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PubMed2002

Probability cuing of target location facilitates visual search implicitly in normal participants and patients with hemispatial neglect.

We explored how variability in the probability of target locations affects visual search in normal individuals and in patients with hemispatial neglect, a deficit in attending to the contralesional side of space. Young and elderly normal participants responded faster when targets appeared in the more probable region than when targets appeared in the less probable region. Similarly, patients were sensitive to the distribution of targets, even in the neglected field. Although the attentional gradient that characterizes neglect was not eliminated, the response facilitation due to the probability distribution was proportionate to that of control participants and equal in magnitude across the neglected field. All participants exploited the uneven distribution of targets to enhance task performance without explicit instructions to do so or awareness of biases in their behavior. These results suggest that attentional orientation and sensitivity to external probabilities are possibly dissociable. An early sensory and a late motor mechanism are postulated as possibly being involved in the observed probability-matching behavior of participants.

Adolescent↗

PubMed2005

C-reactive protein and D-dimer with clinical probability score in the exclusion of pulmonary embolism.

This study evaluated the diagnostic value of C-reactive protein (CRP) combined with a clinical decision rule in the exclusion of pulmonary embolism (PE) and compared this with D-dimer. In 363 consecutive outpatients CRP and D-dimer test were performed and clinical probability of PE was assessed. Patients with D-dimer levels<500 microg/l and clinical probability indicating 'PE unlikely' were followed for 3 months. Ventilation-perfusion scan or spiral computerized tomography was performed in patients with D-dimer levels>or=500 microg/l or clinical probability indicating 'PE likely'. The CRP had a sensitivity of 95.7% [95% confidence interval (CI): 90-100] and negative predictive value (NPV) of 98.4% (96-100). CRP<5 mg/l with clinical probability score indicating 'PE unlikely' (n=108, 30%), had a sensitivity of 96.7% (90-100), a specificity of 43.0% (37-49) and NPV of 99.1% (97-100). D-dimer<500 microg/l with clinical probability score indicating 'PE unlikely' (n=170, 51%), had a sensitivity of 96.7% (90-100), a specificity of 67.9% (62-74) and NPV of 99.4% (98-100). Based on retrospective data it was concluded that a standard CRP test can potentially be used to safely exclude PE, either as a sole test or combined with clinical probability assessment. Prospective studies are needed to confirm these findings.

Adolescent↗

PubMed1975

The prior probability of autosomal linkage.

An expression is derived for the prior probability of linkage between a random trait locus and any one of m random marker loci, and this probability is computed form=1, 10, 20, 30, 50 and 100. A similar expression is derived for two trait loci, and computed for m=1, 10, 20 and 30. When one trait locus and 30 marker loci are being studied, a priori there is over a three-quarter probability that the trait locus should be syntenic with at least one of the markers, and about a one-half probability that there should be a linkage mappable from recombination frequencies. If two traits are studied, then the prior probability that at least one should be syntenic with one of the 30 markers is 0-94, and there is a three-quarter probability that such a linkage should be mappable.

Chromosome Mapping↗

PubMed2006

Clinical probability and alveolar dead space measurement for suspected pulmonary embolism in patients with an abnormal D-dimer test result.

BACKGROUND: Most patients with suspected pulmonary embolism (PE) have a positive D-dimer test and undergo diagnostic imaging. Additional non-invasive bedside tests are required to reduce the need for further diagnostic tests. OBJECTIVES: We aimed to determine whether a combination of clinical probability assessment and alveolar dead space fraction measurement can confirm or exclude PE in patients with an abnormal D-dimer test. METHODS: We assessed clinical probability of PE and alveolar dead space fraction in 270 consecutive in- and outpatients with suspected PE and positive D-dimer. An alveolar dead space fraction < 0.15 was considered normal. PE was subsequently excluded or confirmed by venous compression ultrasonography, spiral computed tomography and a 3-month follow-up. Radiologists were unaware of the results of clinical probability and capnography. RESULTS: PE was confirmed in 108 patients (40%). Capnography had a sensitivity of 68.5% (95% confidence interval [CI]: 58.9-77.1%) and a specificity of 81.5% (95% CI: 74.6-87.1%) for PE. Forty-five patients (16.6%) had both a low clinical probability and normal capnography (sensitivity: 99.1%, 95% CI: 94.9-100%) and 34 patients (12.6%) had both a high clinical probability and abnormal capnography (specificity: 100%, 95% CI: 97.7-100%). CONCLUSION: Capnography alone does not exclude PE accurately. The combination of clinical probability and capnography accurately excludes or confirms PE and avoids further testing in up to 30% of patients.

