Corresponding Coefficient Of Variation Research Articles

Divergent apparent temperature sensitivity of terrestrial ecosystem respiration

Aims Recent studies revealed convergent temperature sensitivity of ecosystem respiration (Re) within aquatic ecosystems and between terrestrial and aquatic ecosystems. We do not know yet whether various terrestrial ecosystems have consistent or divergent temperature sensitivity. Here, we synthesized 163 eddy covariance flux sites across the world and examined the global variation of the apparent activation energy (Ea), which characterizes the apparent temperature sensitivity of and its interannual variability (IAV) as well as their controlling factors. Methods We used carbon fluxes and meteorological data across FLUXNET sites to calculate mean annual temperature, temperature range, precipitation, global radiation, potential radiation, gross primary productivity and Re by averaging the daily values over the years in each site. Furthermore, we analyzed the sites with >8 years data to examine the IAV of Ea and calculated the standard deviation of Ea across years at each site to characterize IAV. Important Findings The results showed a widely global variation of Ea, with significantly lower values in the tropical and subtropical areas than in temperate and boreal areas, and significantly higher values in grasslands and wetlands than that in deciduous broadleaf forests and evergreen forests. Globally, spatial variations of Ea were explained by changes in temperature and an index of water availability with differing contribution of each explaining variable among climate zones and biomes. IAV and the corresponding coefficient of variation of Ea decreased with increasing latitude, but increased with radiation and corresponding mean annual temperature. The revealed patterns in the spatial and temporal variations of Ea and its controlling factors indicate divergent temperature sensitivity of R-e, which could help to improve our predictive understanding of R-e in response to climate change.

Creep behavior of basalt fiber reinforced polymer tendons for prestressing application

This paper investigates the creep behavior of newly developed basalt fiber reinforced polymer (BFRP) tendons for prestressing application. The creep strain to time relationship, creep rate and residual strength were experimentally studied and the relevant mechanism was elaborated. Furthermore, the creep rupture stress was predicted based on statistical analysis. The results show that the creep strain to time relationship of BFRP tendons depends highly on the stress level applied. The creep rate of BFRP tendons under low levels of stress remains in low and steady values. Meanwhile, the residual strength of BFRP after 1000h of sustained load still reaches approximately 95% of its initial tensile strength and the corresponding coefficient of variation (CV) is much less than the original CV. For prestressing application, the creep rupture stress limit for BFRP tendons can be adopted up to 52% of its tensile strength according to the reliability based analysis.

Persistent Fluctuations in Stride Intervals under Fractal Auditory Stimulation

Stride sequences of healthy gait are characterized by persistent long-range correlations, which become anti-persistent in the presence of an isochronous metronome. The latter phenomenon is of particular interest because auditory cueing is generally considered to reduce stride variability and may hence be beneficial for stabilizing gait. Complex systems tend to match their correlation structure when synchronizing. In gait training, can one capitalize on this tendency by using a fractal metronome rather than an isochronous one? We examined whether auditory cues with fractal variations in inter-beat intervals yield similar fractal inter-stride interval variability as isochronous auditory cueing in two complementary experiments. In Experiment 1, participants walked on a treadmill while being paced by either an isochronous or a fractal metronome with different variation strengths between beats in order to test whether participants managed to synchronize with a fractal metronome and to determine the necessary amount of variability for participants to switch from anti-persistent to persistent inter-stride intervals. Participants did synchronize with the metronome despite its fractal randomness. The corresponding coefficient of variation of inter-beat intervals was fixed in Experiment 2, in which participants walked on a treadmill while being paced by non-isochronous metronomes with different scaling exponents. As expected, inter-stride intervals showed persistent correlations similar to self-paced walking only when cueing contained persistent correlations. Our results open up a new window to optimize rhythmic auditory cueing for gait stabilization by integrating fractal fluctuations in the inter-beat intervals.

