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PubMed2004

Comparative effects of pH and Vision herbicide on two life stages of four anuran amphibian species.

Vision, a glyphosate-based herbicide containing a 15% (weight:weight) polyethoxylated tallow amine surfactant blend, and the concurrent factor of pH were tested to determine their interactive effects on early life-stage anurans. Ninety-six-hour laboratory static renewal studies, using the embryonic and larval life stages (Gosner 25) of Rana clamitans, R. pipiens, Bufo americanus, and Xenopus laevis, were performed under a central composite rotatable design. Mortality and the prevalence of malformations were modeled using generalized linear models with a profile deviance approach for obtaining confidence intervals. There was a significant (p < 0.05) interaction of pH with Vision concentration in all eight models, such that the toxicity of Vision was amplified by elevated pH. The surfactant is the major toxic component of Vision and is hypothesized, in this study, to be the source of the pH interaction. Larvae of B. americanus and R. clamitans were 1.5 to 3.8 times more sensitive than their corresponding embryos, whereas X. laevis and R. pipiens larvae were 6.8 to 8.9 times more sensitive. At pH values above 7.5, the Vision concentrations expected to kill 50% of the test larvae in 96-h (96-h lethal concentration [LC50]) were predicted to be below the expected environmental concentration (EEC) as calculated by Canadian regulatory authorities. The EEC value represents a worst-case scenario for aerial Vision application and is calculated assuming an application of the maximum label rate (2.1 kg acid equivalents [a.e.]/ha) into a pond 15 cm in depth. The EEC of 1.4 mg a.e./L (4.5 mg/L Vision) was not exceeded by 96-h LC50 values for the embryo test. The larvae of the four species were comparable in sensitivity. Field studies should be completed using the more sensitive larval life stage to test for Vision toxicity at actual environmental concentrations.

Animals↗

PubMed1999

Properties of threshold model predictions.

Estimation of genetic parameters and accuracy of threshold model genetic predictions were investigated. Data were simulated for different population structures by using Monte Carlo techniques. Variance components were estimated by using threshold models and linear sire models applied to untransformed data, logarithmically transformed data, and transformation to Snell scores. Effects of number of categories (2, 5, and 10), incidence of categories (extreme, moderate, and normal), heritability in the underlying scale (.04, .20, and .50), and data structure (unbalanced and balanced) on accuracy of genetic prediction were investigated. The real importance of using a threshold model was to estimate genetic parameters. An expected heritability of .20 was estimated to be .22 and .10 by a threshold model and a linear model, respectively. Accuracy increased significantly with a larger number of categories, a more normal distribution of incidences, increased heritability, and more balanced data. Even threshold models were shown to be more efficient with more than two categories (e.g., binomial). Transformation of scale did not accomplish the purpose intended.

Animal Husbandry↗

PubMed1980

A method of sire evaluation for dichotomies.

In this paper, the following problems associated with genetic evaluation of categorical traits by linear models are discussed: (1) Scores are arbitrarily assigned to response categories. (2) Mixed model solutions do not incorporate the restriction in the estimation space that the sum of response probabilities must total 1 across categories (3) The variance in the observed scale is not constant and depends on the genotypic value of the candidates for selection. (4) The additive genetic variance in the observed scale depends on the mean incidence of the character in the subpopulations considered in the model. (5) Nonadditive genetic variation is present in the observed scale. (6) Linear relationships fail outside a restricted range of the data. (7) Ranking optimality of best linear predictors is lacking when the conditional expectation of the predictand given the data is not linear. A method of sire evaluation of dichotomies based on a log-linear model is introduced, its properties are discussed and examples are presented.

Animals↗

PubMed1991

Genetic analysis of dystocia in dairy cattle.