Capnography↗

PubMed1997

On the effect of probability distributions of input variables in public health risk assessment.

A central part of probabilistic public health risk assessment is the selection of probability distributions for the uncertain input variables. In this paper, we apply the first-order reliability method (FORM) as a probabilistic tool to assess the effect of probability distributions of the input random variables on the probability that risk exceeds a threshold level (termed the probability of failure) and on the relevant probabilistic sensitivities. The analysis was applied to a case study given by Thompson et al. on cancer risk caused by the ingestion of benzene contaminated soil. Normal, lognormal, and uniform distributions were used in the analysis. The results show that the selection of a probability distribution function for the uncertain variables in this case study had a moderate impact on the probability that values would fall above a given threshold risk when the threshold risk is at the 50th percentile of the original distribution given by Thompson et al. The impact was much greater when the threshold risk level was at the 95th percentile. The impact on uncertainty sensitivity, however, showed a reversed trend, where the impact was more appreciable for the 50th percentile of the original distribution of risk given by Thompson et al. than for the 95th percentile. Nevertheless, the choice of distribution shape did not alter the order of probabilistic sensitivity of the basic uncertain variables.

Benzene↗

PubMed1994

The probability of defibrillation success and the incidence of postshock arrhythmia as a function of shock strength.

The effects of high voltage defibrillation shocks given to six swine were studied to determine if there is a limit to the advantage gained from increasing the shock strength. An endocardial electrode was placed in the right ventricle, and a 114-cm2 cutaneous patch was placed on the left lateral thorax. Monophasic (10 msec) and single capacitor biphasic (5/5 msec) shocks with leading edge voltages of 200, 400, 600, 800, and 990 volts (approximately 2.3-59 J) were tested. For monophasic shocks, the probability of successful defibrillation ranged from 0% at 200 V to 90% at 990 V. The incidence of postshock arrhythmia increased from 0% for successful shocks at 600 V to 67% for successful shocks at 990 V. For biphasic shocks, the probability of success peaked at 97% for the 600-, 800-, and 990-V shocks. The incidence of postshock arrhythmia increased from 8% at 400 V to 55% at 990 V. Although more postshock arrhythmias occurred at lower strengths for biphasic than for monophasic shocks, an efficacy criterion, quantifying the probability of defibrillation success and the probability that a postshock arrhythmia will not occur, was always higher for biphasic shocks. The probability of success never reached 100% for either waveform while the incidence of postshock arrhythmia increased as the shock strength increased. In conclusion, for the catheter-patch electrode configuration, increasing the shock strength does not always improve the probability of success and may increase the incidence of postshock arrhythmia.

Animals↗

PubMed2006

Site occupancy models with heterogeneous detection probabilities.

Models for estimating the probability of occurrence of a species in the presence of imperfect detection are important in many ecological disciplines. In these "site occupancy" models, the possibility of heterogeneity in detection probabilities among sites must be considered because variation in abundance (and other factors) among sampled sites induces variation in detection probability (p). In this article, I develop occurrence probability models that allow for heterogeneous detection probabilities by considering several common classes of mixture distributions for p. For any mixing distribution, the likelihood has the general form of a zero-inflated binomial mixture for which inference based upon integrated likelihood is straightforward. A recent paper by Link demonstrates that in closed population models used for estimating population size, different classes of mixture distributions are indistinguishable from data, yet can produce very different inferences about population size. I demonstrate that this problem can also arise in models for estimating site occupancy in the presence of heterogeneous detection probabilities. The implications of this are discussed in the context of an application to avian survey data and the development of animal monitoring programs.

