A population-based study of visual impairment among pre-school children in Beijing: the Beijing study of visual impairment in children.

PURPOSE: To evaluate the prevalence and causes of visual impairment among Chinese children aged 3 to 6 years in Beijing. DESIGN: Population-based prevalence survey. METHODS: Presenting and pinhole visual acuity were tested using picture optotypes or, in children with pinhole vision < 6/18, a Snellen tumbling E chart. Comprehensive eye examinations and cycloplegic refraction were carried out for children with pinhole vision < 6/18 in the better-seeing eye. RESULTS: All examinations were completed on 17,699 children aged 3 to 6 years (95.3% of sample). Subjects with bilateral correctable low vision (presenting vision < 6/18 correctable to >or= 6/18) numbered 57 (0.322%; 95% confidence interval [CI], 0.237% to 0.403%), while 14 (0.079%; 95% CI, 0.038% to 0.120%) had bilateral uncorrectable low vision (best-corrected vision of < 6/18 and >or= 3/60), and 5 subjects (0.028%; 95% CI, 0.004% to 0.054%) were bilaterally blind (best-corrected acuity < 3/60). The etiology of 76 cases of visual impairment included: refractive error in 57 children (75%), hereditary factors (microphthalmos, congenital cataract, congenital motor nystagmus, albinism, and optic nerve disease) in 13 children (17.1 %), amblyopia in 3 children (3.95%), and cortical blindness in 1 child (1.3%). The cause of visual impairment could not be established in 2 (2.63%) children. The prevalence of visual impairment did not differ by gender, but correctable low vision was significantly (P < .0001) more common among urban as compared with rural children. CONCLUSION: The leading causes of visual impairment among Chinese preschool-aged children are refractive error and hereditary eye diseases. A higher prevalence of refractive error is already present among urban as compared with rural children in this preschool population.

Myopia, spectacle wear, and risk of bicycle accidents among rural Chinese secondary school students: the Xichang Pediatric Refractive Error Study report no. 7.

OBJECTIVE: To study the effect of myopia and spectacle wear on bicycle-related injuries in rural Chinese students. Myopia is common among Chinese students but few studies have examined its effect on daily activities. METHODS: Data on visual acuity, refractive error, current spectacle wear, and history of bicycle use and accidents during the past 3 years were sought from 1891 students undergoing eye examinations in rural Guangdong province. RESULTS: Refractive and accident data were available for 1539 participants (81.3%), among whom the mean age was 14.6 years, 52.5% were girls, 26.8% wore glasses, and 12.9% had myopia of less than -4 diopters in both eyes. More than 90% relied on bicycles to get to school daily. A total of 2931 accidents were reported by 423 participants, with 68 requiring medical attention. Male sex (odds ratio, 1.55; P < .001) and spectacle wear (odds ratio, 1.38; P = .04) were associated with a higher risk of accident, but habitual visual acuity and myopia were unassociated with the crash risk, after adjusting for age, sex, time spent riding, and risky riding behaviors. CONCLUSION: These results may be consistent with data on motor vehicle accidents implicating peripheral vision (potentially compromised by spectacle wear) more strongly than central visual acuity in mediating crash risk.

Corneal biomechanics, refractive error, and axial length in Chinese primary school children.

PURPOSE: Low corneal hysteresis is associated with longer axial length in Chinese secondary school children. The authors sought to explore this association in primary school children. METHODS: LogMAR presenting visual acuity, cycloplegic refractive error, ocular biometry, central corneal thickness (CCT), and corneal hysteresis (CH) was assessed for children in grades 1 to 3 at an academically competitive urban school in Shantou, China. RESULTS: Among 872 eligible children (mean age, 8.6 ± 2.1 years), 651 (74.7%) completed the examination. Among 1299 examined eyes, 111 (8.5%) had uncorrected vision ≤6/12. Mean spherical equivalent refractive error for all eyes was +0.26 ± 1.41 D, and axial length (AL) was 22.7 ± 0.90 mm. CH for the lowest (mean AL, 21.7 ± 0.39 mm), two middle (mean AL, 22.4 ± 0.15 and 22.9 ± 0.15 mm), and highest quartiles (mean AL, 23.7 ± 0.74 mm) of AL were 10.6 ± 2.1 mm Hg, 10.4 ± 2.1 mm Hg, 10.3 ± 2.3 mm Hg, and 10.2 ± 2.3 mm Hg respectively (age- and gender-adjusted Pearson's correlation coefficient r = -0.052; P = 0.001). In generalized estimating equation models adjusting for age, gender, and CCT, lower CH was significantly associated with longer AL (P < 0.001) and more myopic refractive error (P = 0.001). CONCLUSIONS: CH measurement is practical in young children because this is when myopia undergoes its most rapid progression. Prospective follow-up of this cohort at high risk for myopia is under way to determine whether low CH is predictive, or a consequence, of long AL.

