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PubMed1989

Effect of crossover on the statistical power of randomized studies.

Randomized studies involving long-term follow-up are vulnerable to the effects of unplanned crossover. In surgical studies, such crossover usually occurs when control patients become more symptomatic and undergo operation. In several large studies of coronary bypass grafting, crossover ranged from 25% to 38%. The most common way of dealing with this problem is to apply the "intention-to-treat" principle, which analyzes such crossovers with their originally assigned groups. Besides the logical problem of counting a control patient who actually undergoes operation as "nonsurgical," a more subtle problem arises in terms of statistical power. When statistical power is low, a truly effective treatment may be mistakenly labeled as no better than control, causing a potentially valuable form of therapy to be ignored or discarded. This analysis demonstrates that crossover may have a profound effect on the statistical power of randomized studies and presents a method for predicting the effect of such crossover on statistical power.

Coronary Artery Bypass↗
PubMedDate not supplied

Charting patients' course: a comparison of statistics used to summarize patient course in longitudinal and repeated measures studies.

Investigators conducting longitudinal studies of psychiatric illnesses often analyze data based on psychiatric symptom scales that were administered at multiple time points. This study examines the statistical properties of seven indices that summarize patient long-term course. These indices can be used to compare differences between two or more groups or to test for changes in symptoms over time. They may also be treated as outcome measures and correlated with other clinical variables.The performance of each of the seven indices was assessed using data from two large ongoing studies of psychiatric patients: a longitudinal study of affective disorders and a longitudinal study of first-episode psychosis. These two datasets were subjected to bootstrapping techniques in order to calculate both type I error rates and statistical power for each summary statistic. Of the seven indices, Kendall's tau performed the best as a measure of patients' symptom course. Kendall's tau appears to offer more statistical power to detect change in course, yet its average type I error rate was comparable to the other indices.

Adult↗
PubMed2002

Optimizing detection of QTLs retarding aging: choice of statistical model and animal requirements.

Quantitative trait locus (QTL) analysis makes no assumptions about the identity of genes involved in regulating aging. Moreover, it may be used as the first step in identifying such genes and, thus QTL analysis may be instrumental in formulating new hypotheses about aging. Genetic experiments, however, require hundreds to thousands of animals and are very expensive in mammals. Statistical power to detect longevity genes could be improved by excluding accidental, unrelated to aging mortality. While many early deaths are probably accidental, excluding early mortality altogether eliminates the age-related component, too. We used computer simulations to assess the effect of excluding early age-related, mortality on the statistical power of several common tests, such as t-test, Mann-Whitney and chi(2). Surprisingly, even the age-related, Gompertz component of early mortality reduces the statistical power of the t- and Mann-Whitney tests. For example, in a backcross design, to detect a gene slowing down the rate of aging and increasing mouse life span by 10% (P=0.0001; power=0.8), a regular t-test will require 640 mice, all kept for the entire life span and genotyped. If life spans of only 25% of the longest-lived animals from each of the two groups, carrying a putative longevity allele and not carrying it, are compared, population size can be reduced by two-fold, to about 300, and genotyping by seven-fold, to 90. Confirming simulation results, the significance of the effect of caloric restriction on life span increased from P=3.4x10(-5) to 1.1x10(-7), when life spans of only 40% of the longest-lived mice from each of the two groups, ad libitum fed and calorie restricted, were compared. Finally, finding the optimal combination of statistical test, the number of phenotyped and the number of genotyped animals, which would minimize experimental costs was addressed.

Aging↗
PubMed1983

Evaluating the medical literature. Part II: Statistical analysis.

We have attempted to provide sufficient information to enable the reader to verify that the investigator has used an appropriate statistical test for the evaluation of his study data. We have not discussed the actual calculation of the tests presented. With the wide availability of computers and programmable calculators, it is safe to assume that the investigator has performed the necessary mathematics accurately. Instead of assessing the accuracy of these calculations, the reader should verify that the correct statistical test was chosen in the first place. The astute reader may be surprised at how frequently an incorrect test is used. Merely achieving statistical significance does not characterize the author's data as clinically important. Neither does it, in and of itself, prove one agent superior to another nor prove a cause-and-effect relationship between two variables. When appropriately interpreted, however, statistical analysis can be a very useful and powerful tool in helping to arrive at the "truth."

