Detecting statistical outliers in psychophysical data

This paper considers how best to identify statistical outliers in psychophysical datasets, where the underlying sampling distributions are unknown. Eight methods are described, and each is evaluated using Monte Carlo simulations of a typical psychophysical experiment. The best method is shown to be one based on a measure of absolute-deviation known as Sn. This method is shown to be more accurate than popular heuristics based on standard deviations from the mean, and more robust than non-parametric methods based on interquartile range. Matlab code for computing Sn is included

Severity of Visual Field Loss at First Presentation to Glaucoma Clinics in England and Tanzania

Purpose: To compare severity of visual field (VF) loss at first presentation in glaucoma clinics in England and Tanzania. Methods: Large archives of VF records from automated perimetry were used to retrospectively examine vision loss at first presentation in glaucoma clinics in Tanzania (N = 1,502) and England (N = 9,264). Mean deviation (MD) of the worse eye at the first hospital visit was used as an estimate of detectable VF loss severity. Results: In Tanzania, 44.7% {CI95%: 42.2, 47.2} of patients presented with severe VF loss (< −20 dB), versus 4.6% {4.1, 5.0} in England. If we consider late presentation to also include cases of advanced loss (-12.01 dB to -20 dB), then the proportion of patients presenting late was 58.1% {55.6, 60.6} and 14.0% {13.3, 14.7}, respectively. The proportion of late presentations was greater in Tanzania at all ages, but the difference was particularly pronounced among working-age adults, with 50.3% {46.9, 53.7} of 18–65-year-olds presenting with advanced or severe VF loss, versus 10.2% {9.3, 11.3} in England. In both countries, men were more likely to present late than women. Conclusions: Late presentation of glaucoma is a problem in England, and an even greater challenge in Tanzania. Possible solutions are discussed, including increased community eye-care, and a more proactive approach to case finding through the use of disruptive new technologies, such as low-cost, portable diagnostic aids

Can children measure their own vision? A comparison of three new contrast sensitivity tests

PURPOSE: To investigate the feasibility of children measuring their own contrast sensitivity using a range of tablet- and paper-based tests. METHODS: Forty children aged 5-15 years with amblyopia (N = 10), bilateral vision impairment (N = 10) or good vision (N = 20) measured their own vision on a screen-based optotype test (Manifold), a gamified vision test (PopCSF) and a paper-based test (Spotchecks) in a laboratory with minimal supervision. Completion rate, test-retest repeatability, test duration and participants' preferences were recorded for each test. RESULTS: Most participants (36/40) were able to perform all three tests. All tests were correlated with clinically measured visual acuity and contrast sensitivity (p < 0.001). The 95% coefficient of repeatability was 0.30 dB for Manifold, 0.29 dB for PopCSF and 0.13 dB for Spotchecks. All tests differentiated between children with reduced contrast sensitivity and control participants. PopCSF and Spotchecks were also able to differentiate between children with amblyopia and those with good vision. Median test time was 152, 130 and 202 s for Manifold, PopCSF and Spotchecks, respectively. Twenty-two participants preferred the PopCSF test, 10 preferred Spotchecks and 6 preferred Manifold. Thirty-nine out of the 40 children (98%) said they would measure their own vision at home using at least one of these tests every month. CONCLUSIONS: Children and young people can test their own contrast sensitivity with repeatable results. Of these three tests, the most repeatable was Spotchecks, the quickest was PopCSF and participants' favourite was PopCSF. Nearly all of the participants said they would be willing to use at least one of the three tests at home

Glaucoma Home Monitoring Using a Tablet- Based Visual Field Test (Eyecatcher): An Assessment of Accuracy and Adherence Over 6 Months

PURPOSE: To assess accuracy and adherence of visual field (VF) home monitoring in a pilot sample of patients with glaucoma. DESIGN: Prospective longitudinal feasibility and reliability study. METHODS: Twenty adults (median 71 years) with an established diagnosis of glaucoma were issued a tablet perimeter (Eyecatcher) and were asked to perform 1 VF home assessment per eye, per month, for 6 months (12 tests total). Before and after home monitoring, 2 VF assessments were performed in clinic using standard automated perimetry (4 tests total, per eye). RESULTS: All 20 participants could perform monthly home monitoring, though 1 participant stopped after 4 months (adherence: 98% of tests). There was good concordance between VFs measured at home and in the clinic (r = 0.94, P < .001). In 21 of 236 tests (9%), mean deviation deviated by more than ±3 dB from the median. Many of these anomalous tests could be identified by applying machine learning techniques to recordings from the tablets' front-facing camera (area under the receiver operating characteristic curve = 0.78). Adding home-monitoring data to 2 standard automated perimetry tests made 6 months apart reduced measurement error (between-test measurement variability) in 97% of eyes, with mean absolute error more than halving in 90% of eyes. Median test duration was 4.5 minutes (quartiles: 3.9-5.2 minutes). Substantial variations in ambient illumination had no observable effect on VF measurements (r = 0.07, P = .320). CONCLUSIONS: Home monitoring of VFs is viable for some patients and may provide clinically useful data

