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Results for “Models, Statistical”

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At least 973 records · Page 54Linked to original sources

PubMed1997

Interpreting the relation between force and cross-sectional area in human muscle.

The maximum force a muscle can produce depends on its cross-sectional area (CSA). However, the exact interpretation of this relationship has been a matter of controversy. Recently, the controversy has centered on whether the measurements are best correlated using regression analysis or ratio standards. Applying regression analysis to this problem implies that all the experimental error is in the measurement of force. Thus, confusion may arise by failure to take account of errors in the measurement of CSA. Using a statistical model, we show how regression analysis can be misleading as error is introduced into the measurement of CSA as well as that of force. Because neither the errors in force nor CSA can be quantified in the experimental situation, we conclude that ratio standards are less likely to mislead although the accuracy of the result depends on the degree of correlation between force and CSA in the muscle measured.

Biomechanical Phenomena↗

PubMed2003

Counseling men with prostate cancer: a nomogram for predicting the presence of small, moderately differentiated, confined tumors.

PURPOSE: Men diagnosed with clinically localized prostate cancer have a number of treatment options available, including watchful waiting, radical prostatectomy and radiation therapy. With the widespread use of serum prostate specific antigen (PSA) testing, prostate cancers are being diagnosed earlier in their natural history, with many tumors being small and of little health risk to the patient, at least in the short term. To better counsel men diagnosed with prostate cancer, we developed a statistical model that accurately predicts the presence of small moderately differentiated, confined cancer based on clinical variables (serum PSA, clinical stage, prostate biopsy Gleason grade and ultrasound volume) and variables derived from the analysis of systematic biopsies. MATERIALS AND METHODS: The analysis included 409 patients diagnosed by systematic needle biopsy with clinical stages T1c or T2a N0 or NX and M0 or MX prostate cancer who were treated solely with radical prostatectomy at 1 of 2 institutions. Additional biopsy features included number and percentage of biopsy cores involved with cancer and high grade cancer, in addition to total length of biopsy cores involved. Indolent cancer was defined as pathologically organ confined cancer 0.5 cc or less in volume and without poorly differentiated elements. Logistic regression was used to construct several prediction models and the resulting nomograms. RESULTS: Overall 80 (20%) of the patients had indolent cancer. The nomogram predicted the presence of an indolent cancer with discrimination (area under the receiver operating characteristics curves) for various models ranging from 0.64 to 0.79. Calibration of the models appeared good. CONCLUSIONS: Nomograms incorporating pretreatment variables (clinical stage, Gleason grade, PSA and the amount of cancer in a systematic biopsy specimen) can predict the probability that a man with prostate cancer has an indolent tumor. These nomograms have good discriminatory ability and calibration, and may benefit the patient and clinician when the various treatment options for prostate cancer are being considered.

Biomarkers, Tumor↗

PubMed2004

Prospectively packaged lymph node dissections with radical cystectomy: evaluation of node count variability and node mapping.

PURPOSE: Accumulating evidence supports the relationship between an increased number of lymph nodes (LNs) reported following radical cystectomy (RC) and overall outcome. We prospectively evaluated RC cases with transitional cell carcinoma of the bladder to determine which factors may contribute to the variability in the number of reported LNs. MATERIALS AND METHODS: We conducted a prospective evaluation in which 144 patients undergoing RC and pelvic lymph node dissection (PLND) between June 2001 and April 2003 were included. Lymph nodes were processed as individual packets. A standard method of evaluating nodal submissions was used. A mixed statistical model was used with neoadjuvant chemotherapy, node status, pathological stage, bacillus Calmette-Guerin exposure, age and number of days from transurethral resection as the fixed effects. Surgeon and pathologist were treated as random effects. RESULTS: The extended PLND group had a significantly greater lymph node yield (median 22.5 nodes) compared to standard PLND (median 8), however, no staging advantage was observed in the extended dissection group. Only the type of PLND performed was associated with node yield (p <0.001). Subset analysis of patients with unexpected microscopic nodal involvement revealed that 33% had involvement of the common iliac nodes. CONCLUSIONS: In our series only the extent of the lymph node dissection was found to influence node yield significantly after radical cystectomy. Additionally, the observed risk of involvement of the common iliac chain in microscopically node positive cases suggests a need to include this region as part of the PLND for bladder cancer for cases without grossly involved LNs.

