SEARCH · PubMed Health

Results for “Probability”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3Linked to original sources

PubMedDate not supplied

Teaching medical students to estimate probability of coronary artery disease.

The authors conducted a randomized trial of two methods for teaching medical students how to estimate the probability of coronary artery disease in patients with chest pain. Eighty-two students were given a pre-test consisting of written protocols summarizing the histories of 25 patients who had undergone coronary arteriography. The students estimated the likelihood of coronary artery disease for each case and were then randomized to receive one of two short written clinical lessons: a cardiology textbook chapter on interpreting chest pain, or a lesson based on a prediction rule for estimating probability of coronary artery disease. All students were given a post-test similar to the pre-test. Students who were given the textbook lesson showed no change in the accuracy of their probability estimates. Students who were taught the prediction rule significantly improved their probability estimates, as measured by a statistical index of calibration. The authors conclude that traditional teaching methods do not provide students with guidance in estimating disease probability, and that better teaching methods are needed. In this study, a clinical prediction rule fulfilled the need for instruction in probability estimation.

Coronary Disease↗

PubMed1990

Amplitude variations in P300 component due to unpredictable stepwise change of stimulus probability.

The present study investigates whether the amplitude of the P300 in 'normal' subjects adapts to unexpected changes in global stimulus probability. The adaptation level (AL) metrics (in terms of Helson's [1964] theory) was applied to explain the variations in the P300 amplitude. In an oddball paradigm the probability of the deviant tone (target 1000 Hz, 80 dB, 50 ms) occurring randomly in a train of standard ones (non-target 1100 Hz, 80 dB, 50 ms) was changed in a stepwise way from 0.5 to 0.3 and then to 0.1 in 3 blocks without interruption of the stimulus presentation. Two experiments were performed. In the first one the subjects did not receive any prior information about the changes in the stimulus probability, whereas in the second experiment they were aware of such changes. The task always required the subject to silently count the deviants. EEG was recorded monopolarly from Fz, Cz and Pz in a total of 15 subjects. The block averaged P300 increased monotonically with probability, the increase being larger in Expt. II. In contrast to the block averaged P300, the variations in the P300 amplitude following the abrupt change in probability showed a non-linear dependency. On the basis of the AL a simulation of the time course of the P300 variations with probability was performed and a clear similarity between the simulated P300 and the experimental data was found. Some properties of the AL model are discussed.

Acoustic Stimulation↗

PubMed1991

Fixation probability in spatially changing environments.

The fixation probability of a mutant in a subdivided population with spatially varying environments is investigated using a finite island model. This probability is different from that in a panmictic population if selection is intermediate to strong and migration is weak. An approximation is used to compute the fixation probability when migration among subpopulations is very weak. By numerically solving the two-dimensional partial differential equation for the fixation probability in the two subpopulation case, the approximation was shown to give fairly accurate values. With this approximation, we show in the case of two subpopulations that the fixation probability in subdivided populations is greater than that in panmictic populations mostly. The increase is most pronounced when the mutant is selected for in one subpopulation and is selected against in the other subpopulation. Also it is shown that when there are two types of environments, further subdivision of subpopulations does not cause much change of the fixation probability in the no dominance case unless the product of the selection coefficient and the local population size is less than one. With dominance, the effect of subdivision becomes more complex.

Biological Evolution↗

PubMed1985

Delineation of coding areas in DNA sequences through assignment of codon probabilities.

Codon usage tables have been produced for E. coli, yeast, human, and mouse. The nonrandom employment of codons allows assignment of probability values to trinucleotides in any DNA sequence. These values represent the probability that a given trinucleotide is used as a codon in the organism from which the table is derived. For the graphical delineation of coding areas in DNA sequences, a probability is assigned to each trinucleotide equal to its frequency in the codon table. Averaging and smoothing procedures then greatly enhance the detectability of areas of high average codon probability and better represent the mean codon probability. These manipulations increase graphical clarity without altering the overall magnitude of probabilities. Averaging introduces an error of less than 0.5% between "raw" and smoothed data. This graphical delineation of coding sequences does not depend on the presence of punctuation, ribosomal binding sites, etc: moreover the delineation of introns and exons is also possible.

Base Sequence↗

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↗

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↗

PubMed1989

Developmental change in subjective probability during adolescence.

The developmental change in subjective probability during adolescence, an important period for establishing the probability concept, was investigated. 75 Japanese adolescents, from 12 to 23 yr. of age, were asked to make probability judgments for a lottery under 15 conditions. Analysis showed that with increase in age their subjective probability came closer to the objective probability. Discussion of these results took into consideration recent studies on the development of the concept of probability.

Adolescent↗

PubMed1991

[The initial clinical probability in the diagnosis of the hospitalized pediatric patient].