Extended multivariate approach for uncertainty reduction in the assessment of undrained shear strength in clays

Important features of the multivariate approach are discussed, and an extension to this approach is proposed whereby the total uncertainty in site investigation methods due to spatial averaging is assessed prior to its adoption. Results from a site investigation of spatially averaged values of undrained shear strength ([Formula: see text]) and the corresponding coefficient of variation ([Formula: see text]) in Veda sulphide clay were used as a practical illustration of the extended multivariate approach and provide a basis for discussion. The inherent variability and scales of fluctuation for different methods are presented. The study shows the usefulness of the extended multivariate approach for the evaluation of representative values of [Formula: see text] and [Formula: see text] based on results from different methods. It is also a way of implicitly reducing the transformation errors that arise when a property is derived from measurement results. Nevertheless, considerable care must be taken as a much lower COV for one method will have a significant impact on the results.

A new submaximal cycle ergometer test for prediction of VO2max

Maximal oxygen uptake (VO2max) is an important, independent predictor of cardiovascular health and mortality. Despite this, it is rarely measured in clinical practice. The aim of this study was to create and evaluate a submaximal cycle ergometry test based on change in heart rate (HR) between a lower standard work rate and an individually chosen higher work rate. In a mixed population (n = 143) with regard to sex (55% women), age (21-65 years), and activity status (inactive to highly active), a model included change in HR per unit change in power, sex, and age for the best estimate of VO2max. The association between estimated and observed VO2max for the mixed sample was r = 0.91, standard error of estimate = 0.302 L/min, and mean measured VO2max = 3.23 L/min. The corresponding coefficient of variation was 9.3%, a significantly improved precision compared with one of the most commonly used submaximal exercise tests, the Åstrand test, which in the present study was estimated to be 18.1%. Test-retest reliability analysis over 1 week revealed no mean difference in the estimated VO2max (-0.02 L/min, 95% confidence interval: -0.07-0.03). The new test is low-risk, easily administered, and valid for a wide capacity range, and is therefore suitable in situations as health evaluations in the general population.

Variation in omeprazole pharmacokinetics in a random Iranian population: a pilot study

Omeprazole is metabolized in the liver mainly by the polymorphic CYP2C19 enzyme. Considerable ethnic differences have been reported in the pharmacokinetics of omeprazole. The present study was conducted to evaluate the pharmacokinetic parameters of omeprazole after a single oral administration to a random Iranian population. Thirty healthy male subjects, aged 24-31 years, weighing 60-98 kg completed the study. Plasma concentrations of omeprazole were measured over a 12 h period after administration of a single oral dose of 20 mg omeprazole. The pharmacokinetic parameters were calculated from the plasma concentration-time profiles. Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) was used to quantify 5-hydroxyomeprazole. The mean area under the concentration-time curve (AUC) from time zero to infinity (AUC(∞) ) values of omeprazole and the corresponding coefficient of variation (CV%) was 987.3 ng h/ml (65%). In general, most subjects showed a normal distribution. Only one subject showed a very high AUC compared with the corresponding mean AUC level. This subject had the highest half-life and the lowest rate of elimination. The omeprazole metabolic ratio for this subject was 2.9, while for the others it was in the range 0.12-0.56. These results are consistent with previous literature that showed the existence of interindividual variability in omeprazole pharmacokinetics, even within a single ethnic group. Differences in the pharmacokinetics could be due to differences in the genetic make-up of subjects as found in their omeprazole metabolic ratios.

Determination of brachial artery stiffness prior to vascular access creation: reproducibility of pulse wave velocity assessment

Despite routine ultrasound mapping of upper extremity arteries and veins, early thrombosis and nonmaturation remain frequent complications following vascular access (VA) surgery. Besides vascular diameters, brachial artery stiffness is assumed to play an important role; however, reproducibility of measurements has never been established. The purpose of this study was to determine within-session and between-session variabilities of pulse wave velocity (PWV) assessment by using ultrasonography and blood pressure registration. Beat-to-beat changes in brachial artery diameter and pressure were obtained in 21 subjects in measurement sessions on Day 1 and Day 3. Each session consisted of three acquisitions. For each acquisition, systolic and diastolic diameter and pressure were determined and used for calculation of brachial artery PWV. Within-session variability of diameter and pressure, as well as the estimated PWV, was expressed using the intraclass correlation coefficient with corresponding coefficient of variation (CoV). Between-session variability was reported using Bland-Altman analysis in combination with CoV analysis. Significant agreement (P < 0.001) was obtained for all diameter and pressure measurements obtained on Day 1 and Day 3. Within-session CoV of pulse pressure, diastolic diameter and distension were 7.0, 1.6 and 18.3%, respectively. Subsequent estimation of local PWV resulted in a CoV of 10.6%. Between-session CoV was 15.1, 3.8 and 18.9% for pulse pressure, diastolic diameter and distension, respectively. For PWV estimation, this resulted in a CoV of 13.5%. Diameter and pressure can be recorded accurately over the cardiac cycle, and calculations of distensibility, pulse pressure and PWV show a slight to moderate degree of variation. Larger studies elaborating on interindividual differences need to determine the clinical efficacy of PWV measurements prior to VA creation.

