References of "Lambert, Philippe"
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See detailStatistical models for the analysis of controlled trials on acute migraine
Vandenhende, François; Lambert, Philippe ULg

in Pharmaceutical Statistics (2003), 2

pecific efficacy criteria were defined by the International Headache Society for controlled clinical trials on acute migraine. They are derived from the pain profile and the timing of rescue medication ... [more ▼]

pecific efficacy criteria were defined by the International Headache Society for controlled clinical trials on acute migraine. They are derived from the pain profile and the timing of rescue medication intake. We present a methodology to improve the analysis of such trials. Instead of analysing each endpoint separately, we model the joint distribution and derive success rates in any criteria as predictions. We use cumulative regression models for each response at a time and a multivariate normal copula to model the dependence between responses. Parameters are estimated using maximum likelihood. Benefits of the method include a reduction in the number of tests performed and an increase in their power. The method is well suited to dose-response trials from which predictions can be used to select doses and optimize the design of subsequent trials. More generally, our method permits a very flexible modelling of longitudinal series of ordinal data. [less ▲]

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See detailImproved rank-based dependence measures for categorical data
Vandenhende, François; Lambert, Philippe ULg

in Statistics & Probability Letters (2003), 63

We extend rank-based dependence measures like Spearman's rho to categorical data so that the same ±1 limits are always reached under complete dependence. A goodness-of-fit procedure is derived for ... [more ▼]

We extend rank-based dependence measures like Spearman's rho to categorical data so that the same ±1 limits are always reached under complete dependence. A goodness-of-fit procedure is derived for dependence models using copulas. [less ▲]

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See detailA mixture model to assess the effect of a hormonal stimulation on the development of follicles in prepubertal heifers
Lambert, Philippe ULg

in Journal of the Royal Statistical Society : Series C (Applied Statistics) (2002), 4

The goal of this study is to assess the effect of follicle stimulating hormone (FSH) on the growth of follicles in prepubertal heifers. Adams and co-workers have shown that follicles develop in waves as ... [more ▼]

The goal of this study is to assess the effect of follicle stimulating hormone (FSH) on the growth of follicles in prepubertal heifers. Adams and co-workers have shown that follicles develop in waves as in post-pubertal heifers: after a few days, a dominant follicle arises on one of the two ovaries whereas the other subordinate follicles regress.The dominant follicle remains detectable when the next wave occurs. The number and the diameters of these follicles have been measured for 14 days.The same experiment was repeated after a 3-day FSH stimulation. Several large follicles now arise simultaneously on both ovaries together with small follicles which cannot be detected any more when the next wave occurs. Unfortunately, follicles were not individually tracked. Therefore a three-component mixture model was used to describe the potential simultaneous observation of dominant, decaying dominant and subordinate follicles before the FSH stimulation; the same type of model was considered for large, decaying large and small follicles after the hormonal stimulation. Moreover, for technical reasons, only follicles larger than 2 mm are observable. Measurements were thus left truncated.The models fitted are used to quantify the effects of the stimulation and to determine the ideal time point at which follicles should be punctured under ultrasonography to harvest ovocytes that will be fertilized and cultured in vitro. [less ▲]

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See detailA copula based model for multivariate non normal longitudinal data: analysis of a dose titration safety study on a new antidepressant
Lambert, Philippe ULg; Vandenhende, François

in Statistics in Medicine (2002), 21

A new model for multivariate non-normal longitudinal data is proposed. In a first step, each longitudinal series of data corresponding to a given response is modelled separately using a copula to relate ... [more ▼]

A new model for multivariate non-normal longitudinal data is proposed. In a first step, each longitudinal series of data corresponding to a given response is modelled separately using a copula to relate the marginal distributions of the response at each time of observation. In a second step, at each observation time, the conditional (on the past) distributions of each response are related using another copula describing the relationship between the corresponding variables. Note that there is no need to consider the same family of distributions for these response variables. The technique is illustrated in a dose titration safety study on a new antidepressant. The haemodynamic effect on diastolic blood pressure, systolic blood pressure and heart rate is studied. These three responses are measured repeatedly over time on ten healthy volunteers during the dose escalation. The available covariates are sex and the concentration of drug in the plasma at time of measurement. [less ▲]

