Gaussian Frequency Distribution Research Articles

Is there functional collateral flow during vascular occlusion in angiographically normal coronary arteries?

Thus far, it is unknown whether there is functional collateral flow through preexisting anastomoses in patients with angiographically normal coronary arteries. Such preformed coronary collateral vessels could form the basis for subsequently developing protective natural bypasses in the presence of coronary artery disease. Among 100 patients, the collateral flow index (CFI) was measured in coronary arteries without stenotic lesions. The CFI was determined by simultaneous measurement of mean aortic pressure, central venous pressure, and coronary wedge pressure via a sensor-tipped guidewire at the end of a 1-minute balloon occlusion. Patients were divided in 2 groups according to complete angiographic absence (51 patients) or partial presence (49 patients) of stenotic lesions in coronary arteries other than that undergoing collateral measurement. CFI in all patients (61+/-10 years; men/women, 69/31) amounted to 0.18+/-0.08 (range, 0.04 to 0.36). It showed a normal Gaussian frequency distribution; 22 individuals had a CFI > or =0.25, a value that was empirically found to represent well-developed collaterals protective against myocardial ischemia during coronary occlusion. Accordingly, 17 patients did not reveal signs of myocardial ischemia during coronary balloon occlusion, as assessed from an intracoronary ECG, and 26 patients did not experience angina pectoris during occlusion. In humans with angiographically normal coronary arteries, there are functional collateral vessels to the extent that one fifth to one quarter of them do not show signs of myocardial ischemia during brief vascular occlusions.

MAP harmonic retrieval in Gaussian colored noise with prior information

In this paper we deal with the MAP estimation of the parameters of a single complex sinusoid in Gaussian white and colored noise. We analyze the effects of modeling the frequency and amplitude as random variables, deriving the expressions of the MAP estimators, and we analytically compute the CRB's for a tone with Gaussian frequency and amplitude distributions. Moreover, we explicitly derive closed-form expressions of the optimal estimators in the case of Gaussian and Rayleigh distributed amplitude. In particular, we show that, in the colored noise case, the amplitude information can also help improving the performance of the frequency estimator, and vice versa. We propose an efficient FFT-based implementation structure of the optimal estimators, which can be further simplified when the noise can be modeled as an AR process; besides, when the estimators cannot be expressed in closed form, we propose a simple univariate optimization algorithm in order to compute the estimates. Simulation results are reported, which validate the theoretical results and witness the significant performance improvement that can be achieved embodying the parameter priors into the frequency and amplitude estimators.

Sample discrimination of frequency differences with irrelevant context–revisited

This experiment replicated earlier work to provide additional data for post hoc analyses of perceptual weights for conditions with large effects of context on performance. The non-adaptive, 2AFC task was sample discrimination of frequency differences (SD-F) in quiet and with added context stimuli. Listeners were to select the interval in which a pair of target tones was drawn from the higher of two Gaussian frequency distributions (means of 2000 and 2150 Hz). Pairs of context stimuli were added at frequency regions above and below the targets, at distances of 600, 1000 and 1400 Hz. Context stimuli were fixed-frequency tones, noise bands, or random-frequency tones. All stimuli were 100 ms with 5-ms ramps, presented simultaneously. Conditions were tested with and without Gaussian level jitter (standard deviation=3 dB). Mean levels of target and context stimuli were equated or systematically varied across conditions. The results confirm earlier work, with little effect of fixed-frequency tones or noise-band context at any distance from the targets. Random-frequency context tones produced large detrimental effects even at remote distances, due predominantly to effects of the lower-frequency context tone. Level variation had little effect. Different approaches to calculating perceptual weights influences data interpretation. [Work supported by NIDCD.]

An Inverse Gaussian distribution of tree frequencies based on tree size

Forest stand management often depends on data from a single fixed area inventory plot located at random in a forest stand. The plot provides detailed information about tree size distribution but not about per unit area tree frequency distribution unless one assumes a Poisson (POI) distribution. The POI assumption ignores any relationship between a tree's size and its demand for growing space. This study argues for the Inverse Gaussian (IG) distribution as a more realistic model. Maximum likelihood estimates of the IG parameters are obtained from a transformation of tree size data (diameter) to proxies of tree counts. Data from two stands indicated that an IG model was better at predicting the tree frequency distribution than a POI model.

