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Biomedical subjects

Mark Wilson

Publications and source records attributed to Mark Wilson.

At least 19 recordsLinked to original sources
PubMed2006

Does participation in an intervention affect responses on self-report questionnaires?

There has been some concern that participation in an intervention and exposure to a measurement instrument can change participants' interpretation of the items on a self-report questionnaire thereby distorting subsequent responses and biasing results. Differential item functioning (DIF) analysis using item response modeling can ascertain possible differences in item interpretation by testing for differences in item location between groups. The DIF for treatment versus control group differences at post-intervention assessment and the Time 1 and Time 2 differences in a control group were analyzed using data from a dietary change intervention trial for Boy Scouts. The measures included fruit and vegetable (FV) frequency of consumption, preferences and self-efficacy. Treatment-control group DIF at post-intervention assessment was detected in a higher percentage of items for FV frequency than for preference or self-efficacy. Time 1 to Time 2 differences in items for the control group were detected in one item for each of the three scales. Further research will need to clarify whether the obtained DIFs reflected true changes in frequency, preference or self-efficacy or some reinterpretation of items by participants following an intervention or merely after previous exposure to the measure.

Bias↗
PubMed2006

Evaluating the properties of a stage-specific self-efficacy scale for physical activity using classical test theory, confirmatory factor analysis and item response modeling.

The purpose of this paper was to evaluate the psychometric properties of a stage-specific self-efficacy scale for physical activity with classical test theory (CTT), confirmatory factor analysis (CFA) and item response modeling (IRM). Women who enrolled in the Women On The Move study completed a 20-item stage-specific self-efficacy scale developed for this study [n=226, 51.1% African-American and 48.9% Hispanic women, mean age = 49.2 (+/-7.0) years, mean body mass index = 29.7 (+/-6.4)]. Three analyses were conducted: (i) a CTT item analysis, (ii) a CFA to validate the factor structure and (iii) an IRM analysis. The CTT item analysis and the CFA results showed that the scale had high internal consistency (ranging from 0.76 to 0.93) and a strong factor structure. Results also showed that the scale could be improved by modifying or eliminating some of the existing items without significantly altering the content of the scale. The IRM results also showed that the scale had few items that targeted high self-efficacy and the stage-specific assumption underlying the scale was rejected. In addition, the IRM analyses found that the five-point response format functioned more like a four-point response format. Overall, employing multiple methods to assess the psychometric properties of the stage-specific self-efficacy scale demonstrated the complimentary nature of these methods and it highlighted the strengths and weaknesses of this scale.

Adult↗
PubMed2006

Introducing multidimensional item response modeling in health behavior and health education research.

When measuring participant-reported attitudes and outcomes in the behavioral sciences, there are many instances when the common measurement assumption of unidimensionality does not hold. In these cases, the application of a multidimensional measurement model is both technically appropriate and potentially advantageous in substance. In this paper, we illustrate the usefulness of a multidimensional approach to measurement using an empirical example taken from the Behavior Change Consortium. Data from the Treatment Self-Regulation Questionnaire have been analyzed to investigate whether self-regulation can be regarded as a single construct, or if it has multiple dimensions based on the type of regulation or motivation that participants say helps them consider an improvement in healthy behavior. Comparison with consecutive analyses shows the advantages of multidimensional measurement for interpreting participant-reported data.

Health Behavior↗
PubMed2006

Improving measurement in health education and health behavior research using item response modeling: introducing item response modeling.

This paper is the first of several papers designed to demonstrate how the application of item response models in the behavioral sciences can be used to enhance the conceptual and technical toolkit of researchers and developers and to understand better the psychometric properties of psychosocial measures. The papers all use baseline data from the Behavior Change Consortium data archive. This paper begins with an introduction to item response models, including both dichotomous and polytomous versions. The concepts of respondent and item location, model interpretation, standard errors and testing model fit are introduced and described. A sample analysis based on data from the self-efficacy scale is used to illustrate the concepts and techniques.

Data Interpretation, Statistical↗
PubMed2006

Structural studies and polymorphism in amorphous solids and liquids at high pressure.

