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At least 307 records · Page 17Linked to original sources
PubMed1987

Study of the anatomic metric values of the foramen magnum and its relation to sex.

The sagittal and transverse diameters of the foramen magnum were examined in 100 human skulls (74 male, 26 female). The results were statistically analyzed and compared with those obtained by other authors. Our findings show that the sagittal and transverse diameters are clearly different, as well as the total area of the foramen magnum, these parameters being larger in male skulls than in female.

Adult↗
PubMed2005

A new equivalence based metric for predictive check to qualify mixed-effects models.

The main objective of any modeling exercise is to provide a rationale for effective decision making during drug development. The aim of the current simulation experiment was to evaluate the properties of predictive check as a covariate model qualification technique and, more importantly, to introduce and evaluate alternative criteria to qualify models.Original concentration-time profiles (yod) were simulated using a 1-compartment model for an intravenous drug administered to 25 men and 25 women. The typical clearance for male subjects (TVCLm) was assumed to be 5-fold higher than that for female subjects (TVCLf). Fifty such trials under the same design were generated randomly. Predictive check was used as the model qualification tool to study predictive performance of true (males not equal females) and false (males = females) models in the context of maximum likelihood estimation. For each yod, 200 replications were generated to study the properties of a discrepancy variable, a statistic that depends on the model, and a test statistic, a statistic that does not depend on the model. Several qualification criteria were evaluated in assessing predictive performance, such as, predictive p-value (Pp), probability of equivalence (peqv), and probability of rejecting the null hypothesis (data = model) using the Kolmogorov-Smirnov test (pks). The Pp value was calculated using sum of squared errors as a discrepancy variable. For both of the models, the Pp values uniformly ranged between 0 and 1. The pattern of Pp values suggests that qualification of the false model is unlikely. For both of the models, the range of peqv is about 0.95 to 1.0 for concentration at 0.5 hours. However, this is not the case for the concentration at 4 hours, which is primarily dependent on the clearance. The false model (0.35 to 0.50) has poor predictive performance compared with the true model (0.65 to 0.80) using peqv. The pks suggests no difference in the distributions of replicated and original concentrations at all of the time points for both of the models. Discrepancy variables cannot aid in rejecting false models, whereas the use of a test statistic can aid in rejecting false models. However, selection of an informative test statistic is challenging. As far as the qualification criteria are considered, the equivalence-based comparison of a test statistic is more informative than a significance-based comparison. No convincing evidence exists in the literature demonstrating the added advantages of predictive check as a routine model qualification tool over the existing tools, such as diagnostic plots or mechanistic reasoning. However, when a model is to be used for designing a trial, it should at least be able to regenerate the data used to build the model. In such cases, predictive check might offer insights into potential inconsistencies.

Humans↗
PubMed1974

Letter: SI Units.

Explore the source record for details and available documents.

Metric System↗
PubMed1977

SI prefixes.

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Documentation↗
PubMed2005

Atmospheric compensation with a speckle beacon in strong scintillation conditions: directed energy and laser communication applications.

Wavefront control experiments in strong scintillation conditions (scintillation index, approximately equal to 1) over a 2.33 km, near-horizontal, atmospheric propagation path are presented. The adaptive-optics system used comprises a tracking and a fast-beam-steering mirror as well as a 132-actuator, microelectromechanical-system, piston-type deformable mirror with a VLSI controller that implements stochastic parallel gradient descent control optimization of a system performance metric. The experiments demonstrate mitigation of atmospheric distortions with a speckle beacon typical for directed energy and free-space laser communication applications.

Journal Article↗
PubMedDate not supplied

Metric equivalence assessment in cross-cultural research: using an example of the Center for Epidemiological Studies--Depression Scale.

Metric equivalence is a quantitative way to assess cross-cultural equivalences of translated instruments by examining the patterns of psychometric properties based on cross-cultural data derived from both versions of the instrument. Metric equivalence checks at item and instrument levels can be used as a valuable tool to refine cross-cultural instruments. Korean and English versions of the Center for Epidemiological Studies-Depression Scale (CES-D) were administered to 154 Korean Americans and 151 Anglo Americans to illustrate approaches to assessing their metric equivalence. Inter-item and item-total correlations, Cronbach's alpha coefficients, and factor analysis were used for metric equivalence checks. The alpha coefficient for the Korean-American sample was 0.85 and 0.92 for the Anglo American sample. Although all items of the CES-D surpassed the desirable minimum of 0.30 in the Anglo American sample, four items did not meet the standard in the Korean American sample. Differences in average inter-item correlations were also noted between the two groups (0.25 for Korean Americans and 0.37 for Anglo Americans). Factor analysis identified two factors for both groups, and factor loadings showed similar patterns and congruence coefficients. Results of the item analysis procedures suggest the possibility of bias in certain items that may influence the sensitivity of the Korean version of the CES-D. These item biases also provide a possible explanation for the alpha differences. Although factor loadings showed similar patterns for the Korean and English versions of the CES-D, factorial similarity alone is not sufficient for testing the universality of the structure underlying an instrument.

Adolescent↗
PubMed2000

Unit conversion as a source of misclassification in US birthweight data.

OBJECTIVES: This study explains why frequency polygons for US birthweights in 100-g weight classes appear spiky compared with their European counterparts. METHODS: A probability model is used to describe how unit conversion can induce misclassification. Birthweights from the United States and Norway are used to illustrate that misclassification operates in grouped US data. RESULTS: Spikiness represents misclassification that arises when measured birthweights are rounded to the nearer ounce, converted to grams, and then grouped into weight classes. Misclassification is ameliorated, not eliminated, with 200-g weight classes. CONCLUSIONS: Possible biases from misclassification should be carefully evaluated when fitting statistical models to grouped US birthweights.

Bias↗
PubMed2003

Spatial navigation after surgical resection of an acoustic neuroma: pilot study.

OBJECTIVE: To characterize navigation errors made by patients with the absence of vestibular function on one side owing to surgical resection of an acoustic neuroma. METHODS: Seventeen young (18-38 years) and 9 older healthy individuals (67-83 years), as well as 5 patients 2 to 20 months following surgery (37-61 years), were studied. They sidestepped laterally with eyes closed toward memorized targets located 1.25 m to their right or left. They stopped when they judged that they were in front of the target. The position of head and body markers was recorded in three dimensions with a six-camera Vicon 512 system (Oxford Metrics Ltd., Oxford, UK). Navigation errors were (1) distance error, the distance between the end target and a perpendicular line drawn from the sternum to the plane of targets, and (2) deviation, the angle formed between the line joining the initial and end targets and the line joining the subject's shoulders. RESULTS: Mean distance error was 20.9 +/- 22.0 degrees cm in patients, 29.6 +/- 30.3 cm in young healthy subjects, and -1.7 +/- 18.4 cm in older subjects (p < .01 compared with young subjects). Mean deviation was symmetric and 8 degrees and -3 degrees in healthy young and older subjects, respectively. In contrast, patients had a significantly larger deviation when navigating toward the side of their lesion than the intact side (13 degrees +/- 9 degrees versus 3 degrees +/- 9 degrees; p < .01). CONCLUSIONS: Our results suggest that patients with vestibular deficits have impaired ability to control body rotations when walking sideways without vision toward the side of their vestibular lesion.

Adolescent↗