The Gaussian atmospheric transport model and its sensitivity to the joint frequency distribution and parametric variability.

Reconstructed meteorological data are often used in some form of long-term wind trajectory models for estimating the historical impacts of atmospheric emissions. Meteorological data for the straight-line Gaussian plume model are put into a joint frequency distribution, a three-dimensional array describing atmospheric wind direction, speed, and stability. Methods using the Gaussian model and joint frequency distribution inputs provide reasonable estimates of downwind concentration and have been shown to be accurate to within a factor of four. We have used multiple joint frequency distributions and probabilistic techniques to assess the Gaussian plume model and determine concentration-estimate uncertainty and model sensitivity. We examine the straight-line Gaussian model while calculating both sector-averaged and annual-averaged relative concentrations at various downwind distances. The sector-average concentration model was found to be most sensitive to wind speed, followed by horizontal dispersion (sigmaZ), the importance of which increases as stability increases. The Gaussian model is not sensitive to stack height uncertainty. Precision of the frequency data appears to be most important to meteorological inputs when calculations are made for near-field receptors, increasing as stack height increases.

A Model of Inhomogeneous Broadening and Pressure Induced Hole Shifts in the Optical Spectra of Organic Chromophores in Glasses

Zero phonon holes were burned in the S1rS0 absorption bands of organic compounds in ethanol glass at 6 K. The application of He gas pressure (P) leads to the shift of holes with a frequency coefficient d O/dP. The linear dependence of dO/dP on hole position (O) within the inhomogeneous band is of considerable interest, since it can provide the S1rS0 energies of nonsolvated chromophores and the (local) compressibility of the surrounding matrix. However, the extrapolated frequencies O0(P) at which pressure shift should vanish deviate from the actual 0-0 energies of free solutes in a vacuum. The slope (a) of the plot of dO/dP vs O varies between 1.5 10 -5 and 6.2 10 -5 bar -1 for different dyes, and therefore cannot be directly associated with the isothermal compressibility of the matrix. To account for these facts, a model was developed based on the assumption that the solvent shift is a superposition of repulsive, dispersive, electrostatic, and other possible interactions. Each interaction has a specific intermolecular distance ( r) dependence (e.g. r -6 for dispersive and r -12 for repulsive interactions) and is assigned a Gaussian frequency distribution function. It follows from the model that the observed slope a depends on, in addition to the volume compressibility of the matrix, the widths of the constituent Gaussians and the respective power coefficients of r. Both the measured bandwidth and the slope a are also sensitive to the type of correlation between different solvent shift mechanisms. In rhodamine dyes a dipole moment change between the ground and the excited state is responsible for a strong broadening and a shallow slope a. The conformational flexibility of a dicarbocyanine dye HIDCI has similar consequences. The mechanisms leading to a matrix polarity dependent hypsochromism in open chain cyanine dyes, free base tetrapyrroles, and s-tetrazine correspond to a distance dependence of r -6 and are deemed to be of multipolar nature. The magnitude of the hypsochromic solvent shift owing to repulsive forces is probably small, although their contribution to the slope a and the inhomogeneous bandwidth may be relatively large.

Effect of target versus distracter-tone frequency region and variance on sample discrimination of frequency differences

This experiment examined how performance in a complex frequency-discrimination task was influenced by the frequency region and relative frequency variability of target versus distracter tones. In a 2IFC sample-discrimination task, normal-hearing listeners were asked to select the interval containing target tones drawn from the higher of two overlapping Gaussian frequency distributions. The target distributions were placed in one of three frequency regions: ‘‘low’’ (500 Hz), ‘‘middle’’ (1414 Hz), or ‘‘high’’ (4000 Hz). For a particular target region (e.g., low), two distracter tones were then drawn at random and added, one from each of the Gaussian distributions at the two remaining frequency regions (e.g., middle and high). All distributions were equivariant on a logarithmic frequency scale. Based on the CoRE model [R. A. Lutfi and K. A. Doherty, J. Acoust. Soc. Am. 96, 3443–3450 (1994)], it was predicted that tones with greater variance would have a greater influence on performance independent of frequency region. The results indicate that the influence of variance depends on the frequency region of targets versus distractors, with notable individual differences and training effects. [Work supported by NIDCD.]