Self correction of refractive error among young people in rural China: results of cross sectional investigation.

OBJECTIVE: To compare outcomes between adjustable spectacles and conventional methods for refraction in young people. DESIGN: Cross sectional study. SETTING: Rural southern China. PARTICIPANTS: 648 young people aged 12-18 (mean 14.9 (SD 0.98)), with uncorrected visual acuity ≤ 6/12 in either eye. INTERVENTIONS: All participants underwent self refraction without cycloplegia (paralysis of near focusing ability with topical eye drops), automated refraction without cycloplegia, and subjective refraction by an ophthalmologist with cycloplegia. MAIN OUTCOME MEASURES: Uncorrected and corrected vision, improvement of vision (lines on a chart), and refractive error. RESULTS: Among the participants, 59% (384) were girls, 44% (288) wore spectacles, and 61% (393/648) had 2.00 dioptres or more of myopia in the right eye. All completed self refraction. The proportion with visual acuity ≥ 6/7.5 in the better eye was 5.2% (95% confidence interval 3.6% to 6.9%) for uncorrected vision, 30.2% (25.7% to 34.8%) for currently worn spectacles, 96.9% (95.5% to 98.3%) for self refraction, 98.4% (97.4% to 99.5%) for automated refraction, and 99.1% (98.3% to 99.9%) for subjective refraction (P = 0.033 for self refraction v automated refraction, P = 0.001 for self refraction v subjective refraction). Improvements over uncorrected vision in the better eye with self refraction and subjective refraction were within one line on the eye chart in 98% of participants. In logistic regression models, failure to achieve maximum recorded visual acuity of 6/7.5 in right eyes with self refraction was associated with greater absolute value of myopia/hyperopia (P<0.001), greater astigmatism (P = 0.001), and not having previously worn spectacles (P = 0.002), but not age or sex. Significant inaccuracies in power (≥ 1.00 dioptre) were less common in right eyes with self refraction than with automated refraction (5% v 11%, P<0.001). CONCLUSIONS: Though visual acuity was slightly worse with self refraction than automated or subjective refraction, acuity was excellent in nearly all these young people with inadequately corrected refractive error at baseline. Inaccurate power was less common with self refraction than automated refraction. Self refraction could decrease the requirement for scarce trained personnel, expensive devices, and cycloplegia in children's vision programmes in rural China.

School-based approaches to the correction of refractive error in children.

The World Health Organization estimates that 13 million children aged 5-15 years worldwide are visually impaired from uncorrected refractive error. School vision screening programs can identify and treat or refer children with refractive error. We concentrate on the findings of various screening studies and attempt to identify key factors in the success and sustainability of such programs in the developing world. We reviewed original and review articles describing children's vision and refractive error screening programs published in English and listed in PubMed, Medline OVID, Google Scholar, and Oxford University Electronic Resources databases. Data were abstracted on study objective, design, setting, participants, and outcomes, including accuracy of screening, quality of refractive services, barriers to uptake, impact on quality of life, and cost-effectiveness of programs. Inadequately corrected refractive error is an important global cause of visual impairment in childhood. School-based vision screening carried out by teachers and other ancillary personnel may be an effective means of detecting affected children and improving their visual function with spectacles. The need for services and potential impact of school-based programs varies widely between areas, depending on prevalence of refractive error and competing conditions and rates of school attendance. Barriers to acceptance of services include the cost and quality of available refractive care and mistaken beliefs that glasses will harm children's eyes. Further research is needed in areas such as the cost-effectiveness of different screening approaches and impact of education to promote acceptance of spectacle-wear. School vision programs should be integrated into comprehensive efforts to promote healthy children and their families.