Breeding values and genetic parameters for dystocia were estimated in Normande and Holstein breeds. Dystocia scores were related to an underlying continuous variable via a threshold model. The underlying linear model included the effects of calving season, sex of calf by parity of dam, sire of calf, grandsire of calf, dam within maternal grandsire and herd-year effects. Typical results were found for the environmental effects, with a strong influence of dam parity on dystocia, a strong influence of sex of calf, and a small effect of calving season. Herd-year variances were 32 and 40% of the residual variance in the Normande and Holstein breeds, respectively. Heritabilities for the Normande (Holstein) breed were .08 (.07) for direct effects and .11 (.07) for maternal effects. Correlations between sire and grandsire effects were .51 and .36 for the Normande and Holstein breeds, respectively. The corresponding correlations between direct and maternal effects were .15 and -.09. The results of this study show that a complete model for dystocia including the threshold concept and maternal effects can be applied for routine evaluation of dairy AI bulls. Maternal effects are important, and they should be considered in dystocia analysis, especially if nonrandom mating is present. Selection for reducing dystocia in calf and cow effects are not antagonistic.

Animals↗

PubMedDate not supplied

[Ultraviolet radiation doses in Mexican students].

OBJECTIVE: To determine the ultraviolet radiation dose received by children and adolescents at elementary, middle, or high school. MATERIAL AND METHODS: A cohort study was conducted in the City of San Luis Potosí between May 2001 and April 2002. The study population consisted of 80 school male and female children, selected at random, aged 6 to 19 years of age. The dose was quantitated during an entire schoolyear by recording the time each student was exposed to sunlight. Ultraviolet radiation was simultaneously monitored with terrestrial radiometry equipment. Holidays and weekends were excluded. Data were analyzed using univariate analysis and comparative analysis by sex and months of exposure. A regression model was fit to explain the dose of solar exposure. RESULTS: The erythemally weighted UV dose for Mexican schoolchildren averaged 16,456 J/m2/year. Differences by gender were found: 14,264 J/m2/year in females vs. 18,648 J/m2/year in males (Kolmogorov-Smirnov, p = 0.003). No significant differences were found among groups. Significant differences were found among months of exposure (Kruskal-Wallis, p = < 0.0001). Stepwise regression models were fit to find the best model, using generalized linear modeling and the Akaike information criterion, to explain the radiation dose according to month of exposure; the final equation was 587.20 + 438.45(gender)0.16(month)-49.65(month2). The results showed higher radiation doses between March and September. CONCLUSIONS: A theoretical framework is advanced to formulate policies aimed at protecting children in Mexican schools from solar overexposure. Implementing prevention measures at least during the months of greatest exposure (March to June) is in order since 51% of the annual ultraviolet radiation dose is received in this period. By avoiding exposure 8 minutes daily, the radiation dose could be decreased in 39,495 J/m2, which is the dose accumulated in two years. The English version of this paper is available at:http://www.insp.mx/salud/index.html.

Adolescent↗

PubMed2003

Centre and surround responses of marmoset lateral geniculate neurones at different temporal frequencies.

The responses of marmoset lateral geniculate neurones to stimuli that were composed of a sinusoidally modulating centre stimulus and a surround that was modulated in counterphase were measured. The size of the stimulus centre was varied. These measurements were repeated at different temporal frequencies between 1 and 30 Hz. The response amplitudes and phases depended in a characteristic manner on the stimulus centre size. The response behaviour could be modelled by assuming Gaussian responsivity profiles of the cells' receptive field (RF) centres and surrounds and a phase delay in the RF surround responses, relative to the centre, enabling the description of RF centre and surround response characteristics. We found that the RF centre-to-surround phase difference increased linearly with increasing temporal frequency, indicating a constant delay difference of about 4.5 to 6 ms. A linear model, including low-pass filters, a lead lag stage and a delay, was used to describe the mean RF centre and surround responses. The separate RF centre and surround responses were less band pass than the full receptive field responses of the cells. The linear model provided less satisfactory fits to M-cell responses than to those of P-cells, indicating additional nonlinearities.

Animals↗

PubMed1999

Estimation of intraoperative aortocoronary bypass saphenous vein graft circumference from its preoperative compliance.