Measurements of cnidae from sea anemones (Cnidaria: Actiniaria), II: further studies of differences amongst sample means and their taxonomic relevance

Lengths of cnidae, sampled from fresh tissue (tentacles, acontia or column) from the sea anemones Metridium senile (Linnaeus), Cereus pedunculatus (Pennant), Sagartia elegans (Dalyell), Sagartia troglodytes (Price), Anthopleura thallia (Gosse) and Urticina eques (Gosse), were measured. The mean, mode, median, standard deviation, variance, coefficient of variation, minimum, maximum and range of each sample were calculated. Lengths of nematocysts (basitrichs and microbasic p-mastigophores) and spirocysts had normal (Gaussian) frequency distributions, and there was no important within-sample variability. A between-sample (same specimen) difference was detected between mean lengths of microbasic p-mastigophores from acontia in Sagartia elegans. Intraspecific (between-specimen) differences amongst mean lengths of basitrichs from column ectoderm were detected amongst four specimens of Urticina eques, and a difference occurred between the mean lengths of microbasic p-mastigophores from the acontia of two specimens of Sagartia elegans. However, for each of the species U. eques and S. elegans, there was no direct correlation between the sizes of the anemones examined and the mean lengths of their nematocysts. Overall, the results confirm that cnida size, when taken in isolation, is not a reliable taxonomic character for sea anemones. For other purposes, mean cnida measurements may be useful and a rapid sample range test is suggested for analysing differences between them. Methods are also presented for estimating means and standard deviations from data in publications that provide only ranges of cnida sizes and the number of cnidae measured. The present results and conclusions supplement those of Williams (1996, Scientia Marina, 60: 339-351).

An alternative for women initially declining genetic amniocentesis: Individual Down syndrome odds on the basis of maternal age and multiple ultrasonographic markers

OBJECTIVE: Our purpose was to develop a method of calculating the individual odds of Down syndrome on the basis of a combination of maternal age and multiple ultrasonographic parameters that can be used to counsel women at high risk who initially decline amniocentesis. STUDY DESIGN: Maternal age and ultrasonographic biometry data were collected prospectively on 3254 normal and 30 Down syndrome singleton fetuses between 15 and 24 weeks’ gestation. Humerus length data were expressed as multiples of the normal median. Log transformation of the humerus length data permitted their expression in gaussian frequency distributions and the calculation of likelihood ratios for Down syndrome on the basis of humerus length. We also developed likelihood ratios on the basis of the degree of nuchal skinfold thickening and the presence or absence of hyperechoic fetal bowel and hypoplastic fifth digit. RESULTS: The ultrasonographic parameters and maternal age did not significantly correlate with each other and were significant independent predictors of Down syndrome. We therefore calculated the individual odds of Down syndrome by using the product of the age-related risk and the likelihood ratios associated with nuchal thickening, humerus length shortening, and the presence or absence of hyperechoic fetal bowel or fifth digit hypoplasia, respectively. At a Down syndrome risk level of >1:50, a 60.0% detection rate with 4.5% false-positive rate was observed with a screen-positive rate of 5.5%, positive predictive value of 1:10, and odds ratio (95% confidence interval) of 28.4 (12.8 to 64.0). CONCLUSION: This is the first report of individual odds calculation based on multiple midtrimester biometry parameters and maternal age. The screening efficiency is similar to that reported with triple-analyte serum screening. These data are useful for counseling women who are at increased Down syndrome risk and initially decline amniocentesis. (Am J Obstet Gynecol 1998;179:514-9.)

