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At least 1,171 records · Page 65Linked to original sources

PubMed2000

[Prediction of bioconcentration factors of organic compounds in fish by molecular connectivity indices and function correction factors].

Studies on the prediction models of bioconcentration factors(BCF) of 239 organic compounds showed that the BCFs of polar organic compounds could not be accurately predicted solely by the linear model based on molecular connectivity indices. Additionally, the linear model was not suitable for superphilic compounds. By introducing function correction factors to the model, the residual of the prediction model for polar organic compounds was reduced significantly. When the nonlinear estimation was used, the accuracy of the obtained model was further improved. The final prediction model consisted of parameters (0 chi v)2, (2 chi v)1/2, 2 chi, 3 chi c, 0 chi v and 10 correction factors for function groups OH, NH2, NO2, NCOO, etc. The difference between calculated and observed values of logBCF and its causes were analyzed. The robustness of the model was tested by Jackknifed method, based on random grouping and compound classification.

Animals↗

PubMed1987

Disposition of phenytoin in critically ill trauma patients.

Estimates of phenytoin pharmacokinetic variables and protein binding were determined in 10 adult critically ill trauma patients. Each study subject received phenytoin sodium as an intravenous loading dose of 15 mg/kg, followed by an initial intravenous maintenance dose of 6 mg/kg/day. Serial blood samples were obtained throughout the seven-day study period and analyzed for total and unbound serum phenytoin concentrations. The concentration data for each patients were fitted to a one-compartment model with elimination defined by the Michaelis-Menten constant Km and the maximum rate of metabolism (Vmax) and to a one-compartment model with first-order elimination. The Michaelis-Menten model used Bayesian parameter estimation while the linear model used weighted non-linear least-squares regression analysis. Unbound phenytoin fraction ranged from 0.073 to 0.25. Free fraction increased 7% to 108% in 9 of 10 patients (median increase 29%) from day 1 to day 7 of therapy. Variable estimates using the Michaelis-Menten model were as follows: volume of distribution, 0.76 +/- 0.15 L/kg (0.58-1.01 L/kg); Vmax, 568 +/- 197 mg/day (350-937 mg/day); and Km, 4.5 +/- 1.8 mg/L (1.8-6.2 mg/L). These estimates fell within the wide range of values obtained in studies using stable patients or healthy volunteers. The Michaelis-Menten model was significantly less biased and more precise than the linear model. Three of four patients who continued to receive their study maintenance dose had substantially lower measured total serum concentrations of phenytoin than predicted using the study variable estimates.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed2002

Reduction of trauma symptoms following group therapy.

OBJECTIVE: We examined the effect of a group programme for the treatment of adults suffering the sequelae of childhood abuse and/or neglect. Symptom reduction was the indicator of whether this group programme was an effective treatment modality. METHOD: This was an experimental field study using a prepost design and General Linear Model (GLM) analyses. The group programme was conducted for adult survivors of child abuse and neglect and took place at Specialty Clinics, Central Coast Mental Health Services. There were 83 participants, comprising 64 women and 19 men; 56 were in the experimental group, and 17 in the control (waitlist) group. The Trauma Symptom Inventory (TSI) was administered to both experimental and control groups prior to receiving treatment and again three months later. RESULTS: General Linear Model analyses indicated there was a reduction of trauma symptoms for the experimental group on seven of the 10 scales of the TSI and on the three composite scales; no similar reduction was found for the control group. CONCLUSIONS: The study indicates that this group programme may have been an effective treatment modality for abuse survivors. The level of symptomatology was reduced for those in the group programme compared with the control group for whom there were no changes.

Adaptation, Psychological↗

PubMed1990

Review of methods and criteria for the evaluation of bioequivalence studies.

Guidelines for the performance and analysis of bioequivalence studies are not very specific. The advantages and disadvantages of the following methods and tests are discussed: analysis of variance by summation or by use of general linear models, nonparametric procedures, aposteriori probabilities and tests on the normality of residuals and on the variability of the results. Arguments for or against an analysis of data after logarithmic transformation versus analysis of untransformed data are presented. If the confidence intervals lie within certain limits, preparations may be considered equivalent. The criteria leading to those limits are discussed. It is recommended that concentration-dependent data of bioequivalence studies be evaluated by analysis of variance after logarithmic transformation, applying general linear models. Data that by theoretical reasons cannot be normally or log-normally distributed should be analysed by nonparametric methods. Otherwise these methods can only be recommended if a significant deviation from normality has been noted and only for two-way cross-over designs. For a geometric evaluation (after logarithmic transformation) the regions of acceptance should be symmetrical in the logarithm, e.g. (80%, 125%).