Animals↗

PubMed2004

Cytologic diagnosis: expression of probability by clinical pathologists.

BACKGROUND: Clinical pathologists use descriptive terms or modifiers to express the probability or likelihood of a cytologic diagnosis. Words are imprecise in meaning, however, and may be used and interpreted differently by pathologists and clinicians. OBJECTIVES: The goals of this study were to 1) assess the frequency of use of 18 modifiers, 2) determine the probability of a positive diagnosis implied by the modifiers, 3) identify preferred modifiers for different levels of probability, 4) ascertain the importance of factors that affect expression of diagnostic certainty, and 5) evaluate differences based on gender, employment, and experience. METHODS: We surveyed 202 clinical pathologists who were board-certified by the American College of Veterinary Pathologists (Clinical Pathology). Surveys were distributed in October 2001 and returned by e-mail, fax, or surface mail over a 2-month period. Results were analyzed by parametric and nonparametric tests. RESULTS: Survey response rate was 47.5% (n = 96) and primarily included clinical pathologists at veterinary schools (n = 58) and diagnostic laboratories (n = 31). Eleven of 18 terms were used "often" or "sometimes" by >/= 50% of respondents. Broad variability was found in the probability assigned to each term, especially those with median values of 75 to 90%. Preferred modifiers for 7 numerical probabilities ranging from 0 to 100% included 68 unique terms; however, a set of 10 terms was used by >/= 50% of respondents. Cellularity and quality of the sample, experience of the pathologist, and implications of the diagnosis were the most important factors affecting the expression of probability. CONCLUSION: Because of wide discrepancy in the implied likelihood of a diagnosis using words, defined terminology and controlled vocabulary may be useful in improving communication and the quality of data in cytology reporting.

Adult↗

PubMed2001

An algorithm for modifying neurotransmitter release probability based on pre- and postsynaptic spike timing.

The precise times of occurrence of individual pre- and postsynaptic action potentials are known to play a key role in the modification of synaptic efficacy. Based on stimulation protocols of two synaptically connected neurons, we infer an algorithm that reproduces the experimental data by modifying the probability of vesicle discharge as a function of the relative timing of spikes in the pre- and postsynaptic neurons. The primary feature of this algorithm is an asymmetry with respect to the direction of synaptic modification depending on whether the presynaptic spikes precede or follow the postsynaptic spike. Specifically, if the presynaptic spike occurs up to 50 ms before the postsynaptic spike, the probability of vesicle discharge is upregulated, while the probability of vesicle discharge is downregulated if the presynaptic spike occurs up to 50 ms after the postsynaptic spike. When neurons fire irregularly with Poisson spike trains at constant mean firing rates, the probability of vesicle discharge converges toward a characteristic value determined by the pre- and postsynaptic firing rates. On the other hand, if the mean rates of the Poisson spike trains slowly change with time, our algorithm predicts modifications in the probability of release that generalize Hebbian and Bienenstock-Cooper-Munro rules. We conclude that the proposed spike-based synaptic learning algorithm provides a general framework for regulating neurotransmitter release probability.

Action Potentials↗

PubMed1980

Application of conditional probability analysis to the clinical diagnosis of coronary artery disease.

