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PubMed2004

The Global Error Assessment (GEA) model for the selection of differentially expressed genes in microarray data.

MOTIVATION: Microarray technology has become a powerful research tool in many fields of study; however, the cost of microarrays often results in the use of a low number of replicates (k). Under circumstances where k is low, it becomes difficult to perform standard statistical tests to extract the most biologically significant experimental results. Other more advanced statistical tests have been developed; however, their use and interpretation often remain difficult to implement in routine biological research. The present work outlines a method that achieves sufficient statistical power for selecting differentially expressed genes under conditions of low k, while remaining as an intuitive and computationally efficient procedure. RESULTS: The present study describes a Global Error Assessment (GEA) methodology to select differentially expressed genes in microarray datasets, and was developed using an in vitro experiment that compared control and interferon-gamma treated skin cells. In this experiment, up to nine replicates were used to confidently estimate error, thereby enabling methods of different statistical power to be compared. Gene expression results of a similar absolute expression are binned, so as to enable a highly accurate local estimate of the mean squared error within conditions. The model then relates variability of gene expression in each bin to absolute expression levels and uses this in a test derived from the classical ANOVA. The GEA selection method is compared with both the classical and permutational ANOVA tests, and demonstrates an increased stability, robustness and confidence in gene selection. A subset of the selected genes were validated by real-time reverse transcription-polymerase chain reaction (RT-PCR). All these results suggest that GEA methodology is (i) suitable for selection of differentially expressed genes in microarray data, (ii) intuitive and computationally efficient and (iii) especially advantageous under conditions of low k. AVAILABILITY: The GEA code for R software is freely available upon request to authors.

Algorithms↗

PubMed2005

Sensitivity-encoded (SENSE) echo planar fMRI at 3T in the medial temporal lobe.

Parallel imaging techniques are useful for fMRI studies in light of the increasing susceptibility effects at high magnetic field strength. Yet, spatially varying noise amplification constitutes a challenge for the application of these techniques. The medial temporal lobe is particularly vulnerable to susceptibility effect with increasingly strong signal reduction. We present two fMRI studies comparing SENSE single-shot (ssh) echo planar imaging (EPI) at acceleration factors of 2.0, 2.4, 2.7, and 3.0 with conventional sshEPI at TE of 22 and 35 ms. Data were acquired during a learning task which activates the medial temporal lobe bilaterally. Susceptibility related image distortion was markedly reduced with increasing SENSE acceleration. Moreover, in the group results, statistical power increased in the whole brain with SENSE compared to conventional imaging and with a TE of 35 ms compared to 22 ms. Higher SENSE acceleration factors further improved image quality and increased statistical power in the occipital lobe and fusiform gyrus, but not in the medial temporal lobe. We therefore conclude that an sshEPI acquisition protocol with a moderate SENSE acceleration factor of R = 2.0 and TE 35 ms is suitable for the detection of medial temporal activation at 3T.

Adult↗

PubMed1995

Performance of odds ratios obtained with a job-exposure matrix and individual exposure assessment with special reference to misclassification errors.

OBJECTIVES: Individual assessment of exposure by experts and the use of a job-exposure matrix are the two main methods of evaluating past occupational exposures in community-based case-referent studies. The objective of this study was to compare the performance of the estimations of the odds ratio in the two methods. This paper focuses on job-exposure matrices whose entries consist of proportions of persons exposed. METHODS: Simulations were used to compare the variances of the estimations of the odds ratios obtained with the two methods and to study the consequences with respect to bias and the precision of the odds ratios estimated for misclassifications of exposure produced by either the experts or the matrix. RESULTS: When there was no misclassification, the results showed that the precision obtained with the job-exposure matrix was about three times less than that achieved by experts in a large range of practical situations. However, when potential errors of exposure assessment were taken into account, the simulations suggested that the test of the hypothesis OR = 1 against the alternative OR not equal to 1 when exposure was assessed with an unbiased job-exposure matrix had a statistical power close to that obtained when exposure was assessed by an expert with high sensibility and specificity. CONCLUSIONS: The evaluation of exposure with an unbiased job-exposure matrix in studies of the association between exposure and disease had a statistical power close to that expected in practice with a good expert in the large range of practical situations which were investigated.