Temporal Characteristics of NMR Signals from Spin 3/2 Nuclei of Incompletely Disordered Systems

Anisotropic nuclear quadrupole interactions can produce residual quadrupole splitting in the NMR spectra of rapidly moving quadrupolar nuclei in incompletely disordered aqueous heterogeneous systems. Such systems may include hydrated sodium nuclei in biological tissue and biopolymer gels. To describe the NMR signals from such samples, we use a domain model in which each domain is characterized by a quadrupole frequency and a residence time of the nucleus. We show that the signals from each domain after one pulse, the quadrupole echo sequence, and the various multiple quantum filters (MQFs) can be expressed as a linear combination of five different phase coherences. To simulate the effect of various distributions (Pake powder pattern, Gaussian, etc.) of quadrupole frequencies for different domains on the NMR signal, we have written the computer program CORVUS. CORVUS also includes the effects of exchange between different domains using diffusion and random jump models. The results of computer simulations show that the Gaussian and Pake powder pattern quadrupole frequency distributions produce very different phase coherences and observable NMR signals when the exchange rate (1/taue) between different domains is slow. When 1/taue is similar to the root mean square quadrupole frequency (final sigma), the signals from the two distributions are similar. When 1/taue is an order of magnitude greater than final sigma, there is no apparent evidence of quadrupole splitting in the shape of the signal following one pulse, but the residual effects of the quadrupole splitting make a significant contribution to the fast transverse relaxation rate. Therefore, in this case, it is inappropriate to use the observed biexponential relaxation rates to obtain a single correlation time. The quadrupole echo and the various MQF signals contain an echo from the satellite transitions in the presence of quadrupole splitting. The peak of this echo is very sensitive to 1/taue. The time domain analysis of these signals is more direct and less ambiguous than the frequency domain analysis because the echo does not occur at the beginning of data acquisition. The quadrupole echo pulse sequence is the most sensitive detector of residual quadrupole splitting and exchange of sodium ions between different domains. However, if the sample is compartmentalized so that only a fraction of the nuclei have quadrupole splitting, the double quantum magic angle filter (DQ-MA) is more suitable. This is because the DQ-MA signal contains only the contributions from satellite transitions. Use of simulations to analyze signals from various one-pulse, quadrupole echo, and multiple quantum filter pulse sequences can yield information on substrate order and aid in quantitation of multiple quantum filter signals.

Interactions of pairs of target and context tones based on relative frequency relation and degree of frequency uncertainty

Four normal-hearing listeners completed 2IFC sample-discrimination tasks for frequency, in which they judged which of two tone pairs was drawn from the higher of two Gaussian frequency distributions. The center frequencies of the two target distributions were placed at low (400/500 Hz), middle (1128/1410 Hz), or high (3200/4000 Hz) frequency regions. All distributions were equally spaced and equivariant on a logarithmic frequency scale. Pairs of extraneous (to-be-ignored) context tones were added above, flanking, and below the low-, middle-, and high-frequency target regions, respectively. Across conditions, target and context tones used the same frequency regions, but never occupied the same frequency region within condition. Context tones were fixed in frequency, varied between two known frequencies, or were Gaussian distributed. A level jitter was added to derive perceptual weighting functions across stimuli. The results showed little detrimental effect of fixed-frequency context tones, but large individual differences in the effects of known pairs of tones or tones with Gaussian variation. Weighting functions and patterns of frequency interactions among stimuli will be discussed. [Work supported by NIDCD.]

Bayesian approach to recalibration

The Bayesian method of estimating the mean of a Gaussian frequency distribution is applied to the problem of recalibrating a measuring device which incorporates a calibrated reference quantity. It is supposed that the recalibration is performed by measuring a quantity which has been estimated from other sources prior to the measurement. An expression for the joint a posteriori probability distribution for the measurand and reference quantity is derived. This yields a posteriori estimates, associated uncertainties and the covariance for the two quantities. The estimate of the reference quantity so obtained is interpreted as the recalibrated value.