Influence of root structure on root survivorship: an analysis of 18 tree species using a minirhizotron method

AbstractFine root survivorship is an important aspect of root ecology and is known to be influenced by a suite of covariates. However, the relative importance of each covariate on root survivorship is not clear. Here, we used minirhizotron‐based data from 18 woody species to evaluate the relative strength of influence on root survivorship by root diameter, branch order, soil depth, and season of root birth, and to examine how the relationship between each covariate and root survivorship differed across species. We extracted hazard ratio estimates for 16 species from published studies that performed Cox proportional hazards regression analysis, and from our own unpublished data for two Chinese temperate tree species. The mean change in hazard ratio (CHR) and corresponding coefficient of variation for each factor were calculated across species. On average, root diameter and season of root birth had stronger effects on root survivorship than branch order and soil depth. However, the effects of season varied with species and were stronger in temperate forests, whereas the influence of diameter and order were relatively consistent across species. These results suggest that root structures such as diameter and branch order should be carefully considered in root classification and sampling, and adding season of birth (particularly in temperate forests) as a covariate in root survivorship analysis should further enhance our ability in achieving a better understanding of root demography and more accurate estimates of root turnover in forests of different climate zones.

Latitudinal patterns of magnitude and interannual variability in net ecosystem exchange regulated by biological and environmental variables

AbstractOver the last two and half decades, strong evidence showed that the terrestrial ecosystems are acting as a net sink for atmospheric carbon. However the spatial and temporal patterns of variation in the sink are not well known. In this study, we examined latitudinal patterns of interannual variability (IAV) in net ecosystem exchange (NEE) of CO2 based on 163 site‐years of eddy covariance data, from 39 northern‐hemisphere research sites located at latitudes ranging from ∼29°N to ∼64°N. We computed the standard deviation of annual NEE integrals at individual sites to represent absolute interannual variability (AIAV), and the corresponding coefficient of variation as a measure of relative interannual variability (RIAV). Our results showed decreased trends of annual NEE with increasing latitude for both deciduous broadleaf forests and evergreen needleleaf forests. Gross primary production (GPP) explained a significant proportion of the spatial variation of NEE across evergreen needleleaf forests, whereas, across deciduous broadleaf forests, it is ecosystem respiration (Re). In addition, AIAV in GPP and Re increased significantly with latitude in deciduous broadleaf forests, but AIAV in GPP decreased significantly with latitude in evergreen needleleaf forests. Furthermore, RIAV in NEE, GPP, and Re appeared to increase significantly with latitude in deciduous broadleaf forests, but not in evergreen needleleaf forests. Correlation analyses showed air temperature was the primary environmental factor that determined RIAV of NEE in deciduous broadleaf forest across the North American sites, and none of the chosen climatic factors could explain RIAV of NEE in evergreen needleleaf forests. Mean annual NEE significantly increased with latitude in grasslands. Precipitation was dominant environmental factor for the spatial variation of magnitude and IAV in GPP and Re in grasslands.