Analysis of multiple noninvasive tests offers the promise of more accurate diagnosis of coronary artery disease, but discordant test responses can occur frequently and, when observed, result in diagnostic uncertainty. Accordingly, 43 patients undergoing diagnostic coronary angiography were evaluated by noninvasive testing and the results subjected to analysis using Bayes' theorem of conditional probability. The procedures used included electrocardiographic stress testing for detection of exercise-induced ST segment depression, cardiokymographic stress testing for detection of exercise-induced precordial dyskinesis, myocardial perfusion scintigraphy for detection of exercise-induced relative regional hypoperfusion, and cardiac fluoroscopy for detection of coronary artery calcification. The probability for coronary artery disease was estimated by Bayes' theorem from each patient's age, sex, and symptom classification, and from the observed test responses. This analysis revealed a significant linear correlation between the predicted probability for coronary artery disease and the observed prevalence of angiographic disease over the entire range of probability from 0 to 100% (P less than 0.001 by linear regression). The 12 patients without angiographic disease had a mean posttest likelihood of only 7.0 +/- 2.6% despite the fact that 13 of the 60 historical and test responses were falsely "positive." In contrast, the mean posttest likelihood was 94.1 +/- 2.8% in the 31 patients with angiographic coronary artery disease, although 45 of the 155 historical and test responses were falsely "negative." In 8 of the 12 normal patients, the final posttest likelihood was under 10% and in 26 of the 31 coronary artery disease patients, it was over 90%. These estimates also correlated well with the pooled clinical judgment of five experienced cardiologists (P less than 0.001 by linear regression). The observed change in probability for disease for each of the 15 different test combinations correlated with their information content predicted according to Shannon's theorem (P less than 0.001 by linear regression). These results support the use of probability analysis in the clinical diagnosis of coronary artery disease and provide a formal basis for comparing the relative diagnostic effectiveness and cost-effectiveness of different test combinations.

Adult↗

PubMedDate not supplied

The reliability of probability analysis in the prediction of coronary artery disease in two hospitals.

To assess the accuracy of the Bayesian computer program CADENZA for the prediction of coronary artery disease, the authors examined the probabilities generated by the application of this program to the clinical and noninvasive test results of 303 patients in a private referral center and 199 patients in a veterans' hospital. These probabilities were compared with those produced by applying a six-variable discriminant function derived by logistic regression at the private referral center. Two statistical approaches were employed in evaluating the relative performances of the Bayesian program and the discriminant function. The first of these involved the sorting of patients in both test groups into ascending deciles of probability and comparing expected probability with observed angiographic disease prevalence in each decile. The second involved the calculation and comparison of a standardized reliability measure. The latter was significantly lower for the discriminant function both at the private hospital (0.200 for the discriminant function versus -17.5 +/- 1.96 for the Bayesian program) and at the veterans' hospital (-0.8 +/- 1.96 for the discriminant function versus -11.3 for Bayesian program). This suggests that the discriminant function is significantly superior to the Bayesian algorithm CADENZA for predicting coronary artery disease probabilities in subjects who have relatively high pretest disease probabilities.

Bayes Theorem↗

PubMedDate not supplied

Clinical assessment of the probability of coronary artery disease: judgmental bias from personal knowledge.

Probability estimates of angiographic coronary artery disease made by experienced, board-certified staff cardiologists were compared with those of cardiologists in training (fellows). In addition, estimates made before coronary angiography were compared with those made several months later based on written clinical summaries of 15 items of objective clinical and test data. Cardiologists were asked to estimate the probabilities of coronary artery disease, multivessel disease, and triple-vessel or left main disease. The study population consisted of 510 consecutive patients without valvular disease referred for the first time for coronary angiography to three hospitals. Both staff and fellows consistently overestimated the pre-angiographic probability of coronary artery disease. The probabilities estimated from patient summaries were always significantly lower than the pre-angiographic assessments. Only staff cardiologists reliably assessed the probabilities of coronary artery disease during the second assessment (p less than 0.05). Thus, estimates of disease probability based on clinical judgment vary according to the source of information, and these estimates are more accurate when physicians have objective data on hand and do not know the identities of the patients.

Coronary Angiography↗

PubMed1998

Expert panel assessment of appropriateness of abdominal aortic aneurysm surgery: global judgement versus probability estimation.