Influence of Milling or Cutting Procedures on Trace Element Contents of Plant Samples

Several milling and cutting procedures have been tested for their suitability in trace element analysis of non - contaminated plant samples. Barley and wheat were selected because of their low trace element contents, and lucerne in order to test cutting with rotating steel knives. Grains and lucerne plants were distributed as a ring test, in order to yield processed and homogenized samples ready for decomposition and determination. The samples were collected and analyzed for their trace element contents in one lab, in order to avoid differences due to systematic errors of analysis. The trace elements of interest were chosen to detect abrasion of the mills (Fe, Cr, Ni, Mo, Co), contamination from dust (Pb, Cd), and cross contamination from animal feed premixes (Cu, Mn, Zn), which are often processed in the same labs as well. Whereas the results for Mn, Mo, and Cd approached Gaussian frequency distribution, the results for Cr, Ni, and Fe were clearly asymmetrically distributed, presumably due to many contaminated samples. In order to recognize outliers for asymmetric frequency distributions, construction of a symmetrical frequency distribution around the median is proposed, to which Gaussian statistics was applicable. The precision of the subsequent analytical procedure was evaluated with the same, barley sample and among the same labs in a ring test one year afterwards. As a result, the precision of the median- corrected frequency distribution of milling/cutting and of the analysis were within then same range, whereas the scatter of data from elements prone to abrasion like Fe, Cr, Ni, was much larger from the original data set. As sources for contamination, bad maintenance of the milling room, as well as abrasion of rotor and sieve from continuously operating mills (stainless steel - titanium) could be recognized. There was no significant difference between continously and discontinously operating devices, nor between different modes of cleaning, like washing, discarding the first portion, or just blowing and brushing.

Down syndrome screening in an asian population using alpha-fetoprotein and free β-hcg: a report of the taiwan down syndrome screening group

To evaluate whether the strategy of maternal serum screening for Down syndrome, using alpha-fetoprotein (AFP) and free beta-hCG in combination with maternal age, a technique developed in western countries, is applicable to an Asian population. Alpha-fetoprotein and beta-hCG were measured in serum samples from 23 Down syndrome pregnancies and 1748 unaffected singleton Taiwanese (ethnically Chinese) pregnancies at 14-22 weeks' gestation. Gestational age-specific medians and a maternal weight correction formula were established for our own population. Likelihood ratio for Down syndrome pregnancies in relation to multiples of the median (MoM) levels of these analytes were derived from the overlapping gaussian frequency distribution curves for Down syndrome and unaffected pregnancies. The serum AFP and free beta-hCG median MoM values of Down syndrome pregnancies were significantly abnormal in Asian subjects (0.77 and 2.91, respectively), and similar to those of affected pregnancies in white women. The median value of free beta-hCG:AFP MoM ratio (2.97) in Down syndrome pregnancies was significantly higher than that of unaffected pregnancies (1.09). The mean maternal weight during the second trimester in pregnant Asian women (55.2 kg) was markedly lighter than that of white women. At a 5.8% false-positive rate, free beta-hCG identified 47.8% of Down syndrome pregnancies (likelihood ratio 8.2), AFP detected only 13% of the cases (likelihood ratio 2.2), and free beta-hCG:AFP MoM ratio detected 43.5% of the cases (likelihood ratio 7.4). By using a multivariate risk algorithm involving the combination of AFP, free beta-hCG, and maternal age, 56.5% of Down syndrome cases could be detected with a 5.3% false-positive rate (likelihood ratio 10.7). Maternal serum screening strategy using AFP and free beta-hCG in combination with maternal age is feasible in the detection of fetal Down syndrome among Asian women.

Contributions of frequency distribution analysis to the understanding of coronary restenosis. A reappraisal of the gaussian curve.