Repeatability of nerve conduction measurements derived entirely by computer methods

BackgroundNerve conduction studies are an objective, quantitative, and reproducible measure of peripheral nerve function and are widely used in the diagnosis of neuropathies. The purpose of this study is to determine the reliability of nerve conduction parameters derived entirely from computer based data acquisition and waveform cursor assignments and to quantify the relative contributions of test variability sources.MethodsThirty volunteers, some with symptoms suggestive of neuropathies; of these, 29 completed the study. The median, ulnar, deep peroneal, posterior tibial, and sural nerves were evaluated bilaterally at two test sessions 3-7 days apart. Within each session, nerves were tested twice within 10 minutes. The analyzed nerve conduction parameters include motor latencies, motor conduction velocity (CV), compound muscle action potential (CMAP) amplitude, F-wave latencies (minimum, mean and maximum), sensory peak latency (DSL), sensory CV, and sensory nerve action potential (SNAP) amplitude. The primary outcome measure is variance component analysis and the corresponding coefficient of variation (CoV). The between-session-test variance is the sum of within-session variance and between-session variance, quantifying the total variation between test sessions. Additional statistical measures include the intraclass correlation coefficient (ICC) and relative interval variation (RIV).ResultsMotor and sensory latencies, CV and F-wave latency parameters have low between-session-test CoVs, ranging from 4.2% to 9.8%. Amplitude parameters have a higher between-session-test CoVs in the range of 15.6--19.8%. Between-test CoVs are about 30--80% lower than between-session CoVs with the exception of F-wave latency parameters. Between-test ICC values are 0.96 or above for all parameters. Between-session ICC ranges from 0.98 for F-wave latency to 0.77 for sural sensory CV. All latency-related between-session ICCs have a value 0.83 or above. The RIVs are the tightest for F-wave latency parameters and widest for CMAP amplitude parameters. Repeatability in a sub-group of subjects with more severe symptom grades follows the same trend as the overall study population without substantial quantitative differences.ConclusionThe study demonstrates the high repeatability of nerve conduction parameters acquired by modern electrodiagnostic instruments using computer based waveform cursor assignment. The reliability is comparable to benchmark studies in which the nerve conduction measurements were performed manually in controlled multi-center clinical trials. Furthermore, the ranking of reliability, whereby F-wave latencies have the best reproducibility and amplitudes the worst, is also consistent with the benchmark studies.

Diffusion of the Second Messengers in the Cytoplasm Acts as a Variability Suppressor of the Single Photon Response in Vertebrate Phototransduction

The single photon response in vertebrate phototransduction is highly reproducible despite a number of random components of the activation cascade, including the random activation site, the random walk of an activated receptor, and its quenching in a random number of steps. Here we use a previously generated and tested spatiotemporal mathematical and computational model to identify possible mechanisms of variability reduction. The model permits one to separate the process into modules, and to analyze their impact separately. We show that the activation cascade is responsible for generation of variability, whereas diffusion of the second messengers is responsible for its suppression. Randomness of the activation site contributes at early times to the coefficient of variation of the photoresponse, whereas the Brownian path of a photoisomerized rhodopsin (Rh*) has a negligible effect. The major driver of variability is the turnoff mechanism of Rh*, which occurs essentially within the first 2–4 phosphorylated states of Rh*. Theoretically increasing the number of steps to quenching does not significantly decrease the corresponding coefficient of variation of the effector, in agreement with the biochemical limitations on the phosphorylated states of the receptor. Diffusion of the second messengers in the cytosol acts as a suppressor of the variability generated by the activation cascade. Calcium feedback has a negligible regulatory effect on the photocurrent variability. A comparative variability analysis has been conducted for the phototransduction in mouse and salamander, including a study of the effects of their anatomical differences such as incisures and photoreceptors geometry on variability generation and suppression.

Circulating matrix metalloproteinase 9 levels in relation to sampling methods, femoral and carotid atherosclerosis

To examine whether circulating levels of matrix metalloproteinase 9 (MMP-9) were associated with ultrasound-assessed intima-media thickness (IMT) and echolucent plaques in the carotid and femoral arteries. To examine preanalytical sources of variability in MMP-9 concentrations related to sampling procedures. Plasma and serum MMP-9 levels were compared with ultrasound assessed measures of femoral and carotid atherosclerosis, in a cross-sectional study of 61-year-old men (n = 473). Preanalytical sources of variability in MMP-9 levels were examined in 10 healthy subjects. Main outcome measures were circulating levels of MMP-9 in serum and plasma, IMT of the carotid and femoral arteries, and plaque status based on size and echolucency. Research unit at university hospital. Plasma concentrations of total and active MMP-9 were associated with femoral artery IMT independently of traditional cardiovascular risk factors, and were higher in subjects with moderate to large femoral plaques. Plasma MMP-9 concentration was higher in men with echolucent femoral plaques (P = 0.006) compared with subjects without femoral plaques. No similar associations were found for carotid plaques. MMP-9 concentrations were higher in serum than in plasma, and higher when sampling was performed with Vacutainer than with syringe. MMP-9 levels in serum were more strongly associated with peripheral neutrophil count compared with MMP-9 levels in plasma. Plasma MMP-9 levels were associated with atherosclerosis in the femoral artery, and total MMP-9 concentration was higher in men with echolucent femoral plaques. The choice of sample material and sampling method affect the measurements of circulating MMP-9 levels.