Analysis of Variance↗
PubMed1997

Statistical planning and analysis considerations in the evaluation of in vitro alternatives to whole animal use for eye irritation testing.

Reports from the IRAG Working Groups assessing the current status of in vitro alternatives to whole animal eye irritation tests reflect some common approaches. Although each Working Group studied a particular class of assay, typically all Groups evaluated the in vitro alternatives on the basis of correlation with an in vivo test and the statistical significance of that correlation. However, the data furnished to them by the testing organizations had been obtained with little or no standardization of procedures for testing and evaluation of results. This paper presents issues of design and execution of such test programs that are of statistical concern, including objectives of the evaluation process; limitations of correlation; sources of variation and distinction between actual replication and repeated measurements; evaluation of the predictive ability of in vitro tests; association criteria; and other approaches to such evaluation programs. The distinction between statistical significance and toxicological significance is pointed out. Suggestions are presented for standardization of test protocols, test materials and evaluation programs for both in vitro and in vivo tests. A central repository for such standardization, under the aegis of a government agency, professional standards organization, university or research institute, would be a valuable asset. An addendum is furnished to illustrate assertions about a number of factors influencing correlations and their statistical significance.

Animal Testing Alternatives↗
PubMed2003

Assessment of statistic analysis in non-radioisotopic local lymph node assay (non-RI-LLNA) with alpha-hexylcinnamic aldehyde as an example.

The murine local lymph node assay (LLNA) is used for the identification of chemicals that have the potential to cause skin sensitization. However, it requires specific facility and handling procedures to accommodate a radioisotopic (RI) endpoint. We have developed non-radioisotopic (non-RI) endpoint of LLNA based on BrdU incorporation to avoid a use of RI. Although this alternative method appears viable in principle, it is somewhat less sensitive than the standard assay. In this study, we report investigations to determine the use of statistical analysis to improve the sensitivity of a non-RI LLNA procedure with alpha-hexylcinnamic aldehyde (HCA) in two separate experiments. Consequently, the alternative non-RI method required HCA concentrations of greater than 25% to elicit a positive response based on the criterion for classification as a skin sensitizer in the standard LLNA. Nevertheless, dose responses to HCA in the alternative method were consistent in both experiments and we examined whether the use of an endpoint based upon the statistical significance of induced changes in LNC turnover, rather than an SI of 3 or greater, might provide for additional sensitivity. The results reported here demonstrate that with HCA at least significant responses were, in each of two experiments, recorded following exposure of mice to 25% of HCA. These data suggest that this approach may be more satisfactory-at least when BrdU incorporation is measured. However, this modification of the LLNA is rather less sensitive than the standard method if employing statistical endpoint. Taken together the data reported here suggest that a modified LLNA in which BrdU is used in place of radioisotope incorporation shows some promise, but that in its present form, even with the use of a statistical endpoint, lacks some of the sensitivity of the standard method. The challenge is to develop strategies for further refinement of this approach.

Acrolein↗
PubMed1997

Statistical framework for ultrasonic spectral parameter imaging.

This study examines the statistics of ultrasonic spectral parameter images that are being used to evaluate tissue microstructure in several organs. The parameters are derived from sliding-window spectrum analysis of radiofrequency echo signals. Calibrated spectra are expressed in dB and analyzed with linear regression procedures to compute spectral slope, intercept and midband fit, which is directly related to integrated backscatter. Local values of each parameter are quantitatively depicted in gray-scale cross-sectional images to determine tissue type, response to therapy and physical scatterer properties. In this report, we treat the statistics of each type of parameter image for statistically homogeneous scatterers. Probability density functions are derived for each parameter, and theoretical results are compared with corresponding histograms clinically measured in homogeneous tissue segments in the liver and prostate. Excellent agreement was found between theoretical density functions and data histograms for homogeneous tissue segments. Departures from theory are observed in heterogeneous tissue segments. The results demonstrate how the statistics of each spectral parameter and integrated backscatter are related to system and analysis parameters. These results are now being used to guide the design of system and analysis parameters, to improve assays of tissue heterogeneity and to evaluate the precision of estimating features associated with effective scatterer sizes and concentrations.

