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PubMed1987

Choice between sequences of fixed-ratio schedules: effects of ratio values and probability of food delivery.

Pigeons were exposed to schedules of food delivery that consisted of two sequential fixed ratios. When alternative sequences provided two food deliveries per 50 responses, the schedule with the shorter initial fixed-ratio value was consistently preferred. Progressively reducing from 1.0 to .25 the probability of food delivery following completion of the second fixed ratio of the sequence with the shorter initial fixed ratio did not reduce preference for this sequence. Moreover, the sequence with the shorter initial fixed ratio also was preferred when the probability of food delivery following completion of the initial ratio in that sequence was progressively reduced from 1.0 to .5, although preference shifted to the alternative when the probability was reduced to 0. These findings suggest that the length of the initial fixed ratio was a primary determinant of choice. Subsequent manipulations demonstrated, however, that when the initial fixed ratios of the two alternatives were equal, changes in the ratio value and probability of food delivery following completion of the second fixed ratio lawfully affected choice.

Animals↗

PubMed1993

Molar versus local reinforcement probability as determinants of stimulus value.

During one component of a multiple schedule, pigeons were trained on a discrete-trial concurrent variable-interval variable-interval schedule in which one alternative had a high scheduled rate of reinforcement and the other a low scheduled rate of reinforcement. When the choice proportion between the alternatives matched their respective relative reinforcement frequencies, the obtained probabilities of reinforcement (reinforcer per peck) were approximately equal. In alternate components of the multiple schedule, a single response alternative was presented with an intermediate scheduled rate of reinforcement. During probe trials, each alternative of the concurrent schedule was paired with the constant alternative. The stimulus correlated with the high reinforcement rate was preferred over that with the intermediate rate, whereas the stimulus correlated with the intermediate rate of reinforcement was preferred over that correlated with the low rate of reinforcement. Preference on probe tests was thus determined by the scheduled rate of reinforcement. Other subjects were presented all three alternatives individually, but with a distribution of trial frequency and reinforcement probability similar to that produced by the choice patterns of the original subjects. Here, preferences on probe tests were determined by the obtained probabilities of reinforcement. Comparison of the two sets of results indicates that the availability of a choice alternative, even when not responded to, affects the preference for that alternative. The results imply that models of choice that invoke only obtained probability of reinforcement as the controlling variable (e.g., melioration) are inadequate.

Animals↗

PubMed1999

Delay or probability discounting in a model of impulsive behavior: effect of alcohol.

Little is known about the acute effects of drugs of abuse on impulsivity and self-control. In this study, impulsivity was assessed in humans using a computer task that measured delay and probability discounting. Discounting describes how much the value of a reward (or punisher) is decreased when its occurrence is either delayed or uncertain. Twenty-four healthy adult volunteers ingested a moderate dose of ethanol (0.5 or 0.8 g/kg ethanol: n = 12 at each dose) or placebo before completing the discounting task. In the task the participants were given a series of choices between a small, immediate, certain amount of money and $10 that was either delayed (0, 2, 30, 180, or 365 days) or probabilistic (i.e., certainty of receipt was 1.0, .9, .75, .5, or .25). The point at which each individual was indifferent between the smaller immediate or certain reward and the $10 delayed or probabilistic reward was identified using an adjusting-amount procedure. The results indicated that (a) delay and probability discounting were well described by a hyperbolic function; (b) delay and probability discounting were positively correlated within subjects; (c) delay and probability discounting were moderately correlated with personality measures of impulsivity; and (d) alcohol had no effect on discounting.

Adult↗

PubMed2005

On the effects of signaling reinforcer probability and magnitude in delayed matching to sample.

Two experiments examined whether postsample signals of reinforcer probability or magnitude affected the accuracy of delayed matching to sample in pigeons. On each trial, red or green choice responses that matched red or green stimuli seen shortly before a variable retention interval were reinforced with wheat access. In Experiment 1, the reinforcer probability was either 0.2 or 1.0 for both red and green responses. Reinforcer probability was signaled by line or cross symbols that appeared after the sample had been presented. In Experiment 2, all correct responses were reinforced, and the signaled reinforcer durations were 1.0 s and 4.5 s. Matching was more accurate when larger or more probable reinforcers were signaled, independently of retention interval duration. Because signals were presented postsample, the effects were not the result of differential attention to the sample.