Variability in settlement and recruitment of 0-group dab Limanda limanda L. in Port Erin Bay, Irish Sea

Inter-annual variability in settlement and recruitment of 0-group dab Limanda limanda L. was investigated between 1996 and 2003 on Port Erin Bay, Isle of Man. The annual settlement index of post larvae dab (ind ≤ 20 mm) varied considerably between years with a corresponding coefficient of variation of 97%. The timing of the settlement of dab was associated with metamorphosis and varied significantly with temporal variation in estimated spawning activity. Inter-annual variation in the numbers of settling post larvae was strongly associated with stage one larvae abundance indices from adjacent offshore waters. 0-group abundance indices were significantly correlated with the index of settlement abundance, but not however, with larvae indices. These relationships suggest that the strength of the recruiting year-class of 0-group dab in Port Erin Bay is established early in life, during or before the pelagic phase, in the waters west of the Isle of Man. Inter-annual variability in 0-group mortality rates suggests a possible density-dependent process operating during the nursery ground phase.

Characterization of pore-space heterogeneity in sandstone by X-ray computed tomography

Abstract In this study we review some of the recent advances in the application of X-ray computed tomography (X-ray CT) to geomaterials. This non-destructive technique based on density contrasts provides 2D images comparable with micrographs and 3D reconstructions at various resolutions depending on the acquisition setup. Synchrotron images with resolution of a few microns allow for 3D mapping of the pore space and can be used to perform 3D fluid flow simulations. ‘Industrial’ CT systems can provide images of centimetre-sized samples with a resolution of c . 50 µm. This resolution is suitable for studying centimetre-scale structural heterogeneities and compaction localization, as illustrated in recent studies. Starting from images taken at both synchrotron and industrial resolutions in intact samples of porous sandstones, we show that important conclusions on pore-space heterogeneity can be drawn from global and local analysis of the distributions of X-ray attenuation values. The global analysis is used in particular to identify relatively homogeneous materials in which compaction bands are likely to develop. Local analysis performed over small clusters of voxels appears to have more potential for the geometric description of the pore space. We show the existence of a resolution at which the local coefficient of variation of the X-ray distributions reaches a maximum. This resolution, which is about an order of magnitude lower than synchrotron resolution, appears to be related to the pore size, and the corresponding coefficient of variation to the porosity.

Evolution of genome size in fishes: a phylogenetic test of the Hinegardner and Rosen hypothesis

Despite remarkable advances in genomic studies over the past few decades, surprisingly little is known about the processes governing genome evolution at macroevolutionary timescales. In a seminal paper, Hinegardner and Rosen (Am Nat 106:621-644, 1972) suggested that taxa characterized by larger genomes should also display disproportionately stronger fluctuations in genome size. Therefore, according to the Hinegardner and Rosen (HR) hypothesis, there should be a negative correlation between average within-family genome size and its corresponding coefficient of variation (CV), a prediction that was supported by their analysis of the genomes of 275 species of fish. In this study we reevaluate the HR hypothesis using an expanded dataset (2050 genome size records). Moreover, in addition to the use of standard linear regression techniques, we also conducted modern comparative analyses that take into account phylogenetic non-independence. Our analyses failed to confirm the negative relationship detected in the original study, suggesting that the evolution of genome size in fishes might be more complex than envisioned by the HR hypothesis. Interestingly, the frequency distribution of fish genome sizes was strongly skewed, even on a logarithmic scale, suggesting that the dynamics underlying genome size evolution are driven by multiplicative phenomena, which might include chromosomal rearrangements and the expansion of transposable elements.