Carcinoma↗
PubMed1996

Menstrual phase and breast cancer surgery: influence on clinical outcome or pitfall of statistical analysis?

The influence of menstrual status at the time of surgery on the prognosis of women suffering from breast cancer is still discussed controversially. In our patient collective, including 149 patients, we obtained statistically significant results for six different time intervals, indicating that patients who underwent surgery between 11 and 22 days after the last menstrual period (LMP) have a poorer outcome. Focusing on the effect of statistical data evaluation strategy we designed a simulation study to evaluate the amount of type I error (error of a false positive test result) in a multiple testing situation involving a cyclical covariate. Accordingly, we corrected the minimum P-values for the occurring type I error rates. After that correction all six previously significant P-values failed to achieve statistical significance. The impact of different statistical data evaluation strategies in a multiple testing situation is discussed.

Adult↗
PubMed1997

The dynamics of patient visits to a public hospital ED: a statistical model.

Using a public hospital's computerized database, we formulated a statistical model to explain emergency department (ED) patient volume for better staffing and resource allocation. All patients visiting the ED over a 3-year period were included in this retrospective study. Each observation described the total daily number of referrals and was defined by the following variables: day of the week, month of the year, holiday/ weekday, relative order in a 3-year sequence, and number of visits to the ED on that day. The statistical method used to build the model was analysis of covariance. Periodicity in average number of daily visits existed for each of the seasonal factors that were examined, repeating every year during the study period. Based on a graphic analysis, the model was defined and explained 65% of the variance during the 3-year study, with a relatively low standard deviation of error. A statistically significant correlation existed between time-related factors and the number of visits to the ED. This statistical model may prove to be of value for planning emergency services, which operate under stressful, unpredictable situations.

Analysis of Variance↗
PubMed2003

The level of compressive load affects conclusions from statistical analyses to determine whether a lateral meniscal autograft restores tibial contact pressure to normal: a study in human cadaveric knees.

This study addressed the question of whether the level of compressive load would affect the conclusions from statistical analyses aimed at determining how well a lateral meniscal autograft restores tibial contact (as indicated by the maximum contact pressure, mean pressure, and contact area) to that of the intact knee. If statistical analyses indicated that normal tibial contact was not restored with a higher, more physiologic load, then a secondary question was whether an autograft surgically implanted with bone plugs would improve tibial contact compared to that in a meniscectomized knee. Nine, fresh-frozen human cadaveric knees were subjected to a low, non-physiologic compressive load of 400 N and a higher, more physiologic compressive load of 1200 N under three conditions (lateral meniscus intact, lateral meniscus removed and reimplanted as an autograft, and lateral meniscus removed). Contact pressure on the lateral plateau was measured with pressure sensitive film at 0 degrees, 15 degrees, 30 degrees, and 45 degrees of flexion. At 400 N, p-values from statistical analyses indicated that both the maximum and mean pressures with the autograft were comparable to those of the intact knee (p> or =0.685). However, at 1200 N, p-values from statistical analyses indicated that both the maximum and mean pressures with the autograft were significantly greater than those of the intact knee (p< or =0.0001). Therefore studies designed to evaluate tibial contact pressure for a meniscal transplant should use a higher, more physiologic compressive load, because lower loads overestimate the transplant's effectiveness. Although none of the contact variables was restored to normal when the compressive load was increased to 1200 N, all of the contact variables were more normal than those of the meniscectomized knee. Thus, lateral meniscal allografts implanted using bone plugs can significantly improve contact pressure relative to a meniscectomized knee at the time of implantation.