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See detailOn the joint analysis of longitudinal responses and early discontinuation in randomized trials
Vandenhende, François; Lambert, Philippe ULg

in Journal of Biopharmaceutical Statistics (2002), 12

Our focus is on the joint analysis of longitudinal nonnormal responses and early discontinuation in (pre)-clinical trials. Separate models are fitted to the two series (response and discontinuation) to ... [more ▼]

Our focus is on the joint analysis of longitudinal nonnormal responses and early discontinuation in (pre)-clinical trials. Separate models are fitted to the two series (response and discontinuation) to account for covariate and time effects. The serial dependence and the dependence between response and drop-out are also modeled. This is done using particular dependence functions, called copulas. Copulas are used to create a joint distribution with given marginal distributions. Applications are given for the analysis of heart rate/morbidity in toxicology and pain severity/intake of rescue medications in a trial on migraine. Using copulas, the level of dependence between two variables remains invariant to changes in the marginal distribution of either variable. This proves interesting in modeling the association in a longitudinal setting when responses change over time. [less ▲]

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See detailEffect of conventional controlled-rate freezing and vitrification on quality of bovine blasotcysts produced in vitro
Kaidi, Safia; Bernard, S.; Lambert, Philippe ULg et al

in Biology of Reproduction (2001), 65

This study compares the effects of conventional controlled-rate freezing and vitrification on the morphology and metabolism of in vitro-produced bovine blastocysts. Day 7 expanded blastocysts cultured in ... [more ▼]

This study compares the effects of conventional controlled-rate freezing and vitrification on the morphology and metabolism of in vitro-produced bovine blastocysts. Day 7 expanded blastocysts cultured in synthetic oviduct fluid with 5% fetal calf serum were frozen in 1.36 M glycerol, 0.25 M sucrose or vitrified in 25% glycerol, 25% ethylene glycol. Cell alterations and in vitro development were evaluated immediately after thawing or after 72 h. The effect of cryopreservation on inner cell mass and trophectoderm (TE) cell number as well as glucose, pyruvate, and oxygen uptakes, and lactate release by blastocysts were evaluated. Immediately after thawing, blastocysts showed equivalent cell membrane permeabilization after both cryopreservation procedures, while alterations in nuclear staining were more frequent in vitrified embryos. After culture, similar survival and hatching rates were observed. Both procedures decreased cell number immediately after thawing and after 72 h. However, the number of TE cells was lower in frozen embryos than in vitrified ones. In relation to this, frozen blastocysts showed a decrease in glucose, pyruvate, and oxygen uptake, although those parameters were not altered in vitrified embryos. An increased glycolytic activity was also observed in frozen embryos, indicating a stress response to this procedure. [less ▲]

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See detailSmoothed nonparametric maximum likelihood estimation of the risk distribution underlying bonus-malus systems
Denuit, Michel; Lambert, Philippe ULg

in Proceedings of the Casualty Actuarial Society (2001), LXXXVIII

Mixed Poisson distributions are widely used for modeling claim counts when the portfolio is thought to be heterogeneous. The risk (or mixing) distribution then represents a measure of this heterogeneity ... [more ▼]

Mixed Poisson distributions are widely used for modeling claim counts when the portfolio is thought to be heterogeneous. The risk (or mixing) distribution then represents a measure of this heterogeneity. The aim of this paper is to use a variant of the Patilea and Rolin [15] smoothed version of the Simar [20] Non-Parametric Maximum Likelihood Estimator of the risk distribution in the mixed Poisson model. Empirical results based on two data sets from automobile third-party liability insurance demonstrate the relevance of this approach. The design of merit-rating schemes is discussed in the second part of the paper. [less ▲]