Quantifying Variability of Batching Equipment Using Effective Process Times

Process time variability plays a key role in the cycle time of wafers. Several sources of variability can be distinguished. However, identification and measurement of all individual sources is almost impossible. Therefore, in previous work, a new method has been proposed to measure effective process times (EPT) for single-lot machines. The EPT incorporates the various sources of variability. From the measured EPT realizations, the mean and the corresponding coefficient of variation can be computed for queueing performance analysis. This paper follows up on previous work. The EPT quantification approach is generalized toward batching equipment. The batching types of operations are commonly present in the semiconductor industry. The paper proposes a transformation algorithm that transforms lot events into batch events. This enables one to use the previously developed single-lot algorithm also for batch machine workstations. An industry case illustrates the approach using operational data of furnace workstations.

Possible Interethnic Differences in Omeprazole Pharmacokinetics

Considerable ethnic differences have been reported in the incidence of the poor metaboliser (PM) genotype of cytochrome P450 (CYP) 2C19. The frequency of this genotype was found to be much higher in Oriental persons (13-23%) than in American or European populations (3-5%). There are, however, no valid data published for Arabic subjects. The present study was conducted to evaluate pharmacokinetic parameters of omeprazole after a single dose in healthy Jordanian Arabic subjects and to compare the results with data published for other populations. Seventy-four healthy male Jordanian Arabic volunteers contributed to the study, which was performed at Al Essra Hospital in Amman, Jordan. After an overnight fast, omeprazole was administered as a single Losec 20mg capsule. A total of 20 blood samples were collected over a 10-hour period after administration. Omeprazole pharmacokinetic parameters were determined from the plasma concentration-time profiles using the WinNonlin software. Kolmogorov-Smirnov's test and probit plots of omeprazole area under the plasma concentration-time curve (AUC) data were used to analyse the frequency distribution of phenotypic data. The mean pharmacokinetic parameters and their corresponding coefficient of variation (CV%) for peak plasma concentration (Cmax), AUC from time zero to infinity (AUCinfinity), time to reach Cmax (tmax), apparent oral clearance (CL/F) and elimination half-life (t(1/2)) were 314.96 ng/mL (56%), 923.2 ng . h/mL (108.6%), 2.1h (44%), 0.66 L/h/kg (92%) and 1.5 h (56.6%), respectively. Interindividual differences in the current study were high for all pharmacokinetic parameters, yet comparable to CVs reported in nonphenotyped subjects identified within other ethnic groups (40.3-159% for AUC and 39-48.2% for Cmax). The frequency distribution of all parameters, particularly the AUC, was shown to be trimodal. This has proposed the presence of three distinct phenotypes, designated as extensive metabolisers (EMs), slow-extensive metabolisers (SEMs), and PMs, with corresponding frequency of 36.5%, 39.2% and 24.3%, respectively. After stratification, the relative mean AUCs of omeprazole in EMs, SEMs and PMs were 1 : 2.7 : 9.3 (all p < 0.001). Accordingly, the CL/F of omeprazole showed a ratio of 9.8 : 3.6 : 1 for three phenotype groups, respectively. For other pharmacokinetic parameters including Cmax, t1/2, AUC normalised for bodyweight (AUCN), Cmax/dose and AUC/dose, there were also significant differences between the three groups. The current pharmacokinetic study revealed that the majority of the Jordanian Arabics seemed to be more properly classified within the EM phenotype. More specifically, the observed metabolic rates of heterozygous and homozygous Jordanian Arabic EMs were more comparable to those of Caucasian EMs than Oriental EMs. Consequently, higher dosage requirements can be expected among most of the Jordanian Arabics. Yet, the incidence of PMs is significant and they seemed to exhibit a similar pharmacokinetic pattern to Chinese PMs in terms of long-term exposure (clearance and AUC) as well as short-term exposure (Cmax) parameters, after adjustment for dose and bodyweight. Therefore, further clinical application of CYP2C19 polymorphism is anticipated in Jordanian Arabic mixed population, particularly if long-term use of omeprazole is intended.