Bias↗

PubMedDate not supplied

Monitoring the impact of Bt maize on butterflies in the field: estimation of required sample sizes.

The monitoring of genetically modified organisms (GMOs) after deliberate release is important in order to assess and evaluate possible environmental effects. Concerns have been raised that the transgenic crop, Bt maize, may affect butterflies occurring in field margins. Therefore, a monitoring of butterflies was suggested accompanying the commercial cultivation of Bt maize. In this study, baseline data on the butterfly species and their abundance in maize field margins is presented together with implications for butterfly monitoring. The study was conducted in Bavaria, South Germany, between 2000-2002. A total of 33 butterfly species was recorded in field margins. A small number of species dominated the community, and butterflies observed were mostly common species. Observation duration was the most important factor influencing the monitoring results. Field margin size affected the butterfly abundance, and habitat diversity had a tendency to influence species richness. Sample size and statistical power analyses indicated that a sample size in the range of 75 to 150 field margins for treatment (transgenic maize) and control (conventional maize) would detect (power of 80%) effects larger than 15% in species richness and the butterfly abundance pooled across species. However, a much higher number of field margins must be sampled in order to achieve a higher statistical power, to detect smaller effects, and to monitor single butterfly species.

Agriculture↗

PubMed2000

N-acetyltransferase polymorphisms and colorectal cancer: a HuGE review.

The two expressed genes coding for N-acetyltransferase (NAT) activity, NAT1 and NAT2, are located on chromosome 8 at 8p21.3-23.1 and are polymorphic. Both enzymes are capable of N-acetylation, O-acetylation, and N,O-acetylation and are implicated in the activation and detoxification of known carcinogens. Single base-pair substitutions in NAT2 tend to occur in combination with other substitutions within the gene. As yet, less work has been done to characterize NAT1 allelic variants. Various methods for the detection of the reported polymorphisms exist. It is important to select a method that is appropriate to the population being studied. The functional significance of many NAT allelic variants has not been determined. Geographic and ethnic variation in the frequency of NAT2 genotypes associated with fast or intermediate acetylation has been observed. Insufficient data for NAT1 genotypes are available to reveal a clear geographic pattern. No consistent association has been found between acetylator phenotype or genotype and colorectal cancer. The lack of consistency can in part be accounted for by methodological factors, including limited statistical power. Possible interactions between the NAT genes and either environmental exposures or other polymorphic genes encoding xenobiotic metabolizing enzymes have been investigated in only a minority of these studies, and these studies have lacked statistical power to detect interactions.

Acetyltransferases↗

PubMed2000

Polysaccharide vaccines for preventing serogroup A meningococcal meningitis.

BACKGROUND: Controlled trials over two decades ago showed that the polysaccharide vaccine prevented meningococcal meningitis. Subsequent observational studies suggested variations in the level and duration of protection, particularly among young children. OBJECTIVES: To determine the effect of polysaccharide serogroup A vaccine for preventing serogroup A meningococcal meningitis. SEARCH STRATEGY: MEDLINE and the Cochrane Controlled Trials Register. SELECTION CRITERIA: Randomised and other prospective controlled trials. DATA COLLECTION AND ANALYSIS: One reviewer collected data and assessed the methodologic quality of the trials. Data were pooled using the Exact method to assess vaccine efficacy at one, two and three years post-vaccination. MAIN RESULTS: The protective effect within the first year was consistent across all eight trials, including one with participants exclusively under six years of age (in Finland); overall vaccine efficacy was 95% (Exact 95% CI 87%, 99%). The Finnish trial lacked statistical power to assess the effect of a booster dose given to children less than 18 months old. In the three other trials that included children less than six years old (one in Sudan and two in Nigeria), none of the vaccinated children developed meningitis, but the statistical significance of this finding was undetermined. Protection extended into the second (in two studies) and third (in one study) years after vaccination, but these results were also not statistically significant. REVIEWER'S CONCLUSIONS: When compared with current recommendations, the methodological quality and relative incompleteness of the published reports could arguably render the trials invalid for this review. However, it was unlikely that the results of the trials in such diverse settings would have been biased towards a strong and consistent protective effect. Immunogenicity trials were not included in this review. Stage two of the review will assess the vaccine effect from observational studies.