Adult↗

PubMed2006

Number of sexual encounters involving intercourse and the transmission of sexually transmitted infections.

OBJECTIVE: To investigate the impact of the number of sexual encounters involving intercourse in combination with the number of sexual partners on the transmission dynamics of sexually transmitted infections (STIs). STUDY: A statistical model is used for predicting the basic reproductive rate, R(0), which takes both the number of sexual partners and the number of sexual encounters involving intercourse per partner into consideration. The model is then applied to Swedish survey data that includes data on the number of partners over the course of 1 year, as well as the number of encounters per partner during this time. RESULTS: The number of sexual encounters involving intercourse in combination with the number of sexual partners affects the number of secondary infections in a nonlinear way. The basic reproductive rate, R(0), is lower when the heterogeneity in number of encounters is modeled than when homogeneity is assumed. CONCLUSION: The results indicate that it is possible that individuals who have a large number of partners may not, as is often assumed, be the only ones to play a central role as spreaders of STIs. Individuals who have a large number of sexual encounters involving intercourse per partner and have several (but not necessarily a very large number of) partners may also play a significant role in the transmission of STIs.

Contact Tracing↗

PubMed1998

Empirical determinants of measles metapopulation dynamics in England and Wales.

A key issue in metapopulation dynamics is the relative impact of internal patch dynamics and coupling between patches. This problem can be addressed by analysing large spatiotemporal data sets, recording the local and global dynamics of metapopulations. In this paper, we analyse the dynamics of measles meta-populations in a large spatiotemporal case notification data set, collected during the pre-vaccination era in England and Wales. Specifically, we use generalized linear statistical models to quantify the relative importance of local influences (birth rate and population size) and regional coupling on local epidemic dynamics. Apart from the proportional effect of local population size on case totals, the models indicate patterns of local and regional dynamic influences which depend on the current state of epidemics. Birth rate and geographic coupling are not associated with the size of major epidemics. By contrast, minor epidemics--and especially the incidence of local extinction of infection--are influenced both by birth rate and geographical coupling. Birth rate at a lag of four years provides the best fit, reflecting the delayed recruitment of susceptibles to school cohorts. A hierarchical index of spatial coupling to large centres provides the best spatial model. The model also indicates that minor epidemics and extinction patterns are more strongly influenced by this regional effect than the local impact of birth rate.

Disease Outbreaks↗

PubMed2003

Trophic interactions under climate fluctuations: the Atlantic puffin as an example.

Co-occurrence in food requirements of offspring and food availability is a key factor determining breeding success. Prey availability is typically dependent on environmental conditions that are different from those influencing the predator's decision regarding whether or not to initiate breeding, and is not always optimal at the peak of reproduction requirements. We investigated this relationship to understand better what determines the fledging success of the Atlantic puffin (Fratercula arctica). Colony data from Røst (northern Norway) covering a period of 27 years were analysed with parallel data on sea temperature and the size and abundance of the puffins' main prey (the Norwegian spring-spawning herring, Clupea harengus). By fitting statistical models to the fledging success, we found that one effect of climate on this population of Atlantic puffins is indirect and mediated by sea temperature affecting the availability of first-year herring. The best model also demonstrates that the breeding success of the Røst puffins may be quantitatively predicted from the size of first-year herring and sea temperature.

Animal Nutritional Physiological Phenomena↗

PubMed2004

Single molecule acid-base kinetics and thermodynamics.