Adult↗
PubMedDate not supplied

Neural ensemble coding and statistical periodicity: speculations on the operation of the mind's eye.

Statistical periodicity is a statistical property of densities which arises in the description of retarded dynamical systems. This property is particularly attractive as a possible mechanism for the ensemble coding of information in the nervous system because it operates rapidly and has high storage capacity. For a population of neurons which exhibits statistical periodicity, information would not be encoded by the periodicity, but rather by the spatio-temporal distributions of neural activity. Statistical periodicity is discussed in relation to the temporal binding hypothesis and to the occurrence of multistability in neural systems.

Animals↗
PubMed1997

The significance of statistical significance.

Currently, much nursing practice is based on limited evidence, for example, small-scale research, case studies and clinical experience. In a mature science this would be undesirable, but nursing is in the early stages of development as a science, and many of its practices depend on relatively informal knowledge. To encourage the spread of potentially valuable ideas, nurses must be willing to share their clinical experience and journal editors should consider publishing this information. High-quality research is essential to the long-term development of 'evidence-based practice', but it is crucial at the present stage of nursing science that we do not become too concerned with perfect research methodology at the expense of good ideas. This particularly applies to tests of statistical significance. If we accept only information that has demonstrated statistical significance, we risk the dismissal of qualitative research and other information which may be extremely valuable but which have not yet been fully investigated. The aim of this paper is to convince practitioners and journal editors that statistical significance is not the only way to judge clinical importance and to suggest that decisions on what should be submitted and accepted for publication should be based on potential clinical relevance as well as statistical analysis.

Data Interpretation, Statistical↗
PubMed1998

The significance of statistical significance.

Currently, much nursing practice is based on limited evidence, for example, small-scale research, case studies and clinical experience. In a mature science this would be undesirable, but nursing is in the early stages of development as a science, and many of its practices depend on relatively informal knowledge. To encourage the spread of potentially valuable ideas, nurses must be willing to share their clinical experience and journal editors should consider publishing this information. High-quality research is essential to the long-term development of 'evidence-based practice', but it is crucial at the present stage of nursing science that we do not become too concerned with perfect research methodology at the expense of good ideas. This particularly applies to tests of statistical significance. If we accept only information that has demonstrated statistical significance, we risk the dismissal of qualitative research and other information which may be extremely valuable but which have not yet been fully investigated. The aim of this paper is to convince practitioners and journal editors that statistical significance is not the only way to judge clinical importance and to suggest that decisions on what should be submitted and accepted for publication should be based on potential clinical relevance as well as statistical analysis.

Bias↗
PubMed2003

Statistical optimization for immobilized metal affinity purification of secreted human erythropoietin from Drosophila S2 cells.

We used a novel approach to affinity purify human erythropoietin (hEPO) following its secretion from Drosophila melanogaster S2 cells. Immobilized metal affinity purification of hEPO was optimized using a two-step serial statistical optimization strategy. After determining the elution conditions (based on preliminary batch-type purification experiments), the first optimization step considered three purification factors; resin, equilibrium, and washing. The results of this analysis showed that the resin amount was the major factor influencing yield and purity in both model equations and the washing factor lowered the confidence limits of the acquired model equations. The washing conditions were then set based on the results of the first step optimization and the second step then optimized three factors; resin, equilibrium, and elution. The yield and purity of hEPO were then compared following purification using three different approaches; batch-type purification based upon the conditions determined by serial statistical optimization, batch-type purification performed in preliminary experiments, and FPLC column chromatography-type purification. We found that the serial statistical optimization approach provided the best combination of yield and purity. These findings indicate that serial statistical optimization strategies can be successfully employed for immobilized metal affinity protein purification using either batch-type or column approaches.

Animals↗
PubMed2005

The reading, writing, and arithmetic of the medical literature, part 2: critical evaluation of statistical reporting.