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See detailOsmotic behaviour of in vitro produced bovine blastocysts in cryoprotectant solutions as a potential predictive test of survival
Kaidi, Safia; Donnay, I.; Lambert, Philippe ULg et al

in Cryobiology (2000), 41

The osmotic behavior of bovine blastocysts produced in vitro was filmed during exposure to and dilution of cryoprotectant solutions used for vitrification. The relationship between the changes in the ... [more ▼]

The osmotic behavior of bovine blastocysts produced in vitro was filmed during exposure to and dilution of cryoprotectant solutions used for vitrification. The relationship between the changes in the diameter of embryos and their subsequent survival was assessed. Embryos collected on Day 6 and Day 7 postinsemination were exposed to 10% glycerol (GLY) for 5 min, 10% GLY + 20% ethylene glycol (EG) for 5 min, and 25% Gly + 25% EG for 30 s, before dilution in 0.85 M galactose and finally in embryo transfer freezing medium (ETF). Embryos that had a higher probability of survival behaved as perfect osmometers, shrinking, reexpanding, or swelling according to an identical pattern, whereas embryos that deviated from this standard usually did not survive. The initial embryo diameter, duration of shrinkage and expansion in 10% glycerol, duration of reexpansion in ETF, and final embryo diameter were clearly predictive of the ability to hatch after culture in vitro. On a given day postinsemination, larger blastocysts were more likely than smaller blastocysts to survive and hatch after exposure to cryoprotectants with or without vitrification. [less ▲]

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See detailAlveolar clearance in horses with chronic obstructive pulmonary disease
Votion, Dominique ULg; Vandenput, Sandrina ULg; Duvivier, D. H. et al

in American Journal of Veterinary Research (1999), 60

OBJECTIVE: To assess sensitivity of scintigraphic alveolar clearance rate as an indicator of alveolar epithelium damage in horses. ANIMALS: 5 healthy horses (group A) and 5 with chronic obstructive ... [more ▼]

OBJECTIVE: To assess sensitivity of scintigraphic alveolar clearance rate as an indicator of alveolar epithelium damage in horses. ANIMALS: 5 healthy horses (group A) and 5 with chronic obstructive pulmonary disease (COPD; group B). PROCEDURE: Horses underwent clearance rate (k [%/min]) determination. Clearance rate of group-B horses was determined after remission of the disease following 2 months at pasture (remission 1), stabling in a controlled environment (remission 2), and during crisis induced by exposure to moldy hay and straw. Methacholine challenge test was performed at each investigation period to determine nonspecific pulmonary airway hyperresponsiveness. Pulmonary function tests (PFT) also were performed, and cell populations in bronchoalveolar lavage (BAL) fluid were determined on another occasion. RESULTS: Group-B horses had significantly faster mean clearance rate during crisis (k = 4.30+/-0.95%/min), compared with that for remission 1(k = 1.98+/-0.55%/min), which did not differ from the rate in group-A horses (k = 1.95+/-0.33%/min). Despite lack of clinical signs of COPD during remission when stabled in a controlled environment, an intermediate value was found (k = 3.20+/-0.72%/min). CONCLUSIONS: This technique allowed grading of lung damage induced by COPD, whereas use of PFT and determination of BAL fluid cell populations failed to differentiate between remission 1 and remission 2. CLINICAL RELEVANCE: Determination of alveolar clearance rate by use of scintigraphy is a sensitive indicator of lung damage. A modified clearance rate was found despite the lack of clinical and functional changes [less ▲]

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See detailAnalysing financial returns using regression models based on non-symmetric stable distributions
Lambert, Philippe ULg; Lindsey, James ULg

in Journal of the Royal Statistical Society : Series C (Applied Statistics) (1999), 48

The daily evolution of the price of Abbey National shares over a 10-week period is analysed by using regression models based on possibly non-symmetric stable distributions. These distributions, which are ... [more ▼]

The daily evolution of the price of Abbey National shares over a 10-week period is analysed by using regression models based on possibly non-symmetric stable distributions. These distributions, which are only known through their characteristic function, can be used in practice for interactive modelling of heavy-tailed processes. A regression model for the location parameter is proposed and shown to induce a similar model for the mode. Finally, regression models for the other three parameters of the stable distribution are introduced. The model found to fit best allows the skewness of the distribution, rather than the location or scale parameters, to vary over time. The most likely share return is thus changing over time although the region where most returns are observed is stationary. [less ▲]

