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At least 1,045 records · Page 58Linked to original sources
PubMed2006

Electromagnetic fields and female breast cancer.

The possibility that long term exposure to relatively weak power frequency electromagnetic fields (EMF) could increase the risk of breast cancer has been investigated during the past decade. The hypothesis is based on the assumption that magnetic field exposures suppress melatonin production and that melatonin is protective against breast cancer. Most epidemiological studies have indicated little or no overall effect of EMF exposure, but some early studies suggested effects among premenopausal women, particularly for estrogen receptor positive breast tumors. The early studies were often limited by small numbers, crude exposure information and lack of information on confounding factors. In more recent occupational studies, again no overall risk increases were reported, but some studies found increased risks in certain subgroups, although with no consistent pattern across studies. A recent very large occupational study with improved exposure assessment and enough statistical power also for subgroup analyses found no indications of increased risks in any subgroups. Most of the recent well-designed residential studies report no increased risks, and similar findings are reported in the majority of studies of bed heating devices. Overall, the weight of the evidence available today does not suggest an increased risk of breast cancer related to EMF exposure.

Breast Neoplasms↗
PubMed1994

Uniformity of captopril benefit in the SAVE Study: subgroup analysis. Survival and Ventricular Enlargement Study.

The Survival and Ventricular Enlargement (SAVE) Study demonstrated that long-term administration of the angiotensin-converting enzyme inhibitor captopril to recent survivors of myocardial infarction with left ventricular dysfunction resulted in a reduction in cardiovascular mortality and morbidity. Analysis of multiple subgroups demonstrated that baseline demographics (older age) and clinical characteristics (such as prior MI, history of diabetes or hypertension), that have previously been associated with a higher risk of cardiovascular events, were associated with greater end point event rates in SAVE regardless of therapy assignment at the time of randomization. The effectiveness of captopril therapy in reducing cardiovascular mortality and morbidity was examined in multiple subgroups. Although not all subgroups provided adequate statistical power, the benefits of captopril therapy were relatively uniform in the SAVE study. This indicates that the benefits were not confined to one particular subgroup and conversely that targeting of captopril therapy should be to the broadest group, as defined by SAVE entry criteria, to result in a reduction in cardiovascular mortality and morbidity.

Captopril↗
PubMed2005

A mixed-effects quintile-stratified propensity adjustment for effectiveness analyses of ordered categorical doses.

Observational studies can be used to evaluate treatment effectiveness among patients with a broader range of illness severity than typically seen in randomized controlled clinical trials. However, there are several difficulties with observational evaluations including non-equivalent comparison groups, treatment doses and durations that vary widely, and, in longitudinal studies, multiple courses of treatment per subject. A mixed-effects approach to the propensity adjustment for non-equivalent comparison groups is described that can account for each of these perturbations. The strategy involves two stages. First, characteristics that distinguish among subjects who receive various levels of treatment are examined in a model of propensity for treatment intensity using mixed-effects ordinal logistic regression. Second, the propensity-stratified effectiveness of ordered categorical doses is compared in a mixed-effects grouped time survival model of time until recovery. The model is applied in a longitudinal, observational study of antidepressant effectiveness. Then a Monte Carlo simulation study indicates that the strategy has acceptable type I error rates and minimal bias in the estimates of treatment effectiveness. Statistical power exceeds 0.90 for an odds ratio of 1.5 with N = 250 and 500, and is acceptable for an odds ratio of 2.0 with N = 100. Nevertheless, with N = 100, the models that had high intraclass correlation coefficients had greater tendency towards non-convergence. This approach is a useful strategy for observational studies of treatment effectiveness. It is capable of adjusting for selection bias, incorporating multiple observations per subject, and comparing effectiveness of ordinal doses.

Antidepressive Agents↗
PubMed1980

To define sufficient subjects.

The problem of how many subjects are required for a particular investigation can be solved. Techniques developed to allow experimenters to determine the statistical power of their experiments can be adapted to this end provided that the investigator is prepared to define the significance level and, more importantly, the minimum strength of experimental effect that is of interest. The experience described in this article demonstrates that the process demands consideration of a number of factors like the nature of the dependent variable, the form of the decision being made, and the minimum difference between conditions which has practical consequence.

Clinical Trials as Topic↗
PubMed2006

An empirical Bayes method for updating inferences in analysis of quantitative trait loci using information from related genome scans.