Animals↗

PubMed1999

Coding mammograms using the classification "probably benign finding--short interval follow-up suggested".

OBJECTIVE: Many benign breast lesions revealed by mammography show features indicating that the lesions have a high, but not complete, likelihood of being benign. The Breast Imaging Reporting and Data System (BI-RADS) allows radiologists to classify these mammograms as "probably benign finding-short interval follow-up suggested" (category 3). We explored whether certain factors are associated with the use of category 3 in a national cancer detection program. MATERIALS AND METHODS: We analyzed data from the National Breast and Cervical Cancer Early Detection Program, a comprehensive nationwide program that provides cancer screening for low-income and medically underserved women. The study population included all women at least 40 years old who had undergone mammography on or before September 30, 1996 (n = 372,760). RESULTS: Of the 372,760 mammograms, 7.7% were classified as category 3. The probability of receiving a category 3 classification decreased as patients' ages increased. Women who were symptomatic were nearly twice as likely as women who were asymptomatic to receive a category 3 classification, and women whose clinical breast examinations had abnormal findings were more than twice as likely as women with examinations having normal findings to receive a category 3 classification. The percentage of mammograms classified as category 3 by state or tribal organization ranged from 1.4% to 14.0%. CONCLUSION: Several patient variables, including patient symptomatology, were associated with the probability of having a mammogram classified as category 3. One of the most important determinants was where the patient underwent mammography, which suggests that variability exists among radiologists themselves in using this BI-RADS code for "probably benign" mammographic lesions.

Adult↗

PubMed1987

Effects of signal probability on sensory evoked potentials in cats.

The present experiment was designed to follow the evoked potential (EP) changes recorded from the association cortex and A II area of the auditory cortex and from the vertex of the freely moving cat. The EPs elicited by clicks of different probabilities used as warning stimuli during aversive conditioning were analyzed. It was found that the EPs recorded from the auditory cortex and the vertex showed different changes during the aversive conditioning to the rare clicks of 3 and 10% probabilities. The N50 and P100 components of the auditory cortical (A II area) EPs increased significantly at both signal probabilities. On the vetex and association cortical EPs, elicited by the rare signals, a broadly distributed positivity, the P250 wave could be detected. The amplitude increase of the P250 was inversely proportional to the used probability of the signal.

Acoustic Stimulation↗

PubMed2001

The use of sequential probabilities in the segmentation of speech.

The present investigation addresses the possible utility of sequential probabilities in the segmentation of spoken language. In a series of five word- spotting and two control lexical decision experiments, high- versus low-probability consonant-vowel (Experiments 1, 2, 5, and 7) and vowel-consonant (Experiments 1, 3, 4, and 6) strings were presented either in the nonsense contexts of target words (Experiments 1-3) or within the target words themselves (Experiments 4-7). The results suggest that listeners, at least for sequences in the onset position, indeed use sequential probabilities as cues for segmentation. The probability of a sound sequence influenced segmentation more when the sequence occurred within the target words (Experiments 4-7 vs. Experiments 1-3). Furthermore, the effects were reliable only when the sequences occurred in the onset position (Experiments 1, 2, 5, and 7 vs. Experiments 1, 3, 4, and 6).

Humans↗

PubMed2004

A web-based interface to calculate phonotactic probability for words and nonwords in English.

Phonotactic probability refers to the frequency with which phonological segments and sequences of phonological segments occur in words in a given language. We describe one method of estimating phonotactic probabilities based on words in American English. These estimates of phonotactic probability have been used in a number of previous studies and are now being made available to other researchers via a Web-based interface. Instructions for using the interface, as well as details regarding how the measures were derived, are provided in the present article. The Phonotactic Probability Calculator can be accessed at http://www.people.ku.edu/-mvitevit/PhonoProbHome.html.

Humans↗

PubMed2006

Probability model of the inaccuracy of residual stromal thickness prediction to reduce the risk of ectasia after LASIK part I: quantifying individual risk.