OBJECTIVES: 1. To determine the degree of correlation among different physicians concerning their initial diagnosis and 2. identify the degree of correlation between the probability that physicians assing an initial diagnosis and the number of days the patient is hospitalized, laboratory test and X-rays taken of the patient. DESIGN: A comparative questionnaire. STUDY SITE: Pediatric hospitalization Unit of a third level medical care ward of the Mexican Social Security Institute. STUDY UNITS: All new admissions or non-programmed readmitted patients to the hospital during the months of November and December 1990. MAIN MEASUREMENTS: The treating physicians (Staff pediatricians (MB) and third (R3) and second (R2) year residents) were each asked to independently assign a probability (0 to 100) to each of the diagnosis emitted on the day the patient was admitted. When the patient was discharged, the number of days hospitalized as well as the number of laboratory tests and X-rays taken of the patient were added. RESULTS: 106 patients were evaluated, a correlation was gathered between MBs and R3s of 0.79 (P less than 0.001) and among MBs and R2s of 0.83 (P less than 0.001). The correlation between resident physicians was discretely less 0.60 (P less than 0.01). When relating the probability assigned by the physicians and the number of days the patients were hospitalized, associations were observed of 0.31 (P less than 0.05), 0.15 (P less than 0.05) and 0.19 (P = 0.04) for the MB, R3 and R2s respectively. In the case of laboratory test a correlation of 0.38, 0.06 and 0.04 (MB, R3, R2 respectively) was found. None of these correlations were statistically significant. The X-rays showed a significant correlation in cases of the MBs (0.50, P less than 0.05). CONCLUSIONS: The probabilities assigned by the staff physician as well as the resident doctors are closely related and a lesser grade of association is seen when comparing the residents among each other. No tendencies were identified in the correlation of the probability assigned by the residents and the variables analyzed. A consistent relation was seen between the staff physician and high probabilities, longer stays, and greater number of laboratory tests and X-rays.

Adult↗

PubMed1990

An application of probability theory to a group of breath-alcohol and blood-alcohol data.

Many jurisdictions have "per se" driving-while-intoxicated (DWI) status expressed in terms of a blood-alcohol concentration (BAC) standard (in grams per 100 mL or the equivalent). Since breath-alcohol (BrAC) analysis is typically employed to determine BAC, there is often challenge to the use of an assumed 2100:1 conversion ratio. This concern may be relevant in light of considerable data that show a low percentage of cases in which BrAC greater than BAC, and this concern increases when the BrAC is used to predict BAC in the context of "per se" legislation. Probability theory provides a basis for estimating the likelihood of an individual having a BrAC greater than or equal to g/210 L with a corresponding BAC less than 0.10 g/100 mL. Actual field data from the state of Wisconsin (n = 404) were evaluated to determine the probability of this occurrence. The probability for this occurrence involves the multiplication law for independent events. The computed probability from the data was 0.018. The actual number of occurrences where BrAC greater than or equal to 0.10 g/210 L and BAC less than 0.10 g/100 mL was 5, resulting in a probability of 0.012. The concern of having BrAC greater than BAC at the critical "per se" level has a very low probability of occurrence, which thus supports the reasonableness of "per se" DWI legislation based upon a blood-alcohol standard determined by breath-alcohol analysis.

Alcoholic Intoxication↗

PubMed1989

[Variable cycle avoidance program: systematic variation of the probability of a programmed noxious stimulus].

Six albino rats were exposed in three groups to different values of T (a repetitive time cycle at the end of which an electric shock occurs with a specified probability in absence of a first response), (V.g.: P(E-/R) = .75 and .10) on different weeks. When occurs at least one response on each T cycle it has a probability to delete the shock equal to P(E-/R) that changed on different weeks from .50 to .10. In this way, as the Bayesian probability system indicate for exclusive events, Pk = P(E-/R) - [P(E-/R) x P(E-/R)] was the effective probability of shocks when at least one response occur on each T cycle and Pm = P(E-/R) - [P(E-/R) x P(E-/R) x P(1rst Response)] is the modulated probability of shocks when a first response occur on some T cycles. The results indicated that T/Pk (the mean interval between shocks before the experimental session) nor T/Pm (The mean interval between shocks after the experimental session when the subject responds), or its inverse, are reductional continua that explains monotonically ordered changes of the response rate or the mean total number of responses. As a consequence T and P must be dissociated. On the other hand, corrected response rate (Total responses minus Total first responses/minute) is a direct and monotonically increased function of Pk and of 1/T. In this sense is the increased probability with which noxious stimuli impinge on the behavior stream and the time reduction between stimuli, that separated, sustain responding.

Animals↗

PubMed1978

Application of the response probability density function technique to biodynamic models.