Individual genome scans for quantitative trait loci (QTL) mapping often suffer from low statistical power and imprecise estimates of QTL location and effect. This lack of precision yields large confidence intervals for QTL location, which are problematic for subsequent fine mapping and positional cloning. In prioritizing areas for follow-up after an initial genome scan and in evaluating the credibility of apparent linkage signals, investigators typically examine the results of other genome scans of the same phenotype and informally update their beliefs about which linkage signals in their scan most merit confidence and follow-up via a subjective-intuitive integration approach. A method that acknowledges the wisdom of this general paradigm but formally borrows information from other scans to increase confidence in objectivity would be a benefit. We developed an empirical Bayes analytic method to integrate information from multiple genome scans. The linkage statistic obtained from a single genome scan study is updated by incorporating statistics from other genome scans as prior information. This technique does not require that all studies have an identical marker map or a common estimated QTL effect. The updated linkage statistic can then be used for the estimation of QTL location and effect. We evaluate the performance of our method by using extensive simulations based on actual marker spacing and allele frequencies from available data. Results indicate that the empirical Bayes method can account for between-study heterogeneity, estimate the QTL location and effect more precisely, and provide narrower confidence intervals than results from any single individual study. We also compared the empirical Bayes method with a method originally developed for meta-analysis (a closely related but distinct purpose). In the face of marked heterogeneity among studies, the empirical Bayes method outperforms the comparator.

Chromosome Mapping↗
PubMed2005

A combined genomewide linkage scan of 1,233 families for prostate cancer-susceptibility genes conducted by the international consortium for prostate cancer genetics.

Evidence of the existence of major prostate cancer (PC)-susceptibility genes has been provided by multiple segregation analyses. Although genomewide screens have been performed in over a dozen independent studies, few chromosomal regions have been consistently identified as regions of interest. One of the major difficulties is genetic heterogeneity, possibly due to multiple, incompletely penetrant PC-susceptibility genes. In this study, we explored two approaches to overcome this difficulty, in an analysis of a large number of families with PC in the International Consortium for Prostate Cancer Genetics (ICPCG). One approach was to combine linkage data from a total of 1,233 families to increase the statistical power for detecting linkage. Using parametric (dominant and recessive) and nonparametric analyses, we identified five regions with "suggestive" linkage (LOD score >1.86): 5q12, 8p21, 15q11, 17q21, and 22q12. The second approach was to focus on subsets of families that are more likely to segregate highly penetrant mutations, including families with large numbers of affected individuals or early age at diagnosis. Stronger evidence of linkage in several regions was identified, including a "significant" linkage at 22q12, with a LOD score of 3.57, and five suggestive linkages (1q25, 8q13, 13q14, 16p13, and 17q21) in 269 families with at least five affected members. In addition, four additional suggestive linkages (3p24, 5q35, 11q22, and Xq12) were found in 606 families with mean age at diagnosis of < or = 65 years. Although it is difficult to determine the true statistical significance of these findings, a conservative interpretation of these results would be that if major PC-susceptibility genes do exist, they are most likely located in the regions generating suggestive or significant linkage signals in this large study.

Aged↗
PubMed2001

The effect of periodontal treatment on glycemic control in patients with type 2 diabetes mellitus.

BACKGROUND, AIMS: This study was designed to explore the effect of periodontal therapy on glycemic control in persons with type 2 diabetes mellitus (DM). METHODS: 36 patients with type 2 DM (treatment group) received therapy for adult periodontitis during an 18-month period. A 36-person control group was randomly selected from the same population of persons with type 2 DM who did not receive periodontal treatment. RESULTS: These groups were well matched for most of the parameters investigated. During the nine-month observation period, there was a 6.7% improvement in glycemic control in the control group when compared to a 17.1% improvement in the treatment group, a statistically significant difference. Several parameters that could confound or moderate this glycemic control were explored. These included the treatment of non-dental infections, weight and medication changes. No moderating effect was associated with any of these variables. However, there were too few subjects in the study to have the statistical power necessary to assess these possible moderators of glycemic control. CONCLUSIONS: We interpret the data in the study to suggest that periodontal therapy was associated with improved glycemic control in persons with type 2 DM.

Adult↗
PubMed2004

Statistical assessment of changes in ADL dependence: three-graded versus dichotomised scaling.