Acceptability of a home-based visual field test (Eyecatcher) for glaucoma home monitoring: a qualitative study of patients' views and experiences

Objectives To explore the acceptability of home visual field (VF) testing using Eyecatcher among people with glaucoma participating in a 6-month home monitoring pilot study. Design Qualitative study using face-to-face semistructured interviews. Transcripts were analysed using thematic analysis. Setting Participants were recruited in the UK through an advertisement in the International Glaucoma Association (now Glaucoma UK) newsletter. Participants Twenty adults (10 women; median age: 71 years) with a diagnosis of glaucoma were recruited (including open angle and normal tension glaucoma; mean deviation=2.5 to -29.9 dB). Results All participants could successfully perform VF testing at home. Interview data were coded into four overarching themes regarding experiences of undertaking VF home monitoring and attitudes towards its wider implementation in healthcare: (1) comparisons between Eyecatcher and Humphrey Field Analyser (HFA); (2) capability using Eyecatcher; (3) practicalities for effective wider scale implementation; (4) motivations for home monitoring. Conclusions Participants identified a broad range of benefits to VF home monitoring and discussed areas for service improvement. Eyecatcher was compared positively with conventional VF testing using HFA. Home monitoring may be acceptable to at least a subset of people with glaucoma

The model of mortality with incident cirrhosis (MoMIC) and the model of long-term outlook of mortality in dcirrhosis (LOMiC)

The purpose of this study was to produce two statistical survival models in those with cirrhosis utilising only routine parameters, including non-liver-related clinical factors that influence survival. The first model identified and utilised factors impacting short-term survival to 90-days post incident diagnosis, and a further model characterised factors that impacted survival following this acute phase. Data were from the Clinical Practice Research Datalink linked with Hospital Episode Statistics. Incident cases in patients ≥18 years were identified between 1998 and 2014. Patients that had prior history of cancer or had received liver transplants prior were excluded. Model-1 used a logistic regression model to predict mortality. Model-2 used data from those patients who survived 90 days, and used an extension of the Cox regression model, adjusting for time-dependent covariables. At 90 days, 23% of patients had died. Overall median survival was 3.7 years. Model-1: numerous predictors, prior comorbidities and decompensating events were incorporated. All comorbidities contributed to increased odds of death, with renal disease having the largest adjusted odds ratio (OR = 3.35, 95%CI 2.97–3.77). Model-2: covariables included cumulative admissions for liver disease-related events and admissions for infections. Significant covariates were renal disease (adjusted hazard ratio (HR = 2.89, 2.47–3.38)), elevated bilirubin levels (aHR = 1.38, 1.26–1.51) and low sodium levels (aHR = 2.26, 1.84–2.78). An internal validation demonstrated reliability of both models. In conclusion: two survival models that included parameters commonly recorded in routine clinical practice were generated that reliably forecast the risk of death in patients with cirrhosis: in the acute, post diagnosis phase, and following this critical, 90 day phase. This has implications for practice and helps better forecast the risk of mortality from cirrhosis using routinely recorded parameters without inputs from specialists

The Human Touch:Using a Webcam to Autonomously Monitor Compliance During Visual Field Assessments

Purpose: To explore the feasibility of using various easy-to-obtain biomarkers to monitor non-compliance (measurement error) during visual field assessments. Methods: Forty-two healthy adults (42 eyes) and seven glaucoma patients (14 eyes) underwent two same-day visual field assessments. An ordinary webcam was used to compute seven potential biomarkers of task compliance, based primarily on eye gaze, head pose, and facial expression. We quantified the association between each biomarker and measurement error, as defined by (1) test-retest differences in overall test scores (mean sensitivity), and (2) failures to respond to visible stimuli on individual trials (stimuli -3 dB or more brighter than threshold). Results: In healthy eyes, three of the seven biomarkers were significantly associated with overall (test-retest) measurement error (P = 0.003-0.007), and at least two others exhibited possible trends (P = 0.052-0.060). The weighted linear sum of all seven biomarkers was associated with overall measurement error, in both healthy eyes (r = 0.51, P <0.001) and patients (r = 0.65, P <0.001). Five biomarkers were each associated with failures to respond to visible stimuli on individual trials (all P <0.001). Conclusions: Inexpensive, autonomous measures of task compliance are associated with measurement error in visual field assessments, in terms of both the overall reliability of a test and failures to respond on particular trials ("lapses"). This could be helpful for identifying low-quality assessments and for improving assessment techniques (e.g., by discounting suspect responses or by automatically triggering comfort breaks or encouragement). Translational Relevance: This study explores a potential way of improving the reliability of visual field assessments, a crucial but notoriously unreliable clinical measure