When amorphous materials are compressed their structures are expected to change in response to densification. In some cases, the changes in amorphous structure can be discontinuous and they can even have the character of first-order phase transitions. This is a phenomenon referred to as polyamorphism. Most evidence for polyamorphic transitions between low and high density liquids or analogous transformations between amorphous forms of the same substance to date has been indirect and based on the changes in thermodynamic and other structure-related properties with pressure. Recent studies using advanced X-ray and neutron scattering methods combined with molecular dynamics simulations are now revealing the details of structural changes in polyamorphic systems as a function of pressure. Various "two state" or "two species" models are used to understand the anomalous densification behaviour of liquids with melting curve maxima or regions of negative melting slope. Thermodynamic analysis of the two state model leads to the possibility of low- to high-density liquid transitions caused by differences in bulk thermodynamic properties between different amorphous forms and on the degree of cooperativity between low- and high-density structural configurations. The potential occurrence of first-order transitions between supercooled liquids is identified as a critical-like phenomenon. In this tutorial review we discuss the background to polyamorphism, incorporating the experimental observations, simulation studies and the two-state models. We also describe work carried on several systems that are considered to be polyamorphic.

Journal Article↗
PubMed2006

Introducing equating methodologies to compare test scores from two different self-regulation scales.

Standardizing the measurement tools that researchers use to assess the effectiveness of interventions would strengthen our ability to compare results across studies. In practice, however, standardization is difficult to implement, in part, because researchers prefer to use measurement tools that focus specifically on the components of their interventions. This paper demonstrates the usefulness of item response modeling linking methodology in comparing groups of participants who were administered different scales intended to measure the same underlying constructs. The Treatment Self-Regulation Questionnaire (TSRQ) as it relates to diet improvement provided the empirical application to demonstrate how two different scales that measure the same construct can be compared. The results showed that two eight-item TSRQ scales can be linked if they have at least four items in common. As expected, varying the number of linking items did not affect the reliability of the results; however, it significantly affected the relative rating with respect to the 15-item scale. In health behavior and health education research, linking methodologies can be used to compare results across studies that use slightly different versions of a scale to measure the same construct.

Adult↗
PubMed2006

Improving measurement in health education and health behavior research using item response modeling: comparison with the classical test theory approach.

This paper compares the approach and resultant outcomes of item response models (IRMs) and classical test theory (CTT). First, it reviews basic ideas of CTT, and compares them to the ideas about using IRMs introduced in an earlier paper. It then applies a comparison scheme based on the AERA/APA/NCME 'Standards for Educational and Psychological Tests' to compare the two approaches under three general headings: (i) choosing a model; (ii) evidence for reliability--incorporating reliability coefficients and measurement error--and (iii) evidence for validity--including evidence based on instrument content, response processes, internal structure, other variables and consequences. An example analysis of a self-efficacy (SE) scale for exercise is used to illustrate these comparisons. The investigation found that there were (i) aspects of the techniques and outcomes that were similar between the two approaches, (ii) aspects where the item response modeling approach contributes to instrument construction and evaluation beyond the classical approach and (iii) aspects of the analysis where the measurement models had little to do with the analysis or outcomes. There were no aspects where the classical approach contributed to instrument construction or evaluation beyond what could be done with the IRM approach. Finally, properties of the SE scale are summarized and recommendations made.

Data Interpretation, Statistical↗
PubMed2006

The formation of low-dimensional metal trihalide crystals in carbon nanotubes.

Molecular dynamics computer simulation models are employed to study the direct filling of single-walled carbon nanotubes (which vary in diameter) with an archetypal metal trihalide, LaCl3. The use of relatively simple potential models allows the investigation of details of both the atomistic filling mechanism and the thermodynamic factors controlling the formation. The resulting low-dimensional crystallites are analysed with respect to bulk crystal structures and compared to experimental high-resolution transmission-electron-microscopy images by simulation of equivalent micrographs from one of the obtained potential models, resulting in excellent agreement between the simulated and experimental images.