We report a method in which temperature dependent single-molecule fluorescence measurements are used to study the kinetics and thermodynamics of the acid-base interaction in films of photoresist polymer. We use the two distinct fluorescent prototropic forms of Coumarin 6 (C6-->C6+) to indicate the state of the acid-base system. Data are analyzed using a statistical model of the intensity probability distributions, yielding temperature dependent proton exchange rates, which is confirmed through agreement with a simple two-state Monte Carlo model. The temperature dependent rates are used to calculate the activation enthalpy for proton exchange.

Acids↗

PubMed1996

Estimating defibrillation efficacy using combined upper limit of vulnerability and defibrillation testing.

It is frequently necessary, both clinically and in the laboratory, to estimate how strong a stimulus is required to defibrillate. Current techniques for forming such estimates require the repeated induction of ventricular fibrillation (VF) and subsequent attempts at defibrillation (DF testing). DF testing can be time consuming and in the operating room may increase the patient risks. A novel scheme is presented which combines DF testing with upper limit of vulnerability (ULV) testing. ULV testing is a relatively safe procedure which yields data well correlated with defibrillation efficacy. A Bayesian statistical model of combined ULV/DF testing is presented which is both powerful and concise. The model is used in two examples to design minimum rms error protocols and estimators for the DF95 (the stimulus strength which defibrillates 95% of the time). A simulation for humans of one example solution shows that a single VF episode of combined ULV/DF testing (rms error = 23% of the mean DF95) is better than two VF episodes with DF testing alone (25%). The simulation results for a second example are directly compared with laboratory results from six pigs, showing a less than 1.0% average difference between the simulated and measured rms errors.

Algorithms↗

PubMed2004

MAP estimation for hyperspectral image resolution enhancement using an auxiliary sensor.

This paper presents a novel maximum a posteriori estimator for enhancing the spatial resolution of an image using co-registered high spatial-resolution imagery from an auxiliary sensor. Here, we focus on the use of high-resolution panchomatic data to enhance hyperspectral imagery. However, the estimation framework developed allows for any number of spectral bands in the primary and auxiliary image. The proposed technique is suitable for applications where some correlation, either localized or global, exists between the auxiliary image and the image being enhanced. To exploit localized correlations, a spatially varying statistical model, based on vector quantization, is used. Another important aspect of the proposed algorithm is that it allows for the use of an accurate observation model relating the "true" scene with the low-resolutions observations. Experimental results with hyperspectral data derived from the airborne visible-infrared imaging spectrometer are presented to demonstrate the efficacy of the proposed estimator.

Algorithms↗

PubMed2004

Iterative multimodel subimage binarization for handwritten character segmentation.

Existing binarization methods are categorized as either global or local. In this paper, we present a new category, where the image is considered a collection of subimages. Each subimage provides a statistical model for the handwritten characters that can be used to optimize the binarization of other subimages based on gray-level and stroke-run features. The proposed method uses these multimodels to iteratively arrive at the optimal threshold for each subimage. It can be applied to different types of documents where prior knowledge about the noisiness of the subimages is not available. Experimental results showed significant improvement in the binarization quality in comparison with other well-established algorithms.

Algorithms↗

PubMed2004

Motion estimation using Statistical Learning Theory.

This paper describes a novel application of Statistical Learning Theory (SLT) to single motion estimation and tracking. The problem of motion estimation can be related to statistical model selection, where the goal is to select one (correct) motion model from several possible motion models, given finite noisy samples. SLT, also known as Vapnik-Chervonenkis (VC), theory provides analytic generalization bounds for model selection, which have been used successfully for practical model selection. This paper describes a successful application of an SLT-based model selection approach to the challenging problem of estimating optimal motion models from small data sets of image measurements (flow). We present results of experiments on both synthetic and real image sequences for motion interpolation and extrapolation; these results demonstrate the feasibility and strength of our approach. Our experimental results show that for motion estimation applications, SLT-based model selection compares favorably against alternative model selection methods, such as the Akaike's fpe, Schwartz' criterion (sc), Generalized Cross-Validation (gcv), and Shibata's Model Selector (sms). The paper also shows how to address the aperture problem using SLT-based model selection for penalized linear (ridge regression) formulation.