Analysis of Variance↗

PubMed2000

Limb defects associated with major congenital anomalies: clinical and epidemiological study from the International Clearinghouse for Birth Defects Monitoring Systems.

Although limb defects associated with other congenital anomalies are rarely studied, they may provide insights into limb development that may be useful for etiologic studies and public health monitoring. We pooled data from 11 birth defect registries that are part of the International Clearinghouse for Birth Defects Monitoring Systems. We identified 666 infants, born from 1983 through 1993, who had a non-syndromal limb defect plus at least one other major malformation (rate 12.9/100,000 population). We used observed/expected ratios and log-linear models to detect association patterns. We found that specific limb defects occurred with relatively distinct sets of malformations. Preaxial limb defects occurred more frequently with microtia, esophageal atresia, anorectal atresia, heart defects, unilateral kidney dysgenesis, and some axial skeleton defects; postaxial defects with hypospadias; transverse defects with craniofacial defects, micrognathia, ring constrictions, and muscular defects; intercalary defects with omphalocele; split hand/foot with encephalocele; and amelia with anorectal atresia, omphalocele, severe genitalia defects, unilateral kidney dysgenesis, gastroschisis, and ring constriction. Log-linear modeling identified higher order associations among some of these same malformations.

Cleft Palate↗

PubMed2001

Linear processing of spatial cues in primary auditory cortex.

To determine the direction of a sound source in space, animals must process a variety of auditory spatial cues, including interaural level and time differences, as well as changes in the sound spectrum caused by the direction-dependent filtering of sound by the outer ear. Behavioural deficits observed when primary auditory cortex (A1) is damaged have led to the widespread view that A1 may have an essential role in this complex computational task. Here we show, however, that the spatial selectivity exhibited by the large majority of A1 neurons is well predicted by a simple linear model, which assumes that neurons additively integrate sound levels in each frequency band and ear. The success of this linear model is surprising, given that computing sound source direction is a necessarily nonlinear operation. However, because linear operations preserve information, our results are consistent with the hypothesis that A1 may also form a gateway to higher, more specialized cortical areas.

Animals↗

PubMedDate not supplied

A linear oscillator model for circadian rhythms: implications for phase response curves.

The underlying oscillator of circadian rhythms is assumed to be of a linear simple harmonic kind. Both pulse and step perturbations of positive and negative kinds are used to disturb the system theoretically. Phase response and phase-resetting curves are obtained and the results are compared with existing experimental data (namely Drosophila eclosion rhythm). It is demonstrated that the form and mirror-image symmetry of the response curves do appear similar to what is observed in real experiments. This is strongly indicative of the possibility that the underlying oscillator (or oscillators) is (are) not structurally stable.

Animals↗

PubMed1991

A comparison of static and dynamic characteristics between rectus eye muscle and linear muscle model predictions.

The characteristics of a muscle model are analyzed using rectus eye muscle parameter values and compared to rectus eye muscle data. The muscle is modeled as a viscoelastic parallel combination connected to a parallel combination of active state tension generator, viscosity element, and length tension elastic element. Each of the elements is linear and their existence is supported with physiological evidence. The static and dynamic properties of the muscle model are compared to rectus eye muscle data. The length-tension characteristics of the model are in good agreement with the data within the operating region of the muscle. With the muscle model incorporated into a lever system to match the isotonic experiment paradigm, simulation results for this linear system yield a nonlinear force-velocity curve. Moreover, the family of force-velocity curves generated with different stimulus rates reported in the literature match the predictions of the model without parametric changes. The results of this paper are important in studies involving the oculomotor plant and oculomotor neural networks. Additionally, these results may be applicable to other muscles.

Isometric Contraction↗

PubMedDate not supplied

Profiles of cognitive decline in Alzheimer disease.

Most recent studies have used only two observations to estimate the rate of cognitive decline in patients with Alzheimer disease (AD); few have data taken from more than a 2-year period; and none report on autopsy-verified cases. Repeated observations over the complete course of the disease are necessary to quantitatively evaluate hypotheses such as the triphasic linear model of Brooks et al. (1993). The goal of this study is to compare the triphasic linear and quadratic models of decline in a group of 12 AD patients confirmed at autopsy with a group of age- and sex-matched normal control subjects. Both groups were taken from the University of Western Ontario Dementia Study, and the Extended Scale for Dementia was used as the outcome measure. The squared multiple correlation as a measure of goodness of fit suggested the superiority of the more parsimonious quadratic model over the triphasic linear model. Quantitative models more accurately reflect the profiles of change in AD and may prove more sensitive in measuring the effects of drugs on these patterns.