A method has been developed, which we call the "response probability density function technique," which has applications in predicting the probability of injury in a wide range of biodynamic situations. The method, which was developed in connection with sonic boom damage prediction, utilized the probability density function of the excitation force and the probability density function of the sensitivity of the material being acted upon. The method is especially simple to use when both these probability density functions are lognormal. Studies thus far have shown that the stresses from sonic booms, as well as the strengths of glass and mortars, are distributed lognormally. Some biodynamic processes also have lognormal distributions and are, therefore, amenable to modeling by this technique. In particular, this paper discusses the application of the response probability density function technique to the analysis of the thoracic response to air blast and the prediction of skull fracture from head impact.

Animals↗

PubMed1991

Radiation risk of tissue late effects, a net consequence of probabilities of various cellular responses.

Late effects from the exposure to low doses of ionizing radiation are hardly or not at all observed in man probably due to the low values of risk coefficients that preclude statistical analyses of data from populations that are exposed to doses less than 0.2 Gy. In order to arrive at an assessment of potential risk from radiation exposure in the low dose range, the microdosimetry approach is essential. In the low dose range, ionizing radiation generates particle tracks, mainly electrons, which are distributed rather heterogenously within the exposed tissue. Taking the individual cell as the elemental unit of life, observations and calculations of cellular responses to being hit by energy deposition events from low LET type are analysed. It emerges that besides the probability of a hit cell to sustain a detrimental effect with the consequence of malignant transformation there are probabilities of various adaptive responses that equip the hit cell with a benefit. On the one hand, an improvement of cellular radical detoxification was observed in mouse bone marrow cells; another adaptive response pertaining to improved DNA repair, was reported for human lymphocytes. The improved radical detoxification in mouse bone marrow cells lasts for a period of 5-10 hours and improved DNA repair in human lymphocytes was seen for some 60 hours following acute irradiation. It is speculated that improved radical detoxification and improved DNA repair may reduce the probability of spontaneous carcinogenesis. Thus it is proposed to weigh the probability of detriment for a hit cell within a multicellular system against the probability of benefit through adaptive responses in other hit cells in the same system per radiation exposure. In doing this, the net effect of low doses of low LET radiation in tissue with individual cells being hit by energy deposition events could be zero or even beneficial. Since there was no simple additivity of equal effects from repeated exposures to equal doses and because of the potential effect of adaptive cell responses on the spontaneous evolution of malignancy in tissue, the extrapolation of risk with absorbed dose reaching down to zero, does not appear to be generally valid.

Cell Survival↗

PubMedDate not supplied

ERPs to stimuli requiring response production and inhibition: effects of age, probability and visual noise.

Sixty-six normal adults ranging in age from 20 to 85 years were presented with stimuli containing explicit instructions to initiate or to inhibit a motor response (the words 'push' or 'wait'). In one task, the effect of stimulus probability was investigated by varying probability between 0.25 and 0.75 for both Go and No-go stimuli. In another task, the effect of visual noise was investigated by degrading the stimuli with ampersands on half of the trials. Regression analysis was used to examine the effects of age on P3 amplitude and latency for each stimulus type. The effects of stimulus variables on P3, independent of age, were examined by standardizing each subject's data to those expected for a 20 year old. P3 latency to all stimuli and RT to Go stimuli increased with age. The latency of P3s to No-go stimuli was less sensitive to age than Go stimuli. P3 amplitude at Cz and Pz (but not Fz) diminished with age. P3s to Go stimuli were maximal at Pz and earlier than P3s to No-go stimuli. P3s to No-go stimuli were maximal at Cz. These differences between Go and No-go stimuli remained true under visual noise and probability manipulations. Visual noise prolonged the latency of Go and No-go P3. Less probable Go and No-go stimuli elicited larger and later P3s than more probable stimuli. Decreasing the probability of the No-go stimulus enhanced its central distribution.

Adult↗

PubMed1990

Exact computation of pattern probabilities in random sequences generated by Markov chains.

Observed patterns in macromolecular sequences are often considered as words and compared with their probabilities of occurring in random sequences. Calculation of these probabilities, however, often lacks rigour. We have developed an algorithm for exact computation of such probabilities for stochastic sequences that follow a Markov chain model. The method is applicable to the case that a random sequence contains one out of two given patterns P and Q, or both simultaneously. Another application yields the probability function P(x) that a sequence contains pattern P exactly x times. An application to patterns that include wild-card characters yields probabilities for homonucleotide clusters of a given length. We prove the probability of multiple runs of single nucleotides in the SV40 genome to be in accordance with the dinucleotide composition of the sequence, although it is in conflict with mononucleotide composition.

Algorithms↗

PubMed1989

The probability of quantal secretion at release sites in different calcium concentrations in toad (Bufo marinus) muscle.