The aim of this study was to determine if the intraoperative circumference of aortocoronary saphenous vein bypass grafts could be predicted from preoperative measurement with B-mode ultrasound sonography in 50 patients. The circumference of the saphenous vein was measured during stepwise increments of a thigh congestive cuff from 0 to 60 mmHg. The circumference of the corresponding segment of the coronary bypass vein graft was measured intraoperatively with callipers. The intraoperative circumference was higher (11.8+/-2.3 mm) than the preoperative circumference (10.2+/-2.4 mm, P=0.006) matched to its corresponding intraoperative mean arterial pressure (57+/-15 mmHg). The prediction of the intraoperative circumference by estimation from the preoperative pressure-circumference relationship fitted by a linear model (r = 0.412, P = 0.004) did not improve on the preoperative circumference matched by arterial pressure alone (r = 0.429, P = 0.003). The intraoperative circumference of the graft vein exceeded its preoperative circumference by 12%. Prediction of the intraoperative graft vein circumference is underestimated by a linear model of its preoperative compliance.

Aged↗

PubMed1996

[Study clinical application to determine heparin requirements during hemodialysis].

A de nove Weighted Stepwise Linear Model based on Gotch's heparin pharmacokinetic model has been designed to determine the heparin elimination rate constant during hemodialysis (HD). 100 times of HD in 26 patients with unstable dosage requirements of heparin were performed according to the computing method and the new heparin pharmacokinetic Weighted Stepwise Linear Model monitored be program of pocket calculator. The results showed not only stable anticoagulation but also successful prevention of patients's bleeding and avoidance of coagulation in hemodialyzer and blood tubing. The computer diagram presented in this paper is useful for propramming with different computer languages to calculate the heparin dosage requirements of patients undergoing HD.

Computers↗

PubMedDate not supplied

Quantitative prediction of logk of peptides in high-performance liquid chromatography based on molecular descriptors by using the heuristic method and support vector machine.

A new method support vector machine (SVM) and the heuristic method (HM) were used to develop the nonlinear and linear models between the capacity factor (logk) and seven molecular descriptors of 75 peptides for the first time. The molecular descriptors representing the structural features of the compounds only included the constitutional and topological descriptors, which can be obtained easily without optimizing the structure of the molecule. The seven molecular descriptors selected by the heuristic method in CODESSA were used as inputs for SVM. The results obtained by SVM were compared with those obtained by the heuristic method. The prediction result of the SVM model is better than that of heuristic method. For the test set, a predictive correlation coefficient R = 0.9801 and root-mean-square error of 0.1523 were obtained. The prediction results are in very good agreement with the experimental values. But the linear model of the heuristic method is easier to understand and ready to use for a chemist. This paper provided a new and effective method for predicting the chromatography retention of peptides and some insight into the structural features which are related to the capacity factor of peptides.

Artificial Intelligence↗

PubMed2006

The temporal progression of pelvic osteolysis after uncemented total hip arthroplasty.

This study examined the natural progression of pelvic osteolysis post uncemented total hip arthroplasty. Serial radiographs from 145 total hip arthroplasties with minimum 10-year follow-up were examined. Thirty-two lesions were identified in 25 hips and their 2-dimensional radiographic size over time was measured via computer. Lesion growth rate averaged 42.7 +/- 49.0 mm2/y. Linear models were generally a good fit for lesion growth as evidenced by a high mean r2 value (0.77 +/- 0.18); lesions progressed steadily during the first 10 postoperative years. Although lesions were first radiographically evident 5.7 years postarthroplasty, the x-intercept of the linear model, approximating the mean time when the lesions first began to develop, averaged 1.3 years. Because this study expands our understanding of when and how peri-acetabular lesions develop, it can be useful to clinicians in determining how often to see patients postoperatively.

Acetabulum↗

PubMed2002

Estimating the effective degrees of freedom in univariate multiple regression analysis.

The general linear model provides the most widely applied statistical framework for analyzing functional MRI (fMRI) data. With the increasing temporal resolution of recent scanning protocols, and more elaborate data preprocessing schemes, data independency is no longer a valid assumption. In this paper, we revise the statistical background of the general linear model in the presence of temporal autocorrelations. First, when detecting the activation signal, we explicitly account for the temporal autocorrelation structure, which yields a generalized F-test and the associated corrected (or effective) degrees of freedom (DOF). The proposed approach is data driven and thus independent of any specific preprocessing method. Then, for event-related protocols, we propose a new model for the temporal autocorrelations ("damped oscillator" model) and compare this model to another, previously used in the field (first-order autoregressive model, or AR(1) model). In the case of long fMRI time series, an efficient approximation for the number of effective DOF is provided for both models. Finally, the validity of our approach is assessed using simulated and real fMRI data and is compared with more conventional methods.