Aged↗

PubMed2004

The time-dependent distribution of phosphorylated intermediates in native sarcoplasmic reticulum Ca2+-ATPase from skeletal muscle is not compatible with a linear kinetic model.

Quenched-flow mixing was used to characterize the kinetic behavior of the intermediate reactions of the skeletal muscle sarcoplasmic reticulum (SR) Ca-ATPase (SERCA1) at 2 and 21 degrees C. At 2 degrees C, phosphorylation of SR Ca-ATPase with 100 microM ATP labeled one-half of the catalytic sites with a biphasic time dependence [Mahaney, J. E., Froehlich, J. P., and Thomas, D. D. (1995) Biochemistry 34, 4864-4879]. Chasing the phosphoenzyme (EP) with 1.66 mM ADP 10 ms after the start of phosphorylation revealed mostly ADP-insensitive E2P (95% of EP(total)), consistent with its rapid formation from ADP-sensitive E1P. The consecutive relationship of the phosphorylated intermediates predicts a decrease in the proportion of E1P ([E1P]/[EP(total)]) with increasing phosphorylation time. Instead, after 10 ms the proportion of E1P increased and that of E2P decreased until they reached a constant 1:1 stoichiometry ([E1P]:[E2P] approximately 1). At 21 degrees C, phosphorylation displayed a transient overshoot associated with an inorganic phosphate (P(i)) burst, reflecting increased turnover of E2P at the higher temperature. The P(i) burst exceeded the decay of the EP overshoot, suggesting that rephosphorylation of the enzyme occurs before the recycling step (E2 --> E1). This behavior and the reversed order of accumulation of phosphorylated intermediates at 2 degrees C are not compatible with the conventional linear consecutive reaction mechanism: E1 + ATP --> E1.ATP --> E1P + ADP --> E2P --> E2.P(i) --> E1 + P(i). Solubilization of the Ca-ATPase into monomers using the nonionic detergent C(12)E(8) gave a pattern of phosphorylation in which E1P and E2P behave like consecutive intermediates. Kinetic modeling of the C(12)E(8)-solubilized SR Ca-ATPase showed that it behaves according to the conventional Ca-ATPase reaction mechanism, consistent with monomeric catalytic function. We conclude that the nonconforming features of native SERCA1 arise from oligomeric protein conformational interactions that constrain the subunits to a staggered or out-of-phase mode of operation.

Adenosine Diphosphate↗

PubMed2001

Modelling bivariate ordinal responses smoothly with examples from ophthalmology and genetics.

A non-parametric implementation of the bivariate Dale model (BDM) is presented as an extension of the generalized additive model (GAM) of Hastie and Tibshirani. The original BDM is an example of a bivariate generalized linear model. In this paper smoothing is introduced on the marginal as well as on the association level. Our non-parametric procedure can be used as a diagnostic tool for identifying parametric transformations of the covariates in the linear BDM, hence it also provides a kind of goodness-of-fit test for a bivariate generalized linear model. Cubic smoothing spline functions for the covariates are estimated by maximizing a penalized version of the log-likelihood. The method is applied to two studies. The first study is the classical Wisconsin Epidemiologic Study of Diabetic Retinopathy. The second study is a twin study, where the association between the elements of twin pairs is of primary interest. The results show that smoothing on the association level can give a significant improvement to the model fit.

Adolescent↗

PubMed1994

A model of synchronization of motor acts to a stimulus sequence. II. Stability analysis, error estimation and simulations.

In Part I (Mates 1994), a linear model of timing and error-corrections was constructed that aims at an explanation of the mechanisms underlying a subject's performance in an experimental paradigm, in which the task is to synchronize a sequence of motor acts to a sequence of stimuli. The model consists of two error-corrective mechanisms: (1) corrections of period (inverted frequency) of the sequence of responses; (2) corrections of phase shift of that sequence (synchronization error). In this paper, the influence of the physiologically justifiable model variables and of initial conditions on the steady-state response sequence as well as the stability of performance of the model are analyzed. The model is stable for error-correction gains in the range from 0 to 2. Comparison with known empirical data supports the assumption that reasonable values are less than 1. Furthermore, an alternative to the basic linear model is introduced in which the possible character of the process of subjective acquisition of the synchronization error is discussed. On the basis of findings from other experimental paradigms (fusion and order threshold) it can be assumed that the subjective estimate is a nonlinear function of the difference between the temporal central availability of internal representations of the stimulus and response-feedback events. Some other known synchronization data are simulated by the nonlinear modification of the model in this paper. A good fit of the simulation results achieved further justifies the model structure proposed. Finally, the possible effect of the subjective synchronization-error estimation on empirical data is discussed.