Journal Article↗
PubMed2006

The formation of low-dimensional inorganic nanotube crystallites in carbon nanotubes.

The filling of carbon nanotubes, which vary in diameter and morphology, is directly observed by molecular dynamics computer simulation with a potential model which thermodynamically favors a four-coordinate bulk crystal structure. Inorganic nanotube (INT) structures form which are based on percolating hexagonal nets. For small carbon nanotube diameters the filling is shown to proceed via an "internal wetting" mechanism, which depends on the internal carbon nanotube area rather than the free volume. Both single- and double-walled INTs are predicted to form. The atomistic formation mechanisms are discussed and an intermediate structure identified. The INT structures, including the observed intermediate, are discussed by reference to a simple energy landscape. The formation energetics are discussed in terms of a simple analytical model which combines the INT strain energy and the tube-tube interactions. An effective phase diagram, which predicts the INT morphologies as a function of carbon nanotube diameter, is derived and discussed with respect to the analytical model.

Journal Article↗
PubMed2006

Delivery and assessment of endovascular stents to repair aortic coarctation using MR and X-ray imaging.

PURPOSE: To investigate the utility of MR and X-ray imaging for characterizing aortic coarctation and flow, and guiding the endovascular catheter to place a stent to repair the coarctation. MATERIALS AND METHODS: The descending aorta in eight dogs was looped with elastic band and tightened distal to the subclavian artery. Balanced fast field echo (bFFE) and velocity-encoded cine (VEC) MRI sequences were used for device tracking and measuring aortic flow. A T1-weighted fast-field echo sequence (T1-FFE) was used to visualize the coarctation and roadmap the aorta. Nitinol stents were guided by a nitinol guidewire and placed under MR guidance. RESULTS: Aortic coarctation was visible on MR and X-ray imaging. The procedure success rate was 88%. VEC MRI measured the changes in aortic flow (baseline = 1.3 +/- 0.2, coarctation = 0.2 +/- 0.02, and stent placement = 0.8 +/- 0.1 liters/minute). A significant reduction in iliac blood pressure was measured after coarctation, but it was reversed by stent placement. The stent lumen was visible on X-ray fluoroscopy, but not on MRI. CONCLUSION: Stent deployment to repair aortic coarctation is feasible under MR guidance. The combined use of MR and X-ray imaging is effective for anatomic and functional evaluation of aortic coarctation dilation, which may be crucial for optimal therapy.

Alloys↗
PubMed2006

Comparison of virulence gene profiles of Escherichia coli strains isolated from healthy and diarrheic swine.

A combination of uni- and multiplex PCR assays targeting 58 virulence genes (VGs) associated with Escherichia coli strains causing intestinal and extraintestinal disease in humans and other mammals was used to analyze the VG repertoire of 23 commensal E. coli isolates from healthy pigs and 52 clinical isolates associated with porcine neonatal diarrhea (ND) and postweaning diarrhea (PWD). The relationship between the presence and absence of VGs was interrogated using three statistical methods. According to the generalized linear model, 17 of 58 VGs were found to be significant (P < 0.05) in distinguishing between commensal and clinical isolates. Nine of the 17 genes represented by iha, hlyA, aidA, east1, aah, fimH, iroN(E. coli), traT, and saa have not been previously identified as important VGs in clinical porcine isolates in Australia. The remaining eight VGs code for fimbriae (F4, F5, F18, and F41) and toxins (STa, STb, LT, and Stx2), normally associated with porcine enterotoxigenic E. coli. Agglomerative hierarchical algorithm analysis grouped E. coli strains into subclusters based primarily on their serogroup. Multivariate analyses of clonal relationships based on the 17 VGs were collapsed into two-dimensional space by principal coordinate analysis. PWD clones were distributed in two quadrants, separated from ND and commensal clones, which tended to cluster within one quadrant. Clonal subclusters within quadrants were highly correlated with serogroups. These methods of analysis provide different perspectives in our attempts to understand how commensal and clinical porcine enterotoxigenic E. coli strains have evolved and are engaged in the dynamic process of losing or acquiring VGs within the pig population.