Clinical restenosis after balloon angioplasty can be categorized by use of dichotomous terms based on the presence or absence of recurrent myocardial ischemia. In contrast, recent investigations have concluded that late luminal renarrowing, documented through angiographic imaging, occurs to a variable extent in nearly all stenoses. This process has been characterized by a gaussian or normal frequency distribution, with restenosis simply representing an extreme form of this delayed remodeling. In the current study, frequency distribution analysis was used to examine the process of coronary restenosis in a large cohort of patients at risk. Quantitative coronary angiographic analysis was applied to 9279 cineangiograms obtained in 3093 patients before and immediately after angioplasty and after 6-month follow-up. Late loss, defined as the change in minimum lumen diameter of the target stenosis from postdilation to follow-up, did not statistically conform to a normal distribution (P<.0001 by both chi2 statistic and Kolmogorov-Smirnov test), even after the exclusion of the 236 stenoses that displayed total occlusions at follow-up angiography. Examination of deviation from a normal curve revealed an excessively high frequency of stenoses that experienced either little change (0.0+/-0.3 mm) or marked change (1.0 to 2.0 mm) in late loss, with a low frequency of stenoses with intermediate values (0.3 to 1.0 mm). Similarly, although the distribution of percent diameter stenosis of the target lesion was statistically normal immediately after dilation, this gaussian distribution disappeared during the follow-up period. Other angiographic indexes of restenosis also failed to approximate a normal curve. In an attempt to improve the goodness of fit, a probabilistic model of late loss was created on the basis of deconvolution of the observed data distribution. Two theoretical, discrete populations of stenoses were identified, one with and one without overall late luminal narrowing. Unlike the gaussian distribution, this model provided a good representation of the observed data (P=NS for lack of fit). The frequency distributions of angiographic indexes of restenosis often superficially resemble a gaussian curve, an appearance that is artifactually enhanced by the measurement imprecision of current quantitative techniques. Nevertheless, standard indexes of coronary restenosis fail to conform statistically to a normal distribution. The pattern of deviations observed supports the possible existence of discrete subpopulations of lesions, each with a different propensity toward the development of restenosis after coronary intervention.

Maternal serum pregnancy-associated plasma protein a and fetal nuchal translucency thickness for the prediction of fetal trisomies in early pregnancy

To determine if the risk for fetal trisomies during the first trimester of pregnancy can be derived by combining data from maternal serum pregnancy-associated plasma protein A (PAPP-A) and fetal nuchal translucency thickness.Pregnancy-associated plasma protein A was measured in samples from 87 singleton pregnancies with fetal chromosomal abnormalities (45 trisomy 21, 19 trisomy 18, eight trisomy 13, 11 sex chromosome aneuploidies, four triploidies) and 348 chromosomally normal controls at 10-13 weeks' gestation. Likelihood ratios for trisomies 21, 18, and 13 in relation to PAPP-A, in multiples of the normal median (MoM) for crown-rump length, were derived from the overlapping gaussian frequency distribution curves for normal and abnormal pregnancies.In the chromosomally normal group, maternal serum PAPP-A correlated significantly with fetal crown-rump length (r = 0.421, P < .0001). In the chromosomally abnormal group, the median PAPP-A was significantly lower than in the normal controls. The respective median values expressed in MoM for trisomies 21, 18, and 13 and other aneuploidies were 0.5 MoM (90% confidence interval [CI] 0.09-1.67, z = 6.0, P < .001), 0.17 MoM (90% CI 0.06-1.45, z = 6.6, P < .001), 0.25 MoM (90% CI 0.10-0.62, z = 4.5, P < .001), and 0.72 MoM (90% CI 0.09-2.48, z = 2.2, P < .05), respectively. There was no significant linear association between PAPP-A and fetal nuchal translucency thickness in either the chromosomally normal (r = -0.01, P = .89) or abnormal groups (r = -0.19, P = .08).The risks for fetal trisomies at 10-13 weeks' gestation can be derived by combining data on maternal age, maternal serum PAPP-A, and fetal nuchal translucency thickness.