Humans↗

PubMed1996

A critical assessment of the relationship between silicone breast implants and connective tissue diseases.

Concerns regarding the possible role of breast implants (particularly silicone breast implants) in the development of connective tissue diseases were raised by case reports of connective tissue diseases in women with breast implants. Case reports, however, are not appropriate for causation assessment. Within the past few years, epidemiologic studies have begun to appear. Based on a comprehensive literature search, 15 epidemiologic studies on breast implants and connective tissue diseases, which satisfied certain basic epidemiologic requirements, were included in the critical assessment. These studies utilized either the case-control or the cohort study design. Although each individual study was relatively small, and the statistical power to detect a modest risk increase in specific categories of connective tissue diseases was limited, the results of these studies, however, were strikingly consistent, particularly those reported in case-control studies. To increase statistical power and to take the consistency of results into consideration, meta-analyses were used to summarize results from individual studies quantitatively. Based on data from case-control studies, meta-analyses of rheumatoid arthritis, systemic sclerosis (scleroderma), and systemic lupus erythematosus were performed. These case-control studies represented a combined database of approximately 4000 cases of connective tissue diseases, and the power was sufficient to detect a relatively small increase in risk. Based on the meta-analyses, the relative risks (95% confidence intervals) were 0.85 (0.48-1.51) for rheumatoid arthritis, 0.82 (0. 50-1.35) for systemic sclerosis, and 0.33 (0.06-2.03) for systemic lupus erythematosus, indicating that there was no increased risk of connective tissue diseases associated with breast implants. The findings derived from the meta-analyses of case-control studies were supported by results from cohort or prospective studies. It was concluded that epidemiologic data did not provide any evidence for a causal relationship between silicone breast implants and connective tissue diseases.

Arthritis, Rheumatoid↗

PubMed1999

Allowing for missing parents in genetic studies of case-parent triads.

In earlier work, my colleagues and I described a log-linear model for genetic data from triads composed of affected probands and their parents. This model allows detection of and discrimination between effects of an inherited haplotype versus effects of the maternal haplotype, which presumably would be mediated by prenatal factors. Like the transmission disequilibrium test (TDT), the likelihood-ratio test (LRT) based on this model is not sensitive to associations that are due to genetic admixture. When used as a method for testing for linkage disequilibrium, the LRT can be regarded as an alternative to the TDT. When one or both parents are missing, the resulting incomplete triad must be discarded to ensure validity of the TDT, thereby sacrificing information. By contrast, when the problem is set in a likelihood framework, the expectation-maximization algorithm allows the incomplete triads to contribute their information to the LRT without invalidation of the analysis. Simulations demonstrate that much of the lost statistical power can be recaptured by means of this missing-data technique. In fact, power is reasonably good even when no triad is complete-for example, when a study is designed to include only mothers of cases. Information from siblings also can be incorporated to further improve the statistical power when genetic data from parents or probands are missing.

Algorithms↗

PubMed1988

Therapeutic trial of isaxonine in Duchenne muscular dystrophy.

A randomized double-blind therapeutic trial of isaxonine was completed over a 2-year period for 20 ambulant boys with Duchenne muscular dystrophy aged 5 1/2-10 years. The effect of the drug was monitored by measurement of walking times over 28 and 150 ft, motor ability score, MRC score based on 32 muscle groups, and myometry of 7 muscle groups. The drug had no significant effect on the progression of the disease. The trial had statistical power comparable to previous larger-scale multicenter trials. This reflected the low variability in the patients in relation to the magnitude of the overall deterioration. Measurements of muscle force (myometry and MRC score) had much greater statistical power than measurements of function (motor ability score and walking times) as analyzed by our methods. These observations have important implications for the design of future trials.