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See detailOn the appropriateness of marginal models for repeated measurements in clinical trials
Lindsey, James ULg; Lambert, Philippe ULg

in Statistics in Medicine (1998), 17

Although models developed directly to describe marginal distributions have become widespread in the analysis of repeated measurements, some of their disadvantages are not well enough known. These include ... [more ▼]

Although models developed directly to describe marginal distributions have become widespread in the analysis of repeated measurements, some of their disadvantages are not well enough known. These include producing profile curves that correspond to no possible individual, possibly showing that a treatment is superior on average when it is poorer for each individual subject, implicitly generating complex and implausible physiological explanations, including underdispersion in subgroups, and sometimes corresponding to no possible probabilistic data generating mechanism. We conclude that such marginal models may sometimes be appropriate for descriptive observational studies, such as sample surveys in epidemiology, but should only be used with great care in causal experimental settings, such as clinical trials. [less ▲]

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See detailScintigraphical Evaluation of Alveolar Clearance in Horses
Votion, Dominique ULg; Vandenput, Sandrina ULg; Duvivier, D. H. et al

in Veterinary Journal (1998), 156(1), 51-58

This study proposed a standardized method for measuring alveolar epithelium membrane permeability in the horse. The normal rate of clearance (%.min-1) from lung into blood of nebulized 99mTc-DTPA has been ... [more ▼]

This study proposed a standardized method for measuring alveolar epithelium membrane permeability in the horse. The normal rate of clearance (%.min-1) from lung into blood of nebulized 99mTc-DTPA has been established for healthy horses (Group A) compared with values obtained with horses suffering from chronic obstructive pulmonary disease (COPD; Group B). The 99mTc-DTPA clearance was measured in the caudoventral (R1) and in the half caudal (R2) parts of the left lung during different time intervals. The two regions aimed to define the influence of the airways on measured clearance (R2 contained proportionally more conducting airways than R1). It was concluded that a comparison of groups of subjects may be performed in R2 and on data collected during a 20 min period. The normal clearance rate in R2 was 1.80 +/- 0.46%.min-1 (T1/2R2 = 40.99 +/- 12.45 min) in Group A. In Group B, a significantly faster 99mTc-DTPA transfer rate was found (4.17 +/- 0.83%.min-1 or T1/2R2 = 17.17 +/- 3.38min). Bronchoalveolar lavage (BAL) suggested that the increased permeability measured in Group B could be the result of lung inflammatory responses. Our results have demonstrated the ability of the 99mTc-DTPA clearance test to detect alveolar epithelial damage in horses. Furthermore, we were able to show that a regional analysis of the alveolar-capillary barrier integrity may be performed satisfactorily in the equine patient. [less ▲]

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See detailPredictions in overdispersed series of counts using an approximate predictive likelihood
Lambert, Philippe ULg

in Journal of Forecasting (1997), 16

The generalized autoregression model or GARM, originally used to model series of non-negative data measured at irregularly spaced time points (Lambert, 1996a), is considered in a count data context. It is ... [more ▼]

The generalized autoregression model or GARM, originally used to model series of non-negative data measured at irregularly spaced time points (Lambert, 1996a), is considered in a count data context. It is first shown how the GARM can be expressed as a GLM in the special case of a linear model for some transform of the location parameter. The Butler approximate predictive likelihood (Butler, 1986, Rejoinder) is then used to define likelihood prediction envelopes. The width of these intervals is shown to be slightly wider than the Fisher (1959, pp. 128-33) and Lejeune and Faulkenberry (1982) predictive likelihood-based envelopes which assume that the parameters have fixed known values (equal to their maximum likelihood estimates). The method is illustrated on a small count data set showing overdispersion. [less ▲]