OBJECTIVE: To offer suggestions to help improve clinicians' understanding of the statistical analyses in the literature and their use of these methods in their own medical writings. DATA SOURCES: Literature searches began at the National Library of Medicine's online database and were traced to primary sources. STUDY SELECTION: All referenced information in this article was cited from primary sources. RESULTS: Physicians should be able to determine the variables studied and how they were measured, the comparisons that were made, the difference (with 95% confidence interval) between the groups, the exact P value for the difference, the statistical test used in the analysis, whether the data conformed to the assumptions of the test, whether the study had adequate statistical power, and the clinical importance of the difference. CONCLUSION: Clinicians should know how to interpret statistical results so that they can use medical science to its full extent in treating patients.

Biomedical Research↗
PubMed2000

Physical model-based non-rigid registration incorporating statistical shape information.

This paper describes two new atlas-based methods of 2D single modality non-rigid registration using the combined power of physical and statistical shape models. The transformations are constrained to be consistent with the physical properties of deformable elastic solids in the first method and those of viscous fluids in the second, to maintain smoothness and continuity. A Bayesian formulation, based on each physical model, an intensity similarity measure, and statistical shape information embedded in corresponding boundary points, is employed to derive more accurate and robust approaches to non-rigid registration. A dense set of forces arises from the intensity similarity measure to accommodate complex anatomical details. A sparse set of forces constrains consistency with statistical shape models derived from a training set. A number of experiments were performed on both synthetic and real medical images of the brain and heart to evaluate the approaches. It is shown that statistical boundary shape information significantly augments and improves physical model-based non-rigid registration and the two methods we present each have advantages under different conditions.

Brain↗
PubMed2005

Biological validation of differentially expressed genes in chronic lymphocytic leukemia identified by applying multiple statistical methods to oligonucleotide microarrays.

Oligonucleotide microarrays are a powerful tool for profiling the expression levels of thousands of genes. Different statistical methods for identifying differentially expressed genes can yield different results. To our knowledge, no experimental test has been performed to decide which method best identifies genes that are truly differentially expressed. We applied three statistical methods (dChip, t-test on log-transformed data, and Wilcoxon test) to identify differentially expressed genes in previously untreated patients with chronic lymphocytic leukemia (CLL). We used a training set of Affymetrix Hu133A microarray data from 11 patients with unmutated immunoglobulin (Ig) heavy chain variable region (VH) genes and 8 patients with mutated Ig VH genes. Differential expression was validated using semiquantitative real-time polymerase chain reaction assays and by validating models to predict the somatic mutation status of an independent test set of nine CLL samples. The methods identified 144 genes that were differentially expressed between cases of CLL with unmutated compared with mutated Ig VH genes. Eighty genes were identified by Wilcoxon test, 60 by t-test, and 65 by dChip, but only 11 were identified by all three methods. Greater agreement was found between the t-test and the Wilcoxon test. Differential expression was validated by semiquantitative real-time polymerase chain reaction assays for 83% of individual genes, regardless of the statistical method. However, the Wilcoxon test gave the most accurate predictions on new samples, and dChip, the least accurate. We found that all three methods were equally good for finding differentially expressed genes, but they found different genes. The genes selected by the nonparametric Wilcoxon test are the most robust for predicting the status of new cases. A comprehensive list of all differentially expressed genes can only be obtained by combining the results of multiple statistical tests.

Algorithms↗
PubMed1984

A statistical modelling approach to community prevalence data.

Sample survey techniques are often used to assess the prevalence of illness in a community and to determine any variation with environmental, social and demographic factors. Analysis of survey data is often carried out using several elementary statistical procedures. The formulation of a statistical model is an effective way of conducting a unified analysis. The model provides a concise description of the study population and is a most effective way of summarizing community prevalence data. The testing of statistical hypotheses is equivalent to model simplification and is conveniently performed using general procedures. Two alternative statistical models are given for an investigation into the extent of minor psychiatric morbidity in Perth, Western Australia.

Adolescent↗