The aim of this study was to investigate how dichotomising three-graded ADL Staircase data affects the possibility of detecting changes in ADL dependence between different assessment occasions. An authentic two-occasion data set was used as a basis for a simulation experiment. In all, we used four different data treatment principles, all utilising the matched pairing of the data. The first principle utilised a sum score technique, and the second within-person comparisons by means of item-by-item analysis of improvement or deterioration. The third principle used ADL ranks, a novel approach, while the fourth used within-item ranks. Independently of the data treatment principle used, the statistical power of all tests was reduced by 13-24% after dichotomisation, compared to when the three-graded scale was utilised. The results indicate that dichotomising ADL Staircase data results in information loss, and hence in reduced ability to detect changes. The need to consider the purpose of the ADL assessment before reducing the number of scale steps is highlighted. The knowledge generated in this study is useful for practitioners and researchers, aiming at evaluating rehabilitation interventions.

Activities of Daily Living↗
PubMed2002

Nutritional papers in ICU patients: what lies between the lines?

The abundance of literature related to nutritional support reflects its recently recognised role in preventing metabolic complications and gut dysfunction during critical illness. However, some published studies lack relevance to critically ill patients, as a result of the selection of subjects and outcome variables, or flaws in the study design, as well as in the type, composition, timing, route of administration and amount of nutritional support given. This review will highlight these confounding factors by describing two imaginary (but typical) clinical trials and by analysing some studies published. The point at issue is that basic quality requirements, such as the formulation of a prospective hypothesis and the delineation of the effects of the reference treatment, are often lacking in many studies published. Data analysis was often found to be biased by the absence of statistical power calculation and intention-to-treat analysis. Globally, studies designed to assess the effects of nutritional support on the outcome of critically ill patients, rarely fulfil basic quality requirements and should therefore be interpreted cautiously. We suggest simple strategies or study design that will allow important questions to be answered by future clinical trials.

Clinical Trials as Topic↗
PubMed2005

Serotonin-2A and 2C receptor gene polymorphisms in Japanese patients with obstructive sleep apnea.

OBJECTIVE: The serotonin (5-HT) 2A and 2C receptor subtype plays an important role in the maintenance of upper airway stability and normal breathing in obesity. Polymorphisms in the 5-HT 2A receptor gene (HTR2A) and 5-HT 2C receptor gene (HTR2C) are associated with various diseases. The aim of this study was to investigate whether or not the HTR2A/C genotypes are associated with obstructive sleep apnea (OSA). METHODS: The PCR-restriction fragment length polymorphism method was used to determine genotypes of the HTR2A/C gene. The genotype distributions and allele frequencies were statistically analyzed. SUBJECTS: We studied 177 consecutive male patients with excessive daytime somnolence and an apnea plus hypopnea number [apnea-hypopnea index (AHI)] of greater than five per hour of sleep established by full polysomnography. One hundred Japanese men in whom OSA was clinically excluded were randomly selected as a control group. RESULTS: Genotypes and allele frequencies of 102T/C polymorphism of the HTR2A and 796G/C polymorphism of the HTR2C did not differ between controls and patients with OSA. HTR2C polymorphism was considered inappropriate for association studies because of low frequency of the mutant allele. Multiple regression analysis showed that age and body mass index (BMI) were significantly associated with OSA, but HTR2A polymorphisms were not. HTR2A polymorphisms had no significant relationship with AHI or BMI, although further study with more samples will be needed for powerful statistical analyses. CONCLUSIONS: These results indicate that age and BMI, not these polymorphisms, are associated with OSA in this population.

Adult↗
PubMed2004

Serotonin transporter 5HTTLPR polymorphism and affective disorders: no evidence of association in a large European multicenter study.

The available data from preclinical and pharmacological studies on the role of the serotonin transporter (5-HTT) support the hypothesis that a dysfunction in brain serotonergic system activity contributes to the vulnerability to affective disorders (AD). 5-HTT is the major site of serotonin reuptake into the presynaptic neuron, and it has been shown that the polymorphic repeat polymorphism in the 5-HTT promotor region (5-HTTLPR) may affect gene-transcription activity. 5-HTT maps to chromosome 17 at position 17q11.17-q12, and the 5-HTTLPR polymorphisms have been extensively investigated in AD with conflicting results. The present study tested the genetic contribution of the 5-HTTLPR polymorphism in a large European multicenter case-control sample, including 539 unipolar (UPAD), 572 bipolar patients (BPAD), and 821 controls (C). Our European collaboration has led to efforts to optimize a methodology that attenuates some of the major limitations of the case-control association approach. No association was found with primary psychiatric diagnosis (UPAD and BPAD) and with phenotypic traits (family history of AD, suicidal attempt, and presence of psychotic features). Our negative findings are not attributable to the lack of statistical power, and may contribute to clarify the role of 5-HTTLPR polymorphism in AD.