Analysis of Variance↗

PubMed2000

On the linearity of accommodation dynamics.

Available experimental data on the gain and phase lags in the closed-loop accommodation responses to stimuli whose dioptric vergence changes sinusoidally with time are examined. It is shown that both phase lags and gain change almost linearly with the temporal frequency of the stimulus. This implies that the phase lags correspond to a frequency-independent time delay and that, if the system is linear, the impulse response should take a symmetrical (sin x/x)(2) form, with a corresponding standard form of step response. Comparison of this derived step response with the quasi-exponential results found experimentally underlines the limitations of attempting to generalize dynamic accommodation performance from responses obtained with predictable sinusoidally-changing stimuli and the application of a linear model. Consideration of the frequency response which would lead to an exponential step response supports the argument that, although the gain results obtained with sinusoidal stimuli are reasonably representative of those applying with other forms of stimulus, the predictable nature of sinusoidally-changing stimuli makes the derived phase lags inapplicable to non-predictable step or other stimuli. Other limitations in linear models of the response are discussed.

Accommodation, Ocular↗

PubMed2000

A linear material model for fiber-induced anisotropy of the anulus fibrosus.

The anulus fibrosus (AF) is a lamellar, fibrocartilaginous component of the intervertebral disc, which exhibits highly anisotropic behaviors in tension. These behaviors arise from the material's unique collagen structure. We have investigated the use of a linear, fiber-induced anisotropic model for the AF using a quadratic strain energy density formulation with an explicit representation of the collagen fiber populations. We have proposed a representative set of intrinsic material properties using independent datasets of the AF from the literature and appropriate thermodynamic constraints. The model was validated by comparing predictions with previous experimental data for AF behavior and its dependence on fiber angle. The model predicts that compressible effects may exist for the AF, and suggests that physical effects of the equivalent "matrix," "fiber," "fiber-matrix," and "fiber-fiber," interactions may be important contributors to the mechanical behavior of the AF.

Anisotropy↗

PubMed2001

Neurovascular coupling in terms of a control system: validation of a second-order linear system model.

Neurovascular coupling (NC) adapts cerebral blood flow to cortical activity. Functional transcranial Doppler (f-TCD) investigations revealed a typical time course of evoked blood flow responses with an initial overshoot and a stabilization at a lower, but stable, level. This blood flow reaction can be described in terms of a control-system model. We tested reliability and validity of this new approach using different stimulation paradigms. The P2 segment of the posterior cerebral artery was insonated in 14 healthy volunteers and, subsequently, NC was stimulated using two tests, a checkerboard stimulus of 1 s with different repetition rates and a reading test. Data were analyzed according to algorithms of control-system theory. The reading test was used to measure test reliability and side differences. A second-order linear system can describe blood flow regulation of NC. Different stimulation protocols of the checkerboard test and the related evoked blood flow curves could be described by the same control-system model. Further, there were no differences between the checkerboard and reading test nor between right and left side or test and retest of the reading test. NC can be described in a much more detailed manner using control-system analysis. We were able to show that blood flow response due to different visual stimuli follow one common control-system model. Unlike quantification of NC using overshoot, parameters of the control system have smaller SDs, increasing the statistical power and, thereby, usefulness of f-TCD as a diagnostic instrument.

Adult↗

PubMed2003

Genetic change for clinical mastitis in Norwegian cattle: a threshold model analysis.