OBJECTIVE: To compare an expert panel's global assessment of appropriateness of elective surgery for abdominal aortic aneurysms (AAA) with their assessment of the effect of surgery on the probability of 5-year mortality. METHODS: Nine expert panel members rated the appropriateness of 120 scenarios for elective AAA repair on a nine-point scale, and also estimated the 5-year probability of AAA-related death and of non-AAA related death among 30-day survivors of AAA surgery and among patients with unoperated AAA. These probabilities were used to determine differences in 5-year probability of mortality of surgery vs. no surgery for each scenario. Three categories of appropriateness were defined based on these differences: inappropriate (< 0%), equivocal (0-5%), and appropriate (> 5%). RESULTS: The distribution of scenarios was inappropriate (39%), equivocal (12%), and appropriate (49%) based on probability estimates and inappropriate (43%), equivocal (22%), and appropriate (36%) based on global assessment. There was poor agreement between the two methods, with a Kappa coefficient = 0.28 (95% CI: 0.23 to 0.32). Although a higher proportion of scenarios were rated as appropriate using probability estimation rather than global judgment, the level of agreement among members of the panel was similar, Kappa coefficient = 0.07 (95% CI: -0.07 to 0.72). CONCLUSIONS: Experts disagree about the appropriate indications for elective surgery for AAA. Explicit estimates used in a decision analysis may provide a better assessment of appropriate indications than the global judgment of experts. Global assessment of the appropriateness of AAA surgery based on panel members' review of research evidence for increased survival appears to include implicitly their valuation of outcomes.

Aortic Aneurysm, Abdominal↗

PubMedDate not supplied

Prediction of identity by descent probabilities from marker-haplotypes.

The prediction of identity by descent (IBD) probabilities is essential for all methods that map quantitative trait loci (QTL). The IBD probabilities may be predicted from marker genotypes and/or pedigree information. Here, a method is presented that predicts IBD probabilities at a given chromosomal location given data on a haplotype of markers spanning that position. The method is based on a simplification of the coalescence process, and assumes that the number of generations since the base population and effective population size is known, although effective size may be estimated from the data. The probability that two gametes are IBD at a particular locus increases as the number of markers surrounding the locus with identical alleles increases. This effect is more pronounced when effective population size is high. Hence as effective population size increases, the IBD probabilities become more sensitive to the marker data which should favour finer scale mapping of the QTL. The IBD probability prediction method was developed for the situation where the pedigree of the animals was unknown (i.e. all information came from the marker genotypes), and the situation where, say T, generations of unknown pedigree are followed by some generations where pedigree and marker genotypes are known.

Algorithms↗

PubMed2003

An improved probability mapping approach to assess genome mosaicism.

BACKGROUND: Maximum likelihood and posterior probability mapping are useful visualization techniques that are used to ascertain the mosaic nature of prokaryotic genomes. However, posterior probabilities, especially when calculated for four-taxon cases, tend to overestimate the support for tree topologies. Furthermore, because of poor taxon sampling four-taxon analyses suffer from sensitivity to the long branch attraction artifact. Here we extend the probability mapping approach by improving taxon sampling of the analyzed datasets, and by using bootstrap support values, a more conservative tool to assess reliability. RESULTS: Quartets of orthologous proteins were complemented with homologs from selected reference genomes. The mapping of bootstrap support values from these extended datasets gives results similar to the original maximum likelihood and posterior probability mapping. The more conservative nature of the plotted support values allows to focus further analyses on those protein families that strongly disagree with the majority or plurality of genes present in the analyzed genomes. CONCLUSION: Posterior probability is a non-conservative measure for support, and posterior probability mapping only provides a quick estimation of phylogenetic information content of four genomes. This approach can be utilized as a pre-screen to select genes that might have been horizontally transferred. Better taxon sampling combined with subtree analyses prevents the inconsistencies associated with four-taxon analyses, but retains the power of visual representation. Nevertheless, a case-by-case inspection of individual multi-taxon phylogenies remains necessary to differentiate unrecognized paralogy and shared phylogenetic reconstruction artifacts from horizontal gene transfer events.

Bacterial Proteins↗

PubMed2001

Measuring Alzheimer's disease progression with transition probabilities: estimates from CERAD.