Child↗

PubMed2006

[The European Society of Cardiology (ESC) guidelines for percutaneous coronary interventions (PCI). Three case reports].

The European Society of Cardiology (ESC) for the first time issued guidelines for percutaneous coronary interventions (PCI) in spring 2005. The strengths of recommendations stated in the ESC guidelines (as in those of the AHA/ACC [American Heart Association/American College of Cardiology]) are traditionally a combination of recommendation classes (I, IIa, and IIb) and a level of evidence (A, B, or C). This paper explains and discusses selected focal points of the ESC PCI guidelines based on three representative cases from daily practice. 1. Stable coronary artery disease (CAD): PCI in a 53-year-old patient without angina pectoris and proof of myocardial ischemia. With a clear indication of ischemia in the anterior myocardial wall, the ESC PCI guidelines indicated coronary angiography with possible PCI, even without angina pectoris symptoms. Cardiac catheterization showed a 99% proximal LAD stenosis, which was immediately dilated and stented based on the indicated ischemia. According to the ESC PCI guidelines, an intervention is indicated for CAD when a larger ischemic area is clearly evident even in the absence of typical angina (recommendation class I A). 2. ST segment elevation myocardial infarction (STEMI): PCI even after successful thrombolysis. A 70-year-old patient experienced acute substernal pain and immediately went to his nearby hospital. The ECG clearly showed anterior myocardial wall STEMI, which in this hospital without a cardiac cath lab indicated thrombolysis, since it could be initiated within 3 h after the onset of chest pain. Pain relief was evident soon after thrombolysis, combined with a resolution of the ST segment elevations. As suggested by the ESC PCI guidelines, a transfer to a cardiac cath lab took place the next day, where the 50% residual stenosis of the LAD was stented. The ESC PCI guidelines suggest coronary angiography with possible PCI within 1-2 days following successful thrombolysis (recommendation class I A). Thus, even "successful" thrombolysis is not regarded as the final treatment for STEMI. 3. Premature termination of clopidogrel after stent implantation: stent thrombosis with acute myocardial infarction. A 46-year-old patient visited the practice due to increasing dyspnea. 4 months earlier, a Taxus stent had been implanted at a heart center into the second RPLS of the RCX; 3 days later, a Cypher stent was implanted in the LAD. Upon being discharged on a Friday at noon, the patient was advised to see his general practitioner soon to attain a prescription for clopidogrel. The patient was given an appointment at his general practitioner for the following Wednesday afternoon. But on that Wednesday morning the patient went into cardiogenic shock. Although the occluded LAD (stent thrombosis) could be quickly reopened, left ventricular myocardium became severely damaged. Until a cardiac transplantation will be performed, a defibrillator was implanted. This "organizational" gap in clopidogrel administration did not conform to the ESC PCI guidelines: after implantation of any coronary stent, dual antiplatelet treatment (acetylsalicylic acid and clopidogrel) must be consistently administered for at least 4 weeks. After implantation of drug-eluting stents (DES), the ESC PCI guidelines call for clopidogrel administration for at least 6 months; when small vessels, long lesions or a complex anatomy (e. g., bifurcation stenting) are involved, a duration of 1 year or even longer is recommended. The optimal duration of platelet aggregation inhibition following PCI with DES of unprotected left main stem stenoses is unknown at this time. The traditional levels of evidence according to ESC, AHA and ACC criteria (levels A, B, or C) do no longer meet the actual requirements to assess the scientific evidence of randomized PCI trials and registry studies. For example, only two small randomized studies with few patients and insufficient statistical power utilizing a clinically insignificant surrogate endpoint would be enough to attain level of evidence A. Consequently, a new scoring system will be proposed, which considers criteria such as the importance of a primary clinical endpoint, the statistical power achieved, and the presence of an independent external data review and safety monitoring board.