Records of clinical mastitis on 1.6 million first-lactation daughters of 2,411 Norwegian Cattle sires that were progeny tested from 1978 through 1998 were analyzed with a threshold model. The main objective was to infer genetic change for the disease in the population. A Bayesian approach via Gibbs sampling was used. The model for the underlying liability had age at first calving, month x year of calving, herd x 3-year-period, and sire of the cow as explanatory variables. Posterior mean (SD) of heritability of liability to clinical mastitis was 0.066 (0.003). Genetic evaluations (posterior means) of sires both in the liability and observable scales were computed. Annual genetic change of liability to clinical mastitis for progeny tested bulls born from 1973 to 1993 was assessed. The linear regression of mean sire effect on year of birth had a posterior mean (SD) of -0.00018 (0.0004), suggesting a nearly constant genetic level for clinical mastitis. However, an analysis of sire posterior means by birth-year of daughters indicated an approximately constant genetic level in the cow population from 1976 to 1990 (-0.02%/yr), and a genetic improvement thereafter (-0.27%/yr). This reflects more emphasis on mastitis in selection of bulls in recent years. Corresponding results obtained with a standard linear model analysis were -0.01% and -0.23% per year, respectively (regression of sire predicted transmitting ability on birth-year of daughters). Genetic change seems to be slightly understated with the linear model, assuming the threshold model holds true.

Animals↗

PubMed2003

No association of smoke-free ordinances with profits from bingo and charitable games in Massachusetts.

BACKGROUND: Because it is widely played, claims that smoking restrictions will adversely affect bingo games is used as an argument against these policies. We used publicly available data from Massachusetts to assess the impact of 100% smoke-free ordinances on profits from bingo and other gambling sponsored by charitable organisations between 1985 and 2001. METHODS: We conducted two analyses: (1) a general linear model implementation of a time series analysis with net profits (adjusted to 2001 dollars) as the dependent variable, and community (as a fixed effect), year, lagged net profits, and the length of time the ordinance had been in force as the independent variables; (2) multiple linear regression of total state profits against time, lagged profits, and the percentage of the entire state population in communities that allow charitable gaming but prohibit smoking. RESULTS: The general linear model analysis of data from individual communities showed that, while adjusted profits fell over time, this effect was not related to the presence of an ordinance. The analysis in terms of the fraction of the population living in communities with ordinances yielded the same result. CONCLUSION: Policymakers can implement smoke-free policies without concern that these policies will affect charitable gaming.

Gambling↗

PubMed2006

A modeling approach to the analysis of nerve regenerative experiments.

Many experiments aiming at the investigation of nerve repair involve elaborate testing over a certain time period. Data arising from such experiments are often analyzed by time-point. Such a cross-sectional approach is often very inefficient. In this paper, we consider a case study in which repeated measurements of two response variables assumed to be Poisson distributed are obtained. We show how a repeated measures modeling approach, based on generalized linear models, can handle both responses in one model and improve the inference in the nerve repair experiments. The benefits of the model as well as problems that can occur are illustrated and discussed.

Algorithms↗

PubMed2004

Electronic monitoring of variation in drug intakes can reduce bias and improve precision in pharmacokinetic/pharmacodynamic population studies.

Population pharmacokinetic (PK) and pharmacodynamic (PD) studies evaluate drug concentration profiles and pharmacological effects over time when standard drug dosage regimens are assigned. They constitute a scientific basis for the determination of the optimal dosage of a new drug. Population PK/PD analyses can be performed on relatively few measures per patient enabling the study of a sizable sample of patients who take the drug over a possibly long period of time. We expose the problem of bias in PK/PD estimators in the presence of partial compliance with assigned treatment as it occurs in practice. We propose to solve this by recording accurate data on a number of previous dose timings and using timing-explicit hierarchical non-linear models for analysis. In practice, we rely on electronic measures of an ambulatory patient's drug dosing histories. Especially for non-linear PD estimation, we found that not only bias can be reduced, but higher precision can also be retrieved from the same number of data points when irregular drug intake times occur in well-controlled studies. We apply methods proposed by Mentré et al. to investigate the information matrix for hierarchical non-linear models. This confirms that a substantial gain in precision can be expected due to irregular drug intakes. Intuitively, this is explained by the fact that regular takers experience a relatively small range of concentrations, which makes it hard to estimate any deviation from linearity in the effect model. We conclude that estimators of PK/PD parameters can benefit greatly from information that enters through greater variation in the drug exposure process.

Dose-Response Relationship, Drug↗