PURPOSE: To measure the imprecision of microkeratome cuts, preoperative corneal pachymetry, and laser ablation depth and develop a statistical model to describe the probability of the residual stromal bed thickness (RST) after myopic LASIK being significantly thinner than predicted. METHODS: Preoperative corneal thickness, flap thickness, ablation depth, and RST were measured in 36 eyes by a prototype three-dimensional very high-frequency (VHF) 50 MHz digital ultrasound scanning device (<1.2 microm precision), precursor to the commercially available Artemis 2. All eyes had undergone LASIK with the Moria LSK-One microkeratome and the NIDEK EC-5000 excimer laser. Based on the statistically combined uncertainty (standard deviation) and bias (accuracy to intended value) of corneal thickness measurement, flap thickness, and ablation depth, a continuous probability function was devised describing the chance of obtaining an actual RST less than a specified "cut-off". The model was applied using the data collected from the cohort of eyes. The model was also applied using published flap thickness statistics on a series of microkeratomes. RESULTS: Precision (standard deviation) was 0.74 microm for VHF digital ultrasound measurement of pachymetry, 30.3 microm for Moria LSK-One flap thickness, and 11.2 microm for NIDEK EC-5000 ablation depth. Assuming negligible laser ablation depth bias, the model found the probability that the actual RST will be <200 pmicromgiven a target RST of 250 microm is 7.56% with the Moria LSK-One. The model applied to published flap statistics revealed a range of probabilities of leaving <200 microm given a target RST of 250 microm from <0.01% to 33.6%. CONCLUSIONS: The choice of microkeratome, laser, and pachymeter has a significant impact on the variation of the depth of keratectomy and thus on the risk of ectasia. This model together with high-precision microkeratomes, preoperative pachymetry, and knowledge of laser ablation precision would enable surgeons to determine the specific imprecision of RST prediction for individual LASIK cases and minimize the risk of ectasia.

Corneal Diseases↗

PubMedDate not supplied

[The probability of transmission of the human immunodeficiency virus].

The probability of human immunodeficiency virus (HIV) transmission is one of the key epidemiological parameters in the spread of the AIDS epidemic. The understanding of its possible determinants is necessary for a thorough interpretation of epidemic dynamics and for the evaluation of alternative strategies to control the epidemic. We carried out a bibliographic review of the main studies reporting data on the probability of HIV transmission through sexual contacts and parenteral routes (by transfusion of blood and blood products, by occupational exposures among health care workers, by sharing needles among injecting drug users). Different values of probability are associated to different routes of transmission. The observed high variability depends on the viral load, on the biological fluid (blood or mucous secretions) through which the virus is transmitted and on several other factors related to source infectiousness and to individual susceptibility, both common to all routes of exposures and specific to each of them. It is currently too early to forecast how the introduction of highly active antiretroviral treatment, which strongly reduce the viral load in patients with HIV infection, will modify the probability of transmission. Therefore, the compliance with recommended preventive measures still represents an indispensable way to contrast the virus spread.

Female↗

PubMed1999

Superordinate category formation in pigeons: association with a common delay or probability of food reinforcement makes perceptually dissimilar stimuli functionally equivalent.

Training associated pairs of perceptually dissimilar stimulus classes with a common delay or probability of food reinforcement in pigeons. Then, different choice responses were trained to 1 component class in each pair. In a choice test, the untrained class in each pair occasioned the same response as did the choice-trained class. In a 3rd experiment, 2 classes had reinforcement delays of 1 s and 15 s, respectively, and 2 other classes had reinforcement probabilities of 0.1 and 0.9. Then, 1 choice response was reinforced to a class previously associated with a better condition of reinforcement (e.g., 1-s delay or 1.0 probability), and a different response was reinforced to a class previously associated with a worse condition of reinforcement (0.1 probability or 0-s delay). Testing with all classes suggested that categorization was based on the relative reinforcement or hedonic value and not on the parametric details of reinforcement.

Animals↗

PubMed2000

Probability of helicobacter pylori infection based on IgG levels and other covariates using a mixture model.