Aged↗

PubMed2001

Comparing meta-analysis and ecological-longitudinal analysis in time-series studies. A case study of the effects of air pollution on mortality in three Spanish cities.

STUDY OBJECTIVE: The objective of this paper is to introduce a different approach, called the ecological-longitudinal, to carrying out pooled analysis in time series ecological studies. Because it gives a larger number of data points and, hence, increases the statistical power of the analysis, this approach, unlike conventional ones, allows the complementation of aspects such as accommodation of random effect models, of lags, of interaction between pollutants and between pollutants and meteorological variables, that are hardly implemented in conventional approaches. DESIGN: The approach is illustrated by providing quantitative estimates of the short-term effects of air pollution on mortality in three Spanish cities, Barcelona, Valencia and Vigo, for the period 1992-1994. Because the dependent variable was a count, a Poisson generalised linear model was first specified. Several modelling issues are worth mentioning. Firstly, because the relations between mortality and explanatory variables were non-linear, cubic splines were used for covariate control, leading to a generalised additive model, GAM. Secondly, the effects of the predictors on the response were allowed to occur with some lag. Thirdly, the residual autocorrelation, because of imperfect control, was controlled for by means of an autoregressive Poisson GAM. Finally, the longitudinal design demanded the consideration of the existence of individual heterogeneity, requiring the consideration of mixed models. MAIN RESULTS: The estimates of the relative risks obtained from the individual analyses varied across cities, particularly those associated with sulphur dioxide. The highest relative risks corresponded to black smoke in Valencia. These estimates were higher than those obtained from the ecological-longitudinal analysis. Relative risks estimated from this latter analysis were practically identical across cities, 1.00638 (95% confidence intervals 1.0002, 1.0011) for a black smoke increase of 10 microg/m(3) and 1.00415 (95% CI 1.0001, 1.0007) for a increase of 10 microg/m(3) of sulphur dioxide. Because the statistical power is higher than in the individual analysis more interactions were statistically significant, especially those among air pollutants and meteorological variables. CONCLUSIONS: Air pollutant levels were related to mortality in the three cities of the study, Barcelona, Valencia and Vigo. These results were consistent with similar studies in other cities, with other multicentric studies and coherent with both, previous individual, for each city, and multicentric studies for all three cities.

Air Pollutants↗

PubMed2004

Clinical trials in traumatic brain injury: current problems and future solutions.

Over the past decade many neuroprotective agents have been developed with the hope of being able to improve outcome in patients with traumatic brain injury. Unfortunately, none of the phase III trials performed have convincingly demonstrated efficacy in the overall population. A common misconception is that consequently these agents are ineffective. Such has not been proven and some trials show evidence of efficacy in subgroups of the population studied. The negative results, as reported in the overall population, may in part be caused by specific aspects of the TBI population, as well as by aspects of clinical trial design and analysis. Clinical trials in TBI pose several complicated design issues. Methodological challenges relate particularly to heterogeneity of the population and to outcome assessment. Heterogeneity pertains both to the range of pathologies included in TBI, and to prognostic factors, each causing specific problems. Mechanistic and/or prognostic targeting, as well as possibilities for covariate adjustment, are suggested as possible solutions to deal with the problems of heterogeneity. The aim in most trials was to demonstrate a 10% absolute improvement in favorable outcome in patients with head injury. This may be considered overoptimistic and unrealistic in relation to the heterogeneous patient population. Specific problems are further incurred by the use of the dichotomized Glasgow Outcome Scale as primary outcome measure. Optimal statistical power may expected to be present when the point of dichotomization results in a 50:50 distribution of outcome categories. It is proposed to differentiate the point of dichotomization according to prognostic risk profile, in order to maintain statistical power. Solutions described may be expected to enhance chances of demonstrating benefit of potentially effective neuroprotective agents in future studies. The complexity of problems occurring in clinical trial design and analysis in TBI is such that a strong and sustained multidisciplinary input and effort is required from all experts involved in the field of neurotrauma.