Contamination of Indian maize with fumonisin B1and its effects on chicken macrophage

The natural occurrence of fumonisin B 1 (FB 1 ) in Indian maize and its impact on the viability and phagocytosis of chicken peritoneal macrophages in vitro were investigated. FB 1 was found to be present at the levels of 300-366 ppm in Fusarium moniliforme-infected maize grains. The infected kernels in a population of 100 ears showed normal (Gaussian) frequency distribution. FB 1 , extracted from the contaminated kernels, reduced the viability and phagocytic activity of macrophages. These findings imply that FB 1 exposure may become a potential problem in India and may lead to decreased immune responses with consequent susceptibility of a host to infection

Decay of the resonance fluorescence following pulsed excitation of a weakly disordered excitonic system

A study is made of the decay of the resonance fluorescence following pulsed excitation of a weakly disordered system whose optical excitations are Frenkel excitons. The disorder is characterized by a Gaussian distribution of optical transition frequencies with no correlation between different sites. The duration of the resonant pulse is taken to be short in comparison with the reciprocal of the optical linewidth, and the wavelength of the light is assumed to be large in comparison with either the size of the array or the exciton mean free path associated with the disorder. In the limit where σ, the standard deviation of the Gaussian distribution, is much less than the exciton bandwidth, the integrated intensity of the fluorescence decays non-exponentially and is characterized by universal functions of σ x t, where x= 4 3 , 2, and 4 in one, two, and three dimensions, respectively. Analytic approximations to the scaling functions in two and three dimensions are presented.

Effects of age, race, sex, and smoking on prothrombin fragment 1.2 in a healthy population

Prothrombin fragment 1.2 (F1.2) is a biomarker of thrombin generation during blood coagulation and has diagnostic potential for assessing thrombotic risk and monitoring anticoagulation therapy. We used a monoclonal antibody-based immunoassay for plasma F1.2 to establish a well-defined reference interval and to evaluate the effects of age, race, sex, and smoking status on F1.2 concentrations in a healthy population. Plasma samples and demographic information were obtained from 357 healthy individuals. F1.2 concentrations more closely followed a lognormal than a gaussian frequency distribution. In a multiple linear-regression model in which the logarithms of F1.2 concentrations were regressed on age, race, sex, and smoking status, the significant explanatory variables were age and, to a lesser extent, sex and smoking. A segmented "hockey stick" regression model indicated that F1.2 concentrations and age were unrelated for individuals < 44 years old but were positively correlated above that age threshold. The estimated 95% tolerance interval (P = 0.95) for F1.2 in healthy individuals < 44 years old (n = 268) was 0.21-2.78 nmol/L. We conclude that age-matched F1.2 reference intervals may be important for studies evaluating the diagnostic utility of F1.2 measurements, and that the clinical relevance of increased thrombin generation during aging warrants further investigation.

Predicting Fetal Chromosome Anomalies in the First Trimester Using Pregnancy Associated Plasma Protein-A: A Comparison of Statistical Methods

The analysis of the clinical efficiency of a biochemical parameter in the prediction of chromosome anomalies is described, using a database of 475 cases including 30 abnormalities. A comparison was made of two different approaches to the statistical analysis: the use of Gaussian frequency distributions and likelihood ratios, and logistic regression. Both methods computed that for a 5% false-positive rate approximately 60% of anomalies are detected on the basis of maternal age and serum PAPP-A. The logistic regression analysis is appropriate where the outcome variable (chromosome anomaly) is binary and the detection rates refer to the original data only. The likelihood ratio method is used to predict the outcome in the general population. The latter method depends on the data or some transformation of the data fitting a known frequency distribution (Gaussian in this case). The precision of the predicted detection rates is limited by the small sample of abnormals (30 cases). Varying the means and standard deviations (to the limits of their 95% confidence intervals) of the fitted log Gaussian distributions resulted in a detection rate varying between 42% and 79% for a 5% false-positive rate. Thus, although the likelihood ratio method is potentially the better method in determining the usefulness of a test in the general population, larger numbers of abnormal cases are required to stabilise the means and standard deviations of the fitted log Gaussian distributions.