Algorithms↗

PubMed2004

FloatBoost learning and statistical face detection.

A novel learning procedure, called FloatBoost, is proposed for learning a boosted classifier for achieving the minimum error rate. FloatBoost learning uses a backtrack mechanism after each iteration of AdaBoost learning to minimize the error rate directly, rather than minimizing an exponential function of the margin as in the traditional AdaBoost algorithms. A second contribution of the paper is a novel statistical model for learning best weak classifiers using a stagewise approximation of the posterior probability. These novel techniques lead to a classifier which requires fewer weak classifiers than AdaBoost yet achieves lower error rates in both training and testing, as demonstrated by extensive experiments. Applied to face detection, the FloatBoost learning method, together with a proposed detector pyramid architecture, leads to the first real-time multiview face detection system reported.

Algorithms↗

PubMed2006

Fuzzy Markov random fields versus chains for multispectral image segmentation.

This paper deals with a comparison of recent statistical models based on fuzzy Markov random fields and chains for multispectral image segmentation. The fuzzy scheme takes into account discrete and continuous classes which model the imprecision of the hidden data. In this framework, we assume the dependence between bands and we express the general model for the covariance matrix. A fuzzy Markov chain model is developed in an unsupervised way. This method is compared with the fuzzy Markovian field model previously proposed by one of the authors. The segmentation task is processed with Bayesian tools, such as the well-known MPM (Mode of Posterior Marginals) criterion. Our goal is to compare the robustness and rapidity for both methods (fuzzy Markov fields versus fuzzy Markov chains). Indeed, such fuzzy-based procedures seem to be a good answer, e.g., for astronomical observations when the patterns present diffuse structures. Moreover, these approaches allow us to process missing data in one or several spectral bands which correspond to specific situations in astronomy. To validate both models, we perform and compare the segmentation on synthetic images and raw multispectral astronomical data.

Algorithms↗

PubMed2000

SMR analysis of historical follow-up studies with missing death certificates.

The evaluation of epidemiological follow-up studies is frequently based on a comparison of the number O of deaths observed in the cohort from a specified cause with the expected number E calculated from person years in the cohort and mortality rates from a reference population. The ratio SMR = 100 x O/E is called the standardized mortality ratio (SMR). While person years can easily be calculated from the cohort and reference rates are generally available from the national statistical offices or the World Health Organization (WHO), problems can arise with the accessibility of the causes of death of the deceased study participants. However, the information that a person has died may be available, e.g., from population registers. In this paper, a statistical model for this situation is developed to derive a maximum likelihood (ML) estimator for the true (but unknown) number O* of deaths from a specified cause, which uses the known number O of deaths from this cause and the proportion p of all known causes of death among all decreased participants. It is shown that the standardized mortality ratio SMR* based on this estimated number is just SMR* = SMR/p. Easily computable confidence limits can be obtained by dividing the usual confidence limits of the SMR by the opposite limit of the proportion p. However, the confidence level alpha has to be adjusted appropriately.

Biometry↗

PubMed2003

Evolutionary genetics of resistance and tolerance to natural herbivory in Arabidopsis thaliana.