BACKGROUND: To use IgG antibody measurements to detect infection with Helicobacter pylori (H. pylori). one typically defines a cut-off value based on samples of persons presumed to be infected or uninfected. When there are no good 'gold standard' tests to determine infection status, or when laboratory conditions vary, it is useful to have a method based on the IgG measurements themselves to determine infection status. METHODS: We present a two component mixture model to analyse serologic data on H. pylori infection. The mixing proportions correspond to the probability that a latent variable, the true, unknown infection status I of a person, is either 0 (uninfected) or 1 (infected). By using a logistic model for these probabilities, we are able to incorporate covariate information. RESULTS: The model is applied to IgG data from Shandong, China. The distribution of the true infection status given the IgG value and a set of covariates is calculated using the IgG distribution function. An optimal cut-off point is found by minimising the probability of misclassification for the Shandong data. The optimal cut-off point is slightly lower than the pre-defined one. CONCLUSIONS: We contrast results from the mixture model with results from tabulations and from standard logistic regression that are based on fixed cut-points. The mixture model yields information on the probability that a person is truly infected as a function of IgG levels and covariates. In our data, the mixture model indicates that a slightly lower cut-off value than the pre-defined cut-point 1.0 can reduce misclassification rates.

Adult↗

PubMed2001

Mark-recapture and Moran curve estimates of the survival probabilities of an island population of tsetse flies Glossina morsitans morsitans (Diptera: Glossinidae).

A study on populations of Glossina morsitans morsitans Westwood on Antelope Island, Lake Kariba, Zimbabwe provided Jolly-Seber (J-S) mark-recapture estimates of adult survival and Moran curve estimates of the overall survival of all developmental stages. For females, Moran survival estimates derived using ox fly-round catches showed similar trends to, but were more variable than, those calculated from J-S population estimates. Regression of one set on the other removed only 26% of the variance. Undue emphasis should not be placed on small changes in Moran survival estimates based on sequences of catches. Catch data cannot provide Moran estimates of male survival probabilities and no comparison is thus possible with estimates from the J-S data. The J-S and the Moran approaches were combined to estimate survival probabilities of the immature stages of male and female tsetse. The overall survival per three-week period averaged 45% for males and 59% for females, comprising mature adult survivals of 27 and 46%, and pooled survivals of immature stages of 59 and 77%, respectively. The high survival of immature flies is due to the sheltered, inactive nature of pupal life. Adult and overall survival probabilities were highly correlated in males (r(2) = 0.61) but less so in females (r(2) = 0.24) where capture rates were lower and variance in the results greater. Immature and overall survival was more highly correlated for both sexes, with r(2) = 0.77 and 0.53 for males and females respectively. When a fixed pupal mortality of 1% per day was assumed, estimates of the survival of young adult males suggested that these were even lower than the survival of mature flies at the harshest times of the year, but were not markedly different when overall survival was high. Assuming equal mortality in all adults enabled the estimation of pupal survival. These had high variances but there was no evidence of any difference between the survival probabilities of male and female pupae.

Animals↗

PubMed2000

Protein sequence-structure alignment based on site-alignment probabilities.

A protein sequence-structure alignment method for database searches is examined on how effectively this method together with a simple scoring function previously developed can identify compatibilities between sequences and structures of proteins. The scoring function consists of pairwise contact energies, repulsive packing potentials of residues for overly dense arrangement and short-range potentials for secondary structures. Pairwise contact interactions in a sequence-structure alignment are evaluated in a mean field approximation on the basis of probabilities of site pairs to be aligned. Gap penalties are assumed to be proportional to the number of contacts at each residue position, and as a result gaps will be more frequently placed on protein surfaces than in cores. In addition to minimum energy alignments, we use probability alignments made by successively aligning site pairs in order by pairwise alignment probabilities. Results show that the present energy function and alignment method can detect well both folds compatible with a given sequence and, inversely, sequences compatible with a given fold. Probability alignments consisting of most reliable site pairs only can yield small root mean square deviations, and including less reliable pairs increases the deviations. Remarkably, by this method some individual sequence-structure pairs are detected having only 5-20% sequence identity.

17-Hydroxysteroid Dehydrogenases↗

PubMed2002

Influence of patient age and implantation technique on the probability of re-replacement of the homograft aortic valve.