Heterogeneous error-resilient scheme for spectral analysis in ultra-low power wearable electrocardiogram devices

Wearable devices performing advanced bio-signal analysis algorithms are aimed to foster a revolution in healthcare provision of chronic cardiac diseases. In this context, energy efficiency is of paramount importance, as long-term monitoring must be ensured while relying on a tiny power source. Operating at a scaled supply voltage, just above the threshold voltage, effectively helps in saving substantial energy, but it makes circuits, and especially memories, more prone to errors, threatening the correct execution of algorithms. The use of error detection and correction codes may help to protect the entire memory content, however it incurs in large area and energy overheads which may not be compatible with the tight energy budgets of wearable systems. To cope with this challenge, in this paper we propose to limit the overhead of traditional schemes by selectively detecting and correcting errors only in data highly impacting the end-to-end quality of service of ultra-low power wearable electrocardiogram (ECG) devices. This partition adopts the protection of either significant words or significant bits of each data element, according to the application characteristics (statistical properties of the data in the application buffers), and its impact in determining the output. The proposed heterogeneous error protection scheme in real ECG signals allows substantial energy savings (11% in wearable devices) compared to state-of-the-art approaches, like ECC, in which the whole memory is protected against errors. At the same time, it also results in negligible output quality degradation in the evaluated power spectrum analysis application of ECG signals.

Selective Multi-Carrier Index Keying OFDM: Error Propagation Rate with Moment Generating Function

We propose a new selective multi-carrier index keying in orthogonal frequency division multiplexing (OFDM) systems that opportunistically modulate both a small subset of sub-carriers and their indices. Particularly, we investigate the performance enhancement in two cases of error propagation sensitive and compromised deviceto-device (D2D) communications. For the performance evaluation, we focus on analyzing the error propagation probability (EPP) introducing the exact and upper bound expressions on the detection error probability, in the presence of both imperfect and perfect detection of active multi-carrier indices. The average EPP results in closedform are generalized for various fading distribution using the moment generating function, and our numerical results clearly show that the proposed approach is desirable for reliable and energy-efficient D2D applications.

Challenges in mercury speciation and fractionation in soil and sediment

This investigation focused on the development, test and validation of methodologies for mercury fractionation and speciation in soil and sediment. After an exhaustive review of the literature, several methods were chosen and tested in well characterised soil and sediment samples. Sequential extraction procedures that divide mercury fractions according to their mobility and potential availability in the environment were investigated. The efficiency of different solvents for fractionation of mercury was evaluated, as well as the adequacy of different analytical instruments for quantification of mercury in the extracts. Kinetic experiments to establish the equilibrium time for mercury release from soil or sediment were also performed. It was found that in the studied areas, only a very small percentage of mercury is present as mobile species and that mobility is associated to higher aluminium and manganese contents, and that high contents of organic matter and sulfur result in mercury tightly bound to the matrix. Sandy soils tend to release mercury faster that clayey soils, and therefore, texture of soil or sediment has a strong influence on the mobility of mercury. It was also understood that analytical techniques for quantification of mercury need to be further developed, with lower quantification limits, particularly for mercury quantification of less concentrated fractions: water-soluble e exchangeable. Although the results provided a better understanding of the distribution of mercury in the sample, the complexity of the procedure limits its applicability and robustness. A proficiency-testing scheme targeting total mercury determination in soil, sediment, fish and human hair was organised in order to evaluate the consistency of results obtained by different laboratories, applying their routine methods to the same test samples. Additionally, single extractions by 1 mol L-1 ammonium acetate solution, 0.1 mol L-1 HCl and 0.1 mol L-1 CaCl2, as well as extraction of the organometallic fraction were proposed for soil; the last was also suggested for sediment and fish. This study was important to update the knowledge on analytical techniques that are being used for mercury quantification, the associated problems and sources of error, and to improve and standardize mercury extraction techniques, as well as to implement effective strategies for quality control in mercury determination. A different, “non chemical-like” method for mercury species identification was developed, optimised and validated, based on the thermo-desorption of the different mercury species. Compared to conventional extraction procedures, this method has advantages: it requires little to no sample treatment; a complete identification of species present is obtained in less than two hours; mercury losses are almost neglectable; can be considered “clean”, as no residues are produced; the worldwide comparison of results obtained is easier and reliable, an important step towards the validation of the method. Therefore, the main deliverables of this PhD thesis are an improved knowledge on analytical procedures for identification and quantification of mercury species in soils and sediments, as well as a better understanding of the factors controlling the behaviour of mercury in these matrices.