Animals↗
PubMed2005

A density-driven phase transition between semiconducting and metallic polyamorphs of silicon.

Amorphous and crystalline forms of silicon are well-known, tetrahedrally coordinated semiconductors. High-pressure studies have revealed extensive polymorphism among various metallic crystal structures containing atoms in six-, eight- and 12-fold coordination. Melting silicon at ambient or high pressure results in a conducting liquid, in which the average coordination is greater than four (ref. 3). This liquid cannot normally be quenched to a glass, because of rapid crystallization to the diamond-structured semiconductor. Solid amorphous silicon is obtained by synthesis routes such as chemical or physical vapour deposition that result in a tetrahedrally bonded semiconducting state. It has long been speculated that the amorphous solid and the liquid could represent two polymorphic forms of the amorphous state that are linked by density- or entropy-driven transformations. Such polyamorphic transitions are recognized to occur among several different types of liquid and glassy systems. Here we present experimental evidence for the occurrence of a density-driven polyamorphic transition between semiconducting and metallic forms of solid amorphous silicon. The experiments are combined with molecular dynamics simulations that map the behaviour of the amorphous solid on to that of the liquid state.

Computer Simulation↗
PubMed2005

The construct of internalization: conceptualization, measurement, and prediction of smoking treatment outcome.

BACKGROUND: Depression symptoms and diagnoses are associated with failure to quit smoking in most studies, but not all. METHOD: A new measure of internalization (i.e. symptoms of depression or anxiety, or poor mood) was created to investigate whether internalization would predict smoking cessation in 549 smokers from three randomized clinical trials with inconsistent findings. RESULTS: Predicted item locations based on a map of the construct of internalization agreed with empirical locations based on item response theory. Internalization was highly correlated with neuroticism. Logistic regressions showed that internalization improved upon the predictions of other affect-related measures. High baseline internalization decreased abstinence from smoking at end of treatment and 3 months thereafter. History of major depression (single-episode or recurrent) failed to predict abstinence. CONCLUSIONS: The broad, dimensional construct of internalization as conceptualized herein appears to be an important predictor of smoking cessation.

Adult↗
PubMed2005

A conceptual and psychometric framework for distinguishing categories and dimensions.

An important, sometimes controversial feature of all psychological phenomena is whether they are categorical or dimensional. A conceptual and psychometric framework is described for distinguishing whether the latent structure behind manifest categories (e.g., psychiatric diagnoses, attitude groups, or stages of development) is category-like or dimension-like. Being dimension-like requires (a) within-category heterogeneity and (b) between-category quantitative differences. Being category-like requires (a) within-category homogeneity and (b) between-category qualitative differences. The relation between this classification and abrupt versus smooth differences is discussed. Hybrid structures are possible. Being category-like is itself a matter of degree; the authors offer a formalized framework to determine this degree. Empirical applications to personality disorders, attitudes toward capital punishment, and stages of cognitive development illustrate the approach.

Capital Punishment↗
PubMed2004

Raman spectra of ionic liquids: interpretation via computer simulation.

Theoretical Raman spectra of the complex-forming ionic liquids LaCl3 and ScCl3, derived from molecular dynamics computer simulations, are presented. These simulations, which use polarizable ion interaction models, have previously been shown to predict structural properties in excellent agreement with diffraction experiments. The dependence of the polarizability of the melt on the ionic positions, which determines the Raman spectrum through the time dependence of the polarizability correlation function, is modeled on the basis of ab initio electronic structure calculations carried out on alkali chlorides. New simulation techniques are introduced in order to allow the spectrum to be calculated with acceptable statistics. The calculated spectra are in semiquantitative agreement with experimental data. The distinctive bands which appear in the spectra of such complex melts are linked to the vibrations of the transient coordination complexes which form in these melts and new interpretations for the origin of several well-known features are proposed. The simulations thus enable a link between the structure of a melt as perceived through Raman spectroscopy and through diffraction experiments to be made.

Journal Article↗