Brain Injuries↗

PubMed2004

The effect of measurement unreliability on sleep and respiratory variables.

INTRODUCTION: Unreliability associated with scoring sleep variables is a potentially problematic issue in clinical and research studies. When scoring unreliability is unrecognized, it can contribute to the following: increase variability in the measures of interest, decrease a study's ability to detect important relationships, attenuate correlation coefficients, and increase clinical trial costs. METHODS: This paper first models the relationship between scoring variability and reliability in commonly studied sleep variables. The paper then models the relationship between unreliability and sample size requirements and statistical power. Standard methods are used to model reliability using the intraclass correlation coefficient. RESULTS: The analysis shows that when scoring unreliability is minimized (i.e., scoring reliability is maximized), correlation coefficients are more robust, sample size requirements are reduced, statistical power is increased, and clinical trial costs are reduced. DISCUSSION: When scoring unreliability is recognized, research studies can compensate by increasing the number of research subjects studied; however, it is at the cost of increasing the costs of research and exposing greater numbers of subjects to possible study risks. An effective solution is to implement rigorous initial and ongoing training efforts to maintain high inter-rater and intra-rater reliability coefficients.

Humans↗

PubMed1993

Calcium and colorectal epithelial cell proliferation: a preliminary randomized, double-blinded, placebo-controlled clinical trial.

BACKGROUND: Colonic epithelial cell proliferation is increased in patients at high risk for colon cancer. Calcium administration has ameliorated the proliferative changes in rodents, and findings in small, uncontrolled clinical trials have suggested similar effects in humans. PURPOSE: This preliminary, double-blind, randomized clinical trial was designed 1) to investigate whether supplemental calcium will reduce colonic epithelial cell proliferation in patients with sporadic adenomas who consume a high-fat, Western-style diet; 2) to determine the sample size (number of scorable crypts per person) needed to achieve adequate statistical power; and 3) to evaluate the feasibility of full-scale clinical trials. METHODS: Twenty-one sporadic adenoma patients were treated daily with placebo or 1200 mg of supplemental calcium. To determine colonic epithelial cell proliferation, we used tritiated thymidine labeling of colon crypt epithelial cells in rectal biopsy specimens and calculated the percentage of labeled cells (labeling index [LI]). Two pathology technician "readers" independently scored each specimen, and inter-reader reliability was determined. Subjects remained on their usual diet during the study, and intake of calories, calcium, total fat, and vitamin D did not differ substantially among them. We calculated curves for statistical power to determine the number of scorable crypts needed per person for detection of a statistically significant difference (P < .05) of 1.0% in mean LI. RESULTS: The pooled baseline LI was 4.7%. In the calcium-treated group, the LI increased 0.6% (proportional increase, 12.8%); in the placebo-treated group, it decreased 0.5% (proportional decrease, 10.6%). The difference between change in the mean LI from baseline to 8 weeks' follow-up in the placebo group versus the calcium group was not statistically significant. The intraclass correlation coefficient for inter-reader reliability for the baseline LI was .66. Analyses indicated scoring eight crypts sufficient for estimates of the LI adequate for between-group comparisons, a level achieved in 81% of biopsy specimens. CONCLUSIONS: Calcium carbonate supplements delivering 1200 mg elemental calcium daily may not decrease colonic epithelial cell proliferation over an 8-week period in sporadic adenoma patients. In future trials measuring the LI, consideration should be given to ensuring adequate numbers of scorable crypts and to the impact of inadequate biopsy procedures, labeling failure, reader reliability, and participant withdrawal. Our findings support the feasibility of a full-scale clinical trial to further study the relationships among dietary calcium, colonic epithelial cell proliferation, and colorectal cancer.

Adenoma↗

PubMed2001

Towards the genetic basis of periodic catatonia: pedigree sample for genome scan I and II.