Resistance and tolerance are widely viewed as two alternative adaptive responses to herbivory. However, the traits underlying resistance and tolerance remain largely unknown, as does the genetic architecture of herbivory responses and the prevalence of genetic trade-offs. To address these issues, we measured resistance and tolerance to natural apical meristem damage (AMD) by rabbits in a large field experiment with recombinant inbred lines (RILs) of Arabidopsis thaliana (developed from a cross between the Columbia x Landsberg erecta ecotypes). We also measured phenological and morphological traits hypothesized to underlie resistance and tolerance to AMD. Recombinant inbred lines differed significantly in resistance (the proportion of replicates within an RIL that resisted herbivory), and early flowering plants with tall apical inflorescences were more likely to experience damage. Tolerance (the difference in fitness between the damaged and undamaged states), also differed significantly among RILs, with some lines overcompensating for damage and producing more fruit in the damaged than undamaged state. Plastic increases in basal branch number, basal branch height, and senescence date in response to damage were all associated with greater tolerance. There was no evidence for a genetic trade-off between resistance and tolerance, an observation consistent with the underlying differences in associated morphological and phenological characters. Selection gradient analysis detected no evidence for direct selection on either resistance or tolerance in this experiment. However, a statistical model indicates that the pattern of selection on resistance depends strongly on the mean level of tolerance, and selection on tolerance depends strongly on the mean level of resistance. These observations are consistent with the hypothesis that selection may act to maintain resistance and tolerance at intermediate levels in spatially or temporally varying environments or those with varying herbivore populations.

Adaptation, Biological↗

PubMed1994

The chronic myeloid leukaemias: guidelines for distinguishing chronic granulocytic, atypical chronic myeloid, and chronic myelomonocytic leukaemia. Proposals by the French-American-British Cooperative Leukaemia Group.

We have reviewed our experience with four of the entities that are included under the generic term chronic myeloid leukaemia (CML), namely the classic Ph+ CGL, both BCR+ and BCR-, aCML and CMML. We have developed a statistical model that confirms that CGL, aCML and CMML can be distinguished from each other with reasonable success employing five quantitative parameters (WBC, percentage immature granulocytes, percentage monocytes, percentage basophils, percentage erythroid precursors in bone marrow) and one qualitative parameter (granulocytic dysplasia). It is hoped that these detailed recommendations will enable investigators to improve their diagnostic accuracy. This should permit more uniform comparisons of molecular biologic and clinical studies.

Bone Marrow↗

PubMed2005

Pollen flow in the wildservice tree, Sorbus torminalis (L.) Crantz. II. Pollen dispersal and heterogeneity in mating success inferred from parent-offspring analysis.

Knowing the extent of gene movements from parents to offspring is essential to understand the potential of a species to adapt rapidly to a changing environment, and to design appropriate conservation strategies. In this study, we develop a nonlinear statistical model to jointly estimate the pollen dispersal kernel and the heterogeneity in fecundity among phenotypically or environmentally defined groups of males. This model uses genotype data from a sample of fruiting plants, a sample of seeds harvested on each of these plants, and all males within a circumscribed area. We apply this model to a scattered, entomophilous woody species, Sorbus torminalis (L.) Crantz, within a natural population covering more than 470 ha. We estimate a high heterogeneity in male fecundity among ecological groups, both due to phenotype (size of the trees and flowering intensity) and landscape factors (stand density within the neighbourhood). We also show that fat-tailed kernels are the most appropriate to depict the important abilities of long-distance pollen dispersal for this species. Finally, our results reveal that the spatial position of a male with respect to females affects as much its mating success as ecological determinants of male fecundity. Our study thus stresses the interest to account for the dispersal kernel when estimating heterogeneity in male fecundity, and reciprocally.

Conservation of Natural Resources↗

PubMed1988

Prediction of outcome shortly after delivery for the very low birthweight (less than or equal to 1500 g) infant.

Obstetric and perinatal data for 387 very low birthweight infants (less than or equal to 1500 g) who were admitted to a neonatal intensive care unit were used to derive and test a multivariate statistical model for predicting shortly after birth the risk of death before hospital discharge. Using the gestational age, the Apgar score at 5 minutes and the presence or absence of respiratory distress, the model correctly predicted outcome for 94% of survivors but for only 53% of deaths. The model successfully identified a subgroup with a low (less than 1 in 20) predicted risk of death. This subgroup comprised over one-third of the infants but included 47% of the survivors and only 5% of the deaths. The probabilities derived from the model are tabulated and may help paediatricians in maternity units without neonatal intensive care facilities to decide which very low birthweight infants to transfer for additional care.

Delivery, Obstetric↗