Humans↗

PubMed2005

The Maudsley long-term follow-up of child and adolescent depression. Predicting costs in adulthood.

BACKGROUND: Little is known about the long-term economic impact of childhood depression. The aim of this study is to identify and examine the links between the characteristics of children with depression and the costs of services used in adulthood. METHODS: Subjects (N=149) who had attended psychiatric services in South London for depression were followed up on average 20.7 years later. Sociodemographic and illness characteristics were recorded in childhood and service use in adulthood was measured. Costs were calculated and multiple regression models were developed to explain variations in cost, with a comparison between ordinary least squares estimation and a generalised linear model. RESULTS: Service use and cost data were available on 140 subjects. The mean annual cost was 890 pounds Sterling (range 0 pounds Sterling-7532 pounds Sterling). Predictors of cost variations in both models were age at initial referral, level of childhood anxiety, and the presence of comorbid conduct disorder. In the ordinary least squares model, a family history of psychiatric illness was inversely related to cost, whilst in the generalised linear model there was an inverse link between peer/sibling relationship problems and costs. The models could explain 24 % and 20% of cost variation, respectively. CONCLUSIONS: It is possible to explain a reasonable amount of variation in adult service costs from factors describing the characteristics of children at the time of receiving care.

Adolescent↗

PubMed2005

Methods for confidence interval estimation of a ratio parameter with application to location quotients.

BACKGROUND: The location quotient (LQ) ratio, a measure designed to quantify and benchmark the degree of relative concentration of an activity in the analysis of area localization, has received considerable attention in the geographic and economics literature. This index can also naturally be applied in the context of population health to quantify and compare health outcomes across spatial domains. However, one commonly observed limitation of LQ is its widespread use as only a point estimate without an accompanying confidence interval. METHODS: In this paper we present statistical methods that can be used to construct confidence intervals for location quotients. The delta and Fieller's methods are generic approaches for a ratio parameter and the generalized linear modelling framework is a useful re-parameterization particularly helpful for generating profile-likelihood based confidence intervals for the location quotient. A simulation experiment is carried out to assess the performance of each of the analytic approaches and a health utilization data set is used for illustration. RESULTS: Both the simulation results as well as the findings from the empirical data show that the different analytical methods produce very similar confidence limits for location quotients. When incidence of outcome is not rare and sample sizes are large, the confidence limits are almost indistinguishable. The confidence limits from the generalized linear model approach might be preferable in small sample situations. CONCLUSION: LQ is a useful measure which allows quantification and comparison of health and other outcomes across defined geographical regions. It is a very simple index to compute and has a straightforward interpretation. Reporting this estimate with appropriate confidence limits using methods presented in this paper will make the measure particularly attractive for policy and decision makers.

Computer Simulation↗

PubMed2007

[Analysis of variance of repeated data measured by water maze with SPSS].

OBJECTIVE: To introduce the method of analyzing repeated data measured by water maze with SPSS 11.0, and offer a reference statistical method to clinical and basic medicine researchers who take the design of repeated measures. METHODS: Using repeated measures and multivariate analysis of variance (ANOVA) process of the general linear model in SPSS and giving comparison among different groups and different measure time pairwise. RESULTS: Firstly, Mauchly's test of sphericity should be used to judge whether there were relations among the repeatedly measured data. If any (P<or=0.05), multivariate ANOVA should be taken next, or Greenhouse-Geisser corrected results should be taken. Treated effect could be evaluated by estimating between-subject variance. Repeated measurement effect or its interactive effect with treated group could be evaluated by estimating within-subject variance. The method of Bonferroni should be used to do pairwise comparisons of the repeatedly measured data in different measurement time of each treated group. With multivariate ANOVA, data in different treated group of each measurement time could be compared pairwise. CONCLUSION: The repeated measures process of the general linear model is suitable for variance analysis of repeatedly measured data. SPSS statistical package is available to fulfil this process.

Analysis of Variance↗

PubMed2006

The effect of nonlinear human visual system components on performance of a channelized Hotelling observer in structured backgrounds.

