect accounted for the nonsignificant differences between CB and other groups (i.e. CB > RT approximately ER approximately C). Significant differences existed in isoinertial strength among the trained athletes (i.e. CB > RT > ER approximately C), while isometric and isokinetic strength were not significantly different among any of the trained groups. However, the ES transformation of data demonstrated that large differences existed between resistance-trained groups and ER for isometric and isokinetic strength (i.e. CB approximately RT > ER approximately C). A statistically significant negative correlation (P < 0.001) was found between MHC type IIb isoforms and isoinertial strength index (r = -0.68). The MHC type IIa proteins were positively related to all the strength measures considered (r = 0.51 0.61; P < 0.001). These data demonstrated different patterns of MHC isoform expression among the different groups of athletes and it is suggested that these differences on occasion may affect the expression of strength.

Adult↗
PubMed1996

A comparison of t test, F test, and coherence methods of detecting steady-state auditory-evoked potentials, distortion-product otoacoustic emissions, or other sinusoids.

Sinusoids in background noise can conveniently be detected using unsegmented power spectra, comparing power at the signal frequency to average power at several neighbor frequencies. In this case, the F test is preferable to t tests based on rms or dB values, because of the skewed distributions of rms and dB when signal-to-noise ratio (SNR) = 0. F-test performance improves as the number of frequencies increases, to about 15, but can be degraded if the background noise is not white, with a slope exceeding about 10 dB for the range of frequencies sampled. Segment analysis, using magnitude-squared coherence (MSC) or related statistics, has equivalent statistical power; MSC and F each yield unbiased SNR estimates that have identical distributions when SNR = 0. Selection of F or MSC for detection of sinusoids will usually be a matter of convenience.

Evoked Potentials, Auditory↗