OBJECTIVES: To estimate annual transition probabilities (i.e., the likelihood that a patient will move from one disease stage to another in a given time period) for AD progression. Transition probabilities are estimated by disease stages (mild, moderate, severe) and settings of care (community, nursing home), accounting for differences in age, gender, and behavioral symptoms as well as the length of time a patient has been in a disease stage. METHODS: Using data from the Consortium to Establish a Registry for Alzheimer's Disease (CERAD), the authors employed a modified survival analysis to estimate stage-to-stage and stage-to-nursing home transition probabilities. To account for individual variability, a Cox proportional hazards model was fit to the CERAD data to estimate hazard ratios for gender, age (50 to 64, 65 to 74, and more than 75 years), and level of behavioral symptoms (low/high, according to responses to the Behavioral Rating Scale for Dementia) for each of the key stage-to-stage and stage-to-nursing home transitions. RESULTS: The transition probabilities underscore the rapid progression of patients into more severe disease stages and into nursing homes and the differences among population subgroups. In general, male gender, age under 65, and high level of behavioral symptoms were associated with higher transition probabilities to more severe disease stages. Disease progression is roughly constant as a function of the time a patient has spent in a particular stage. CONCLUSIONS: Transition probabilities provide a useful means of characterizing AD progression. Economic models of interventions for AD should consider the varied course of progression for different population subgroups, particularly those defined by high levels of behavioral symptoms.

Aged↗

PubMed2001

Clinical perception of disease probability associated with Bethesda System diagnoses.

The degree to which clinical perceptions of Papanicolaou smear sensitivity contribute to patient mismanagement is uncertain. A voluntary, anonymous questionnaire was mailed to 350 obstetricians/gynecologists (OGYNs) and 350 other primary care providers (PCPs) located in Pennsylvania or Ohio. The clinicians estimated the probability of no disease, dysplasia, and invasive carcinoma for 1 of 7 Bethesda System diagnoses. Differences in probability estimates between provider types and between the clinicians and medical literature data were measured. The response rate was 22.7%. Compared with published values, clinicians estimated similar disease probabilities for many diagnoses. However, for some diagnoses, the probability estimates differed considerably from published values (e.g., overestimation of dysplasia and invasive carcinoma for benign diagnoses and underestimation of dysplasia for some dysplasia diagnoses), and such errors could contribute to patient mismanagement. OGYNs generally were more accurate in probability estimates than PCPs. Methods to convey more accurately these diagnostic disease probabilities should be examined.

Attitude of Health Personnel↗

PubMed2002

International variations in hip fracture probabilities: implications for risk assessment.

It is recommended that intervention thresholds should be based on absolute fracture risk, but there is a large variation in hip fracture incidence from different regions of the world. The aim of this study was to examine heterogeneity of hip fracture probability in different regions from recent estimates of hip fracture incidence and mortality to adjust intervention thresholds. Ten-year probabilities of hip fracture were computed in men and women at 10-year intervals from the age of 50 years and lifetime risks at the age of 50 years from the hazard functions of hip fracture and death. Lifetime risk at the age of 50 years varied from 1% in women from Turkey to 28.5% in women from Sweden. High lifetime risks in women were associated with high lifetime risks in men (r = 0.83). There also were significant correlations of 10-year risk at any age between men and women. Ten-year probability was standardized to that of men and women from Sweden (set at 1.0). There was a 15-fold range in 10-year probability from 1.24 in Norway to 0.08 in Chile. Countries were categorized by 10-year probabilities comprising very high risk (Norway, Iceland, Sweden, Denmark, and the United States), high risk (China [Taiwan [TW]], Germany, Switzerland, Finland, Greece, Canada, The Netherlands, Hungary, Singapore, Italy, United Kingdom, Kuwait, Australia, and Portugal), medium risk (China [Hong Kong [HK]], France, Japan, Spain, Argentina, and China), and low risk (Turkey, Korea, Venezuela, and Chile). The categorization of hip fracture probabilities can be used to adjust intervention thresholds based on age, sex, and relative risk from a reference population such as Sweden.

Aged↗