Bipolar Disorder↗
PubMed2002

Calculation of the minimum number of replicate spots required for detection of significant gene expression fold change in microarray experiments.

MOTIVATION: We present statistical methods for determining the number of per gene replicate spots required in microarray experiments. The purpose of these methods is to obtain an estimate of the sampling variability present in microarray data, and to determine the number of replicate spots required to achieve a high probability of detecting a significant fold change in gene expression, while maintaining a low error rate. Our approach is based on data from control microarrays, and involves the use of standard statistical estimation techniques. RESULTS: After analyzing two experimental data sets containing control array data, we were able to determine the statistical power available for the detection of significant differential expression given differing levels of replication. The inclusion of replicate spots on microarrays not only allows more accurate estimation of the variability present in an experiment, but more importantly increases the probability of detecting genes undergoing significant fold changes in expression, while substantially decreasing the probability of observing fold changes due to chance rather than true differential expression.

Analysis of Variance↗
PubMed2005

Sample size determination for the false discovery rate.

MOTIVATION: There is not a widely applicable method to determine the sample size for experiments basing statistical significance on the false discovery rate (FDR). RESULTS: We propose and develop the anticipated FDR (aFDR) as a conceptual tool for determining sample size. We derive mathematical expressions for the aFDR and anticipated average statistical power. These expressions are used to develop a general algorithm to determine sample size. We provide specific details on how to implement the algorithm for a k-group (k > or = 2) comparisons. The algorithm performs well for k-group comparisons in a series of traditional simulations and in a real-data simulation conducted by resampling from a large, publicly available dataset. AVAILABILITY: Documented S-plus and R code libraries are freely available from www.stjuderesearch.org/depts/biostats.

Algorithms↗
PubMed2026

Phylogenomic subsampling and upsampling for efficient evolutionary analyses of big data.

Long runtimes, high memory demands, and reliance on high-performance computing impede phylogenomic analyses. We review a scalable phylogenomic subsampling with upsampling (PSU) framework to address this challenge, which reduces runtime and memory requirements by orders of magnitude. In PSU, small subsamples of sites from a concatenated alignment are analyzed, which are expanded by upsampling before inference, and the resulting inferences are aggregated to obtain evolutionary estimates. PSU harnesses the fact that the computational cost of maximum likelihood analysis is strongly influenced by the number of distinct site patterns in the concatenated alignment, whereas statistical power depends primarily on the amount of evolutionary information represented by the total number of sites and substitutions. By reducing the former while restoring the latter through upsampling, PSU can approximate many full-alignment analyses at substantially lower computational cost. Analysis of simulated and empirical datasets shows that PSU can accurately estimate bootstrap support values, select the optimal substitution model, test evolutionary hypotheses, and infer branch lengths, divergence times, and associated uncertainty measures. PSU also provides distributions of inferred clade support across independent subsamples, enabling detection of conflicting phylogenetic signals that may remain hidden in conventional bootstrap analysis of concatenated alignments. Automated tuning of subsample size, the number of subsamples, and the number of upsampling replicates make PSU practical. We suggest that PSU is a general approach for scalable phylogenomic inference using a broad range of statistical methods. By enabling analyses of genome-scale alignments on commodity hardware, PSU broadens research access and reduces environmental and infrastructural costs of big-data phylogenomics.

Phylogeny↗
PubMed1998

Cumulative risk adjusted mortality chart for detecting changes in death rate: observational study of heart surgery.

OBJECTIVE: To detect changes in mortality after surgery, with allowance being made for variations in case mix. DESIGN: Observational study of postoperative mortality from January 1992 to August 1995. SETTING: Regional cardiothoracic unit. SUBJECTS: 3983 patients aged 16 and over who had open heart operations. MAIN OUTCOME MEASURES: Preoperative risk factors and postoperative mortality in hospital within 30 days were recorded for all surgical heart operations. Mortality was adjusted for case mix using a preoperative estimate of risk based on additive Parsonnet factors. The number of operations required for statistical power to detect a doubling of mortality was examined, and control limits at a nominal significance level of P=0.01 for detection of an adverse trend were determined. RESULTS: Total mortality of 7.0% was 26% below the Parsonnet predictor (P<0.0001). There was a highly significant variation in annual case mix (Parsonnet scores 8.7-10.6, P<0.0001). There was no significant variation in mortality after adjustment for case mix (odds ratio 1-1.5, P=0.18) with monitoring by calendar year. With continuous monitoring, however, nominal 99% control limits based on 16 expected deaths were crossed on two occasions. CONCLUSIONS: Hospital league tables for mortality from heart surgery will be of limited value because year to year differences in death rate can be large (odds ratio 1.5) even when the underlying risk or case mix does not change. Statistical quality control of a single series with adjustment for case mix is the only way to take into account recent performance when informing a patient of the risk of surgery at a particular hospital. If there is an increase in the number of deaths the chances of the next patient surviving surgery can be calculated from the last 16 deaths.