Linear model observers based on statistical decision theory have been used successfully to predict human visual detection of aperiodic signals in a variety of noisy backgrounds. However, some models have included nonlinearities such as a transducer or nonlinear decision rules to handle intrinsic uncertainty. In addition, masking models used to predict human visual detection of signals superimposed on one of two identical backgrounds (masks) usually include a number of nonlinear components in the channels that reflect properties of the firing of cells in the primary visual cortex (V1). The effect of these nonlinearities on the ability of linear model observers to predict human signal detection in real patient structured backgrounds is unknown. We evaluate the effect of including different nonlinear human visual system components into a linear channelized Hotelling observer (CHO) using a signal known exactly but variable (SKEV) task. In particular, we evaluate whether the rank order of two compression algorithms (JPEG versus JPEG 2000) and two compression encoder settings (JPEG 2000 default versus JPEG 2000 optimized) based on model observer signal detection performance in X-ray coronary angiograms is altered by inclusion of nonlinear components. The results show: 1) the simpler linear CHO model observer outperforms CHO model with the nonlinear components; 2) the rank order of model observer performance for the compression algorithms/parameters does not change when the nonlinear components are included. For the present task and images, the results suggest that the addition of the nonlinearities to a channelized Hotelling model may add complexity to the model observers without great impact on rank order evaluation of image processing and/or acquisition algorithms.

Algorithms↗

PubMed1996

Caries prevalence and salt fluoridation among 9-year-old schoolchildren in Strasbourg, France.

The 9-year-old group (236 children) of an epidemiological study carried out in 1991 in Strasbourg on children aged 6 to 15 years was selected with the aim of determining if the caries prevalence reduction observed could be related to the use of salt fluoridation (FS) introduced in France in 1987. From these 236 children, 143 answered a questionnaire which showed that 36 of them were fluoridated salt users and 107 were not. The dft index was significantly lower in the FS consumers which showed 35.5% dft reduction compared to the non-FS-consuming children. When using the Generalized Linear Model, this reduction was significant (P = 0.03). Although lower in the FS group, the DMFT and DMFS indices showed no statistical significant difference. It appeared that 72.2% of the users took simultaneously fluoride tablets but no dental fluorosis was observed. The use of fluoride tablets had a significant effect on the DMFS (P = 10(-2)). The children who consumed FS used more frequently fluoridated mouthrinses (P = 10(-3)) and had more frequent professional application of fluoridated gel and varnishes than non-consumers (P = 0.02). The DMFS index increased with the number of meals (P = 10(-6)), which was the most significant variable entered into the Generalized Linear Model. The children who brushed their teeth once a day had a DMFS value 2.6 times higher than those who brushed regularly three times a day (P = 10(-3)). The DMFS value was 4.4 times higher among the children who brushed their teeth irregularly when compared with those who brushed three times a day (P = 10(-2)).

Cariostatic Agents↗

PubMed2001

Effects of development time, biomass and ferromanganese oxides on nickel sorption by stream periphyton.

Procedurally defined periphyton frequently includes substantial quantities of hydrous iron (Fe) and manganese (Mn) oxides. As these oxides are strong sorbers of heavy metals, their presence may complicate estimation of metal bioaccumulation by periphyton. We examined the relationship between nickel (Ni) sorption and the development time, biomass, and Fe and Mn oxide content of stream periphyton. Development time, the time during which periphyton accrued on submerged tile substrata, was used to provide variation in biomass, Fe and Mn levels. Stream periphyton from four development times was exposed to Ni for 2 h in the laboratory, and then ashed. Development time was significantly associated with ash-free dry mass (AFDM), Fe and Mn levels (ANOVA, P < or = 0.003). Ni extracted by a mild reductant (hydroxylamine hydrochloride) was significantly associated with development time, and with AFDM, Fe and Mn levels (linear models, P < or = 0.0002). A subsequent acid digestion yielded similar associations with the same variables (linear models, P < or = 0.0001). For both extractions, AFDM was significantly and positively correlated with Fe (r = 0.68 and 0.89) and with Mn (r = 0.77 and 0.93) (Spearman rank, P < or = 0.005). These data demonstrate the importance of periphyton development time in influencing both metal sorption and levels of biomass and ferromanganese oxides. The data also suggest that metal contaminant levels in periphyton should not be attributed automatically to biotic sorption. Periphyton metal-accumulation studies conducted where ferromanganese oxide concentrations are elevated should address the potential metal-sorbing roles of Fe and Mn oxides within the periphyton matrix.

Absorption↗