In a genome-wide linkage study, we mapped two major susceptibility loci for periodic catatonia, a phenotype with qualitative disturbances of the psychomotor sphere and a morbidity risk of 26.9% in first-degree relatives of index cases, to chromosome 15q15, and to chromosome 22q13 using nonparametric as well as parametric (autosomal dominant model) analyses. The study included 12 multiplex pedigrees with 135 individuals, among them 57 affected persons. A second genome scan is in progress investigating four families with 21 affected individuals, aiming to confirm linkage results. Age at onset patterns as well as the clinical outcome were similar among affected individuals in both sets of families. Within the pedigrees we observed no physical diseases segregating with periodic catatonia. Under the assumption of genetic homogeneity, the statistical power to detect LOD scores > or = 2.0 was 98.5% in the first set of families, and 57.9% in the second set. Thus, the panel of multiplex pedigrees segregating periodic catatonia seems to represent a homogenous clinical sample, and possesses sufficient statistical power to delineate and confirm linkage to major genetic loci for periodic catatonia.

Adult↗

PubMed2006

Meta-analysis for microarray studies of the genetics of complex traits.

In comparison to other complex disease traits, alcoholism and alcohol abuse are influenced by the combined effects of many genes that alter susceptibility, phenotypic expression and associated morbidity, respectively. Many genetic studies, in both animal models and humans, have identified genetic intervals containing genes that influence alcoholism or behavioral responses to ethanol. Concurrently, a growing number of microarray studies have identified gene expression differences related to ethanol drinking or other ethanol behaviors. However, concerns about the statistical power of these experiments, combined with the complexity of the underlying phenotypes, have greatly hampered the identification of candidate genes underlying ethanol behaviors. Meta-analysis approaches using recent compilations of large datasets of microarray, behavioral and genetic data promise improved statistical power for detecting the genes or gene networks affecting ethanol behaviors and other complex traits.

Alcohol Drinking↗

PubMed2001

Using Johnson's transformation and robust estimators with heteroscedastic test statistics: an examination of the effects of non-normality and heterogeneity in the non-orthogonal two-way ANOVA design.

The present study proposes a procedure that combines Johnson's transformation and the trimmed means method to deal with the problem of non-normality. An approximate test such as the Alexander-Govern test or Welch-James type test is then employed to deal with the heterogeneity of cell variance in the non-orthogonal two-way fixed effects completely randomized design. Both unweighted and weighted means analyses are considered. The empirical Type I error rates and the statistical power for comparing population means are investigated by Monte Carlo simulation. The simulated results show that Johnson's transformation with trimmed mean and the approximate test is valid in terms of Type I error rate control, and that the magnitude of the statistical power for non-normal distributions is better than that of conventional methods.

Analysis of Variance↗

PubMed2004

Monitoring recovery after injury: procedures for deriving the optimal test window.

Researchers studying the impact of treatments designed to facilitate recovery after neural injury face competing demands. On the one hand, because treatment effects often emerge slowly over days, and because researchers seek evidence of stable long-term effects, it is common practice to observe experimental subjects for many weeks after treatment. On the other hand, the cost of performing studies and the need to evaluate a multitude of alternative treatment procedures requires optimal efficiency, pushing researchers towards shorter test procedures. With these issues in mind, researchers have appeared to derive a test window based on previously published methodologies and inspection of their recovery curves, with testing terminated after the recovery curve reaches asymptote (approaches a slope of 0). An alternative procedure is introduced here that evaluates the stability of the data set over time. Using correlational techniques, researchers can determine whether (1) testing should be continued for additional days; or (2) equivalent statistical power can be achieved in fewer days. This provides a rational decision rule to help researchers balance competing demands. Applying these techniques to a procedure that evaluates the impact of acute treatments on recovery from spinal cord injury, it is shown that equal statistical power can be achieved in half the time, greatly increasing the efficiency with which alternative treatments can be evaluated.

Animals↗