BACKGROUND AND AIM OF THE STUDY: Results of valve rereplacement (reoperation) in 898 patients undergoing aortic valve replacement with cryopreserved homograft valves between 1975 and 1998 are reported. The study aim was to provide estimates of unconditional probability of valve reoperation and cumulative incidence function (actual risk) of reoperation. METHODS: Valves were implanted by subcoronary insertion (n = 500), inclusion cylinder (n = 46), and aortic root replacement (n = 352). Probability of reoperation was estimated by adopting a mixture model framework within which estimates were adjusted for two risk factors: patient age at initial replacement, and implantation technique. RESULTS: For a patient aged 50 years, the probability of reoperation in his/her lifetime was estimated as 44% and 56% for non-root and root replacement techniques, respectively. For a patient aged 70 years, estimated probability of reoperation was 16% and 25%, respectively. Given that a reoperation is required, patients with non-root replacement have a higher hazard rate than those with root replacement (hazards ratio = 1.4), indicating that non-root replacement patients tend to undergo reoperation earlier before death than root replacement patients. CONCLUSION: Younger patient age and root versus nonroot replacement are risk factors for reoperation. Valve durability is much less in younger patients, while root replacement patients appear more likely to live longer and hence are more likely to require reoperation.

Adolescent↗

PubMedDate not supplied

[Frequency of acquired helplessness and initial probability of showing an elaborated reaction].

The defense reflex of random performance of instrumental reaction (RPIR) had been produced in rats with different probabilities which were equal to 0.5; 0.25; 0.125 and 0.05 in respective experimental groups. Learning of animals was found to take place much faster when probabilities of RPIR were equal to 0.125 and 0.25 as compared with 0.5 and 0.05 values. One third rats which learned at 0.05 and 0.5 probabilities did not produce the reflex; the decrease in aroused, fall of body weight and other disturbances were observed and these findings may evident development of acquired helplessness about. It is supposed that optimal circumstances are occurred at different probabilities of RPIR conditions.

Animals↗

PubMed2004

[Implication of probability analysis for diagnosis of stable coronary diseases. Comparison of non-invasive diagnostic tests].

UNLABELLED: A prospective collection of clinical (history), electrocardiography (ECG), exercise electrocardiography (EE), dobutamine stress echocardiography (SE) and catheterization data was performed in 551 patients with chest pain regarded as definite or probable stable angina pectoris. All patients (65% male) without prior history of myocardial infarction undergone clinical evaluation: chest pain classification according to Diamond, ECG, EE (regarded as positive on the basis of > or = 1 mm ST-depression), SE (ischemia was defined as new or worsening wall motion abnormalities using a 16-segment model) and coronary angiography (CA):CAD was defined as > or = 50% narrowing of at least one major vessel. Sensitivity and specificity of ECG, EE, SE was calculated: and results are as follows respectively: 23% and 87%, 93% and 21%, 85% and 69%. On the basis of these results with implication of probability analysis two algorithms were developed by computer program. They incorporated pretest variables: age, gender, chest pain classification, ECG and results of one or two non-invasive tests: EE and (or) SE. The sensitivity and specificity of the algorithm is 96% and 44%. CONCLUSIONS: 1. Chest pain characteristics remains an effective tool for estimating probability of CAD. 2. The diagnostic value of resting electrocardiography in stable CAD is low. 3. SE has comparably sensitivity but significantly higher specificity than EE. 4. We demonstrated that when the likelihood of CAD is high on the basis of initial evaluation, diagnostic non-invasive testing is not indicated before CA. 5. When the probability is intermediate or low, implementation of first choice test should be different in women (SE) and men (EE).

Adult↗

PubMed2005

The probability of parallel evolution.

How often will natural selection drive parallel evolution at the DNA sequence level? More precisely, what is the probability that selection will cause two populations that live in identical environments to substitute the same beneficial mutation? Here I show that, under fairly general conditions, the answer is simple: if a wild-type sequence can mutate to n different beneficial mutations, replicate populations will on average fix the same mutation with probability P = 2/(n + 1). This probability, which is derived using extreme value theory, is independent of most biological details, including the length of the gene in question and the precise distribution of fitness effects among alleles. I conclude that the probability of parallel evolution under natural selection is nearly twice as large as that under neutrality.

Adaptation, Biological↗