Adolescent↗
PubMed2003

Prediction of precipitation-induced phlebitis: a statistical validation of an in vitro model.

To avoid phlebitis, new intravenous (IV) parenterals are often screened by injection into animals. This method is not only expensive and time consuming, it is also detrimental to the animals. An alternate method, focusing on precipitation as the cause, uses an in vitro dynamic injection model that requires less money and time and reduces the need for live models. Validation of the dynamic injection apparatus, for predicting mechanical phlebitis, is established. Twenty-one currently marketed IV products were injected into isotonic Sorenson's phosphate buffer flowing at 5 mL/min. The resulting opacities, produced by precipitation, are measured in an ultraviolet flow cell. These opacity data, coupled with literature reports on phlebitis occurrence, were used to generate a logistic regression that indicates the probability of phlebitis given an opacity value measured by the apparatus. Regression results are supported by a receiver operator characteristic curve that establishes the most ideal cut-off opacity value. This opacity value provides the highest combined sensitivity (statistical power) and specificity while minimizing false-positive and false-negative results. Both analyses show that an opacity value of 0.003 best delineates phlebitic and nonphlebitic products. Measures of sensitivity (0.83), specificity (0.93), positive predictive value (0.93), and negative predictive value (0.78) indicate the model's predictive accuracy and reliability. These results support the use of the dynamic model in place of animals for preliminary phlebitis testing of new IV injectables.

Chemical Precipitation↗
PubMed1998

Evaluation of different partial AUCs as indirect measures of rate of drug absorption in comparative pharmacokinetic studies.

The performance of different partial AUCs, including partial AUC from zero to t(max) of the reference formulation (AUC(r)) and partial AUC from zero to tmax of test or reference formulation, whichever occurs earliest (AUC(e), as indirect measures of rate of absorption have been evaluated using simulated experiments. The performance of these metrics relative to C(max), t(max) and C(max)/AUC(infinity) was further assessed using the results of actual studies involving a Glaxo drug. The normalised metrics AUC(r)/AUC(infinity) and AUC(e)/AUC(infinity) have also been evaluated. Our provisional conclusions were: (1) AUC(r)/AUC(infinity) and AUC(e)/AUC(infinity) had greater statistical power than C(max) and the non-normalised partial AUCs at detecting true differences in rate of absorption. Using real data, the performance of AUC(e)/AUC(infinity) was poor, however, the performance of AUC(r)/AUC(infinity) was good; (2) C(max)/AUC(infinity) was more precisely estimated than AUC(r)/AUC(infinity) or AUC(e)/AUC(infinity) and may be a superior metric for assessing absorption rates of highly variable drugs.

Area Under Curve↗
PubMed2000

A test for linkage and association in general pedigrees: the pedigree disequilibrium test.

Family-based tests of linkage disequilibrium typically are based on nuclear-family data including affected individuals and their parents or their unaffected siblings. A limitation of such tests is that they generally are not valid tests of association when data from related nuclear families from larger pedigrees are used. Standard methods require selection of a single nuclear family from any extended pedigrees when testing for linkage disequilibrium. Often data are available for larger pedigrees, and it would be desirable to have a valid test of linkage disequilibrium that can use all potentially informative data. In this study, we present the pedigree disequilibrium test (PDT) for analysis of linkage disequilibrium in general pedigrees. The PDT can use data from related nuclear families from extended pedigrees and is valid even when there is population substructure. Using computer simulations, we demonstrated validity of the test when the asymptotic distribution is used to assess the significance, and examined statistical power. Power simulations demonstrate that, when extended pedigree data are available, substantial gains in power can be attained by use of the PDT rather than existing methods that use only a subset of the data. Furthermore, the PDT remains more powerful even when there is misclassification of unaffected individuals. Our simulations suggest that there may be advantages to using the PDT even if the data consist of independent families without extended family information. Thus, the PDT provides a general test of linkage disequilibrium that can be widely applied to different data structures.

Alleles↗