Rate adaptation for wireless video streaming based on error statistics

This paper presents a new rate-control algorithm for live video streaming over wireless IP networks, which is based on selective frame discarding. In the proposed mechanism excess 'P' frames are dropped from the output queue at the sender using a congestion estimate based on packet loss statistics obtained from RTCP feedback and from the Data Link (DL) layer. The performance of the algorithm is evaluated through computer simulation. This paper also presents a characterisation of packet losses owing to transmission errors and congestion, which can help in choosing appropriate strategies to maximise the video quality experienced by the end user. Copyright © 2007 Inderscience Enterprises Ltd.

Evaluation of the H.264 scalable video coding in error prone IP networks

The Joint Video Team, composed by the ISO/IEC Moving Picture Experts Group (MPEG) and the ITU-T Video Coding Experts Group (VCEG), has standardized a scalable extension of the H.264/AVC video coding standard called Scalable Video Coding (SVC). H.264/SVC provides scalable video streams which are composed by a base layer and one or more enhancement layers. Enhancement layers may improve the temporal, the spatial or the signal-to-noise ratio resolutions of the content represented by the lower layers. One of the applications, of this standard is related to video transmission in both wired and wireless communication systems, and it is therefore important to analyze in which way packet losses contribute to the degradation of quality, and which mechanisms could be used to improve that quality. This paper provides an analysis and evaluation of H.264/SVC in error prone environments, quantifying the degradation caused by packet losses in the decoded video. It also proposes and analyzes the consequences of QoS-based discarding of packets through different marking solutions.

The Effect of Changes to the Method of Estimating the Pollen Count from Aerobiological Samples

Pollen data have been recorded at Novi Sad in Serbia since 2000. The adopted method of producing pollen counts has been the use of five longitudinal transects that examine 19.64% of total sample surface. However, counting five transects is time consuming and so the main objective of this study is to investigate whether reducing the number to three or even two transects would have a significant effect on daily average and bi-hourly pollen concentrations, as well as the main characteristics of the pollen season and long-term trends. This study has shown that there is a loss of accuracy in daily average and bi-hourly pollen concentrations (an increase in % ERROR) as the sub-sampling area is reduced from five to three or two longitudinal transects. However, this loss of accuracy does not impact on the main characteristics of the season or long-term trends. As a result, this study can be used to justify changing the sub-sampling method used at Novi Sad from five to three longitudinal transects. The use of two longitudinal transects has been ruled out because, although quicker, the counts produced: (a) had the greatest amount of % ERROR, (b) altered the amount of influence of the independent variable on the dependent variable (the slope in regression analysis) and (c) the total sampled surface (7.86%) was less than the minimum requirement recommended by the European Aerobiology Society working group on Quality Control (at least 10% of total slide area).

On the structure and performance of a novel blind source separation based carrier phase synchronization error compensator

In this paper we carry out a detailed performance analysis of a novel blind-source-seperation (BSS) based DSP algorithm that tackles the carrier phase synchronization error problem. The results indicate that the mismatch can be effectively compensated during the normal operation as well as in the rapidly changing environments. Since the compensation is carried out before any modulation specific processing, the proposed method works with all standard modulation formats and lends itself to efficient real-time custom integrated hardware or software implementations.

Essays on Applications of the Factor Model

Thesis (Ph.D.)--University of Washington, 2013

The Impact of Uncertainty Shocks under Measurement Error: A Proxy SVAR approach

A growing literature considers the impact of uncertainty using SVAR models that include proxies for uncertainty shocks as endogenous variables. In this paper we consider the impact of measurement error in these proxies on the estimated impulse responses. We show via a Monte-Carlo experiment that measurement error can result in attenuation bias in impulse responses. In contrast, the proxy SVAR that uses the uncertainty shock proxy as an instrument does not su¤er from this bias. Applying this latter method to the Bloom (2009) data-set results in impulse responses to uncertainty shocks that are larger in magnitude and more persistent than those obtained from a recursive SVAR.