1. The number of quanta secreted from visualized release sites along terminal branches at toad (Bufo marinus) neuromuscular junctions in different extracellular concentrations of calcium ions. [Ca2+]o, and during facilitation was determined. Terminal branches were visualized by prior staining with the fluorescent dye, 3-3 diethyloxardicarbocyanine iodide (DiOC2(5)). 2. Increasing [Ca2+]o between 0.25 and 0.4 mM gave a similar proportional increase in the mean quantal content of the end-plate potential recorded with an extracellular electrode (me) at all sites along terminal branches. Thus the length constant for the experimental decline in me along terminal branches (the quantal length constant) remained constant with an increase in [Ca2+]o. The increase in m with [Ca2+]o followed a fourth power relation at both proximal and distal release sites. 3. The increase in me with [Ca2+]o was almost entirely due to an increase in the binomial probability for secretion, pe, which increased as the third to fourth power of [Ca2+]o. However, at higher [Ca2+]o there was an increasing tendency for the binomial parameter ne to increase. It is shown that when ne increases by about 1 there is very little change in pe, suggesting that the new release site recruited at high [Ca2+]o has a relatively low probability for secretion. 4. Test impulses gave a similar proportional increase in me following a conditioning impulse at all sites along terminal branches. The quantal length constant remained constant for both conditioning and test values of me. The increase in me for the test impulse increased linearly with an increase in me for the conditioning impulse at all release sites. 5. Facilitation of me declined exponentially with an increase in the test-conditioning interval. The time constant for this decline (30-40 ms) was similar at both proximal and distal release sites. Changes in facilitation of me were almost entirely due to changes in pe except at very short test-conditioning intervals of about 10 ms. At these intervals ne frequently increased by about 1 and there was very little change in pe. Again, this suggests that additional release sites recruited at short intervals have relatively low probability for secretion. 6. The results indicate that relatively low probability release sites exist in close juxtaposition to relatively high probability release sites which themselves decline in probability along the length of terminal branches.

Animals↗

PubMedDate not supplied

Relation of physicians' predicted probabilities of pneumonia to their utilities for ordering chest x-rays to detect pneumonia.

To investigate the relation between physicians' predicted probabilities of pneumonia and their utilities for ordering chest x-rays to detect pneumonia, the authors studied 52 physicians who ordered chest x-rays of 886 patients presenting to an emergency department with fever or respiratory complaints. Physicians estimated the probability of pneumonia prior to obtaining the results of the chest x-ray. Utilities were assessed by asking physicians to consider a hypothetical patient presenting with acute respiratory symptoms, with unknown chest x-ray status, and to rank on a scale from +50 ("best thing I could do") to -50 ("worst thing I could do") their rating scale utilities for not diagnosing pneumonia and not ordering a chest x-ray when the patient had pneumonia (i.e., missing a pneumonia), and for diagnosing pneumonia and ordering a chest x-ray when the patient did not have pneumonia (i.e., ordering an unnecessary x-ray). The utility for ordering an unnecessary x-ray was negatively correlated with average predicted probability (r = -0.1495, p = 0.29), whereas the utility for missing a pneumonia was positively correlated with average predicted probability (r = 0.2254, p = 0.11), although the correlations were not statistically significant. Relative chagrin, defined as the difference in these utilities, was significantly inversely correlated with average predicted probability (r = -0.2992, p less than 0.035), even after adjusting for the prevalence of pneumonia seen by each physician (partial r = -0.42, p less than 0.0027). It is concluded that physicians who experienced greater regret over missing a pneumonia than over ordering an unnecessary x-ray estimated lower probabilities of pneumonia for patients for whom they ordered x-rays.(ABSTRACT TRUNCATED AT 250 WORDS)

Cost-Benefit Analysis↗

PubMed1989

Intuitive reasoning about probability: theoretical and experimental analyses of the "problem of three prisoners".

Among various Bayesian problems of probability, the "problem of three prisoners" (Lindley, 1971; Mosteller, 1965) is an especially good example which illustrates the drastic discrepancy between intuitive reasoning and mathematical formal reasoning about probability. In particular, it raises intriguing questions concerning the mathematical and cognitive relevance of factors such as prior probabilities and the context in which certain information is given. In the current paper, we report a new version of the problem which turned out to be even more counterintuitive. This new version was also designed so that different inferential schemes would lead to separate estimates of posterior probability. The data obtained from questionnaires and theoretical analyses of the original and modified problems suggest that: (1) The psychological processes of intuitive reasoning are qualitatively different from mathematical reasoning. (2) The tendency to neglect prior probabilities (Tversky & Kahneman, 1974, 1982) is not always the critical factor for illusory judgments. (3) Intuitive judgments can be categorized by several, distinctive propositional beliefs from which the judgments are apparently derived. We call these prototypical, crude beliefs "subjective theorems," and discuss their nature and roles in the current paper.

Adult↗