Contribution of Sampling to the Variability of Pesticide Residue Data

The uneven distribution of pesticide residues among the treated objects leads to an inevitable variability of pesticide residue levels measured in the samples, which may significantly contribute to the combined uncertainty of the analytical results. A total of 8844 unit-crop residue data derived from 57 lots and 19 field trials were evaluated to determine the characteristic features of residue distribution in unit crops and composite samples. The average residue levels and the corresponding coefficient of variation (CV) values obtained for individual units taken from a given lot showed wide variation from lot to lot. There was no significant difference between the CVs of residue levels in sample sets of various unit crops or composite sample populations of different sizes taken from various crops. The CV values for levels of residues taken from individual lots followed normal distribution. Very good correlation was found between the CVs of the parent and sample populations. The experimentally obtained values were very close to those expected on the basis of the central limit theorem. The estimated typical relative standard uncertainties of sampling medium-size crops for pesticide residue analysis in the cases of sample sizes of 5, 10, and 25 were 37, 25, and 16%, respectively.

Spatial and temporal variability of the daily rainfall regime in Catalonia (northeastern Spain), 1950–2000

AbstractSpatial and temporal patterns in the daily rainfall regime of Catalonia (northeastern Spain) recorded for the 1950–2000 period are analysed from several points of view, including the irregularity of the time series in terms of entropy, the Mann–Kendall test for time trends, a principal component analysis (PCA), an average linkage (AL) clustering algorithm and, finally, a power spectrum analysis, which includes a comparison of white‐noise and Markovian red‐noise hypotheses. The analyses are based on three monthly variables derived from the amounts recorded on a daily basis: the average daily rainfall and the standard deviation of the daily rainfall for each month, together with the corresponding coefficient of variation. The joint spatial–temporal variability is manifested by the irregularity index, which is characterized by relevant values in all cases and gradients from the north (Pyrenees and Pre‐Pyrenees mountain ranges) to the south (Ebro Valley) and to the Mediterranean coast. The interpretation of the factor scores derived from the PCA and of the clusters obtained from the AL algorithm also describes the complex spatial distribution of the daily rainfall regime, given that the effects of atmospheric circulation patterns on rainfall regimes are conditioned by the complex orography of Catalonia and its proximity to the Mediterranean Sea. The factor loadings associated with the PCA also suggest a distinction between hot, cold and mild seasons. Finally, it is worth noting that monthly series are usually accompanied by white background noise and, in a few cases, signs of Markovian behaviour and some significant periodicities, which are generally of less than 10 months and which change from one cluster to another. Copyright © 2004 Royal Meteorological Society

Contributions of pulmonary perfusion and ventilation to heterogeneity in V(A)/Q measured by PET.

To estimate the contributions of the heterogeneity in regional perfusion (Q) and alveolar ventilation (V A) to that of ventilation-perfusion ratio (V A/Q), we have refined positron emission tomography (PET) techniques to image local distributions of Q and V A per unit of gas volume content (sQ and sV A, respectively) and V A/Q in dogs. sV A was assessed in two ways: 1) the washout of 13NN tracer after equilibration by rebreathing (sV A(i)), and 2) the ratio of an apneic image after a bolus intravenous infusion of 13NN-saline solution to an image collected during a steady-state intravenous infusion of the same solution (sV A(p)). SV A(p) was systematically higher than sV A(i) in all animals, and there was a high spatial correlation between sQ and sV A(p) in both body positions (mean correlation was 0.69 prone and 0.81 supine) suggesting that ventilation to well-perfused units was higher than to those poorly perfused. In the prone position, the spatial distributions of sQ, sV A(p), and V A/Q were fairly uniform with no significant gravitational gradients; however, in the supine position, these variables were significantly more heterogeneous, mostly because of significant gravitational gradients (15, 5.5, and -10%/cm, respectively) accounting for 73, 33, and 66% of the corresponding coefficient of variation (CV)2 values. We conclude that, in the prone position, gravitational forces in blood and lung tissues are largely balanced out by dorsoventral differences in lung structure. In the supine position, effects of gravity and structure become additive, resulting in substantial gravitational gradients in sQ and sV A(p), with the higher heterogeneity in V A/Q caused by a gravitational gradient in sQ, only partially compensated by that in sV A.