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See detailModelling irregularly sampled profiles of nonnegative dog triglyceride responses under different distributional assumptions
Lambert, Philippe ULg

in Statistics in Medicine (1996), 15

General methodology for modelling series of non-negative data observed at unequally spaced times is developed. The parameterization enables both the importance of the serial association, as well the order ... [more ▼]

General methodology for modelling series of non-negative data observed at unequally spaced times is developed. The parameterization enables both the importance of the serial association, as well the order of this dependence to be expressed. An example is given where the effects of three fibre based diets on dog triglyceride profiles are analysed and compared. Many different types of models based on common distributions such as the normal, exponential, gamma, Weibull and log-normal observations are presented. Comparison of possibly non-nested models fitted on the same data set is made using the Akaike criterion. [less ▲]

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See detailModelling of repeated series of count data measured at unequally spaced times
Lambert, Philippe ULg

in Journal of the Royal Statistical Society : Series C (Applied Statistics) (1996), 45

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See detailAbility of Bronchodilators to Prevent Bovine Experimental Respiratory Distress
Genicot, Bruno; Lambert, Philippe ULg; Votion, Dominique ULg et al

in Veterinary Research (1995), 26(4), 276-283

This cross-over trial involving 6 Blue Belgian double-muscled calves aimed to investigate whether ipratropium bromide inhaled alone or in combination with clenbuterol hydrochloride could prevent the ... [more ▼]

This cross-over trial involving 6 Blue Belgian double-muscled calves aimed to investigate whether ipratropium bromide inhaled alone or in combination with clenbuterol hydrochloride could prevent the dramatic clinical and pulmonary disturbances that are observed during an experimentally induced bronchoconstriction. Inhaled bronchodilators significantly influenced the clinical and functional responses of bovines subjected to a 5-hydroxytryptamine perfusion. However, despite a mean (standard error) wash-out period of 11.2 (3.1) and 7.5 (0.9) d after the 1st and the 2nd one-day challenges, respectively, first-order carry-over effects (ie those remaining from the previous treatment), effects of the period during which the treatment was allocated and interaction effects did not allow a definitive interpretation of overall treatment differences. [less ▲]

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See detailDynamic generalized linear models and repeated measurements
Lindsey, James ULg; Lambert, Philippe ULg

in Journal of Statistical Planning & Inference (1995), 47

The dynamic generalized linear model for non-normal data is extended for use in repeated measurements, when series of observations are available for more than one individual. Examples are given for count ... [more ▼]

The dynamic generalized linear model for non-normal data is extended for use in repeated measurements, when series of observations are available for more than one individual. Examples are given for count and duration data. [less ▲]

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See detailPhysical Properties of Particles of Ipratropium and Clenbuterol Generated by Equipment Suitable for the Inhalation of Drugs by Calves
Genicot, Bruno; Lapp, K.; Close, Roland et al

in Research in Veterinary Science (1995), 59(2), 164-167

When solutions of ipratropium and clenbuterol were atomised at 300 kPa and 450 kPa in equipment suitable for the inhalation of drugs by calves, the numbers, velocities and diameters of the particles ... [more ▼]

When solutions of ipratropium and clenbuterol were atomised at 300 kPa and 450 kPa in equipment suitable for the inhalation of drugs by calves, the numbers, velocities and diameters of the particles produced were similar. When the pressure was increased to 600 kPa more of the particles were less than 2 microns in diameter and fewer were more than 7 microns in diameter, the fractions of the total mass of the solution generated in these size ranges were similarly increased and decreased, and the velocities of the particles were increased. At any given pressure, the numbers of particles of different sizes, and the proportions of the total mass generated, were similar for the solutions of ipratropium and clenbuterol, but a solution of saline produced more particles with a diameter less than 3 microns. Particles from the solution of ipratropium had the highest velocity and particles from the solution of clenbuterol had the lowest velocity. [less ▲]

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See detailEvaluation of the performances of an aerosol delivery equipment suitable for calves under different conditions.
Génicot, B; Peckova, M; Lambert, Philippe ULg et al

in Yule, A. J.; Dumouchel, C. (Eds.) 6th International Conference on Liquid Atomization and Spray Systems (1994)

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