Refinement and preliminary evaluation of two tablet-based tests of real-world visual function

PurposeTo describe, refine, evaluate, and provide normative control data for two freely available tablet-based tests of real-world visual function, using a cohort of young, normally-sighted adults. MethodsFifty young (18-40 years), normally-sighted adults completed tablet-based assessments of (1) face discrimination and (2) visual search. Each test was performed twice, to assess test-retest repeatability. Post-hoc analyses were performed to determine the number of trials required to obtain stable estimates of performance. Distributions were fitted to the normative data to determine the 99% population-boundary for normally sighted observers. Participants were also asked to rate their comprehension of each test.ResultsBoth tests provided stable estimates in around 20 trials (~1-4 min), with only a further reduction of 14%-17% in the 95% Coefficient of Repeatability (CoR95) when an additional 40 trials were included. When using only ~20 trials: median durations for the first run of each test were 191 s (Faces) and 51 s (Search); test-retest CoR95 were 0.27 d (Faces) and 0.84 s (Search); and normative 99% population-limits were 3.50 d (Faces) and 3.1 s (Search). No participants exhibited any difficulties completing either test (100% completion rate), and ratings of task-understanding were high (Faces: 9.6 out of 10; Search: 9.7 out of 10).ConclusionsThis preliminary assessment indicated that both tablet-based tests are able to provide simple, quick, and easy-to-administer measures of real-world visual function in normally-sighted young adults. Further work is required to assess their accuracy and utility in older people and individuals with visual impairment. Potential applications are discussed, including their use in clinic waiting rooms, and as an objective complement to Patient Reported Outcome Measures (PROMs).</div

Contrast Sensitivity and Night Driving in Older People: Quantifying the Relationship Between Visual Acuity, Contrast Sensitivity, and Hazard Detection Distance in a Night-Time Driving Simulator

Purpose(i) To assess how well contrast sensitivity (CS) predicts night-time hazard detection distance (a key component of night driving ability), in normally sighted older drivers, relative to a conventional measure of high contrast visual acuity (VA); (ii) To evaluate whether CS can be accurately quantified within a night driving simulator.Materials and MethodsParticipants were 15 (five female) ophthalmologically healthy adults, aged 55–81 years. CS was measured in a driving simulator using Landolt Cs, presented under static or dynamic driving conditions, and with or without glare. In the dynamic driving conditions, the participant was asked to simultaneously maintain a (virtual) speed of 60 km/h on a country road. In the with glare conditions, two calibrated LED arrays, moved by cable robots, simulated the trajectories and luminance characteristics of the (low beam) headlights of an approaching car. For comparison, CS was also measured clinically (with and without glare) using a Optovist I instrument (Vistec Inc., Olching, Germany). Visual acuity (VA) thresholds were also assessed at high and low contrast using the Freiburg Visual Acuity Test (FrACT) under photopic conditions. As a measure of driving performance, median hazard detection distance (MHDD) was computed, in meters, across three kinds of simulated obstacles of varying contrast.ResultsContrast sensitivity and low contrast VA were both significantly associated with driving performance (both P &lt; 0.01), whereas conventional high contrast acuity was not (P = 0.10). There was good correlation (P &lt; 0.01) between CS measured in the driving simulator and a conventional clinical instrument (Optovist I). As expected, CS was shown to decrease in the presence of glare, in dynamic driving conditions, and as a function of age (all P &lt; 0.01).ConclusionContrast sensitivity and low contrast VA predict night-time hazard detection ability in a manner that conventional high contrast VA does not. Either may therefore provide a useful metric for assessing fitness to drive at night, particularly in older individuals. CS measurements can be made within a driving simulator, and the data are in good agreement with conventional clinical methods (Optovist I)

The Human Touch: Using a Webcam to Autonomously Monitor Compliance During Visual Field Assessments

Purpose: To explore the feasibility of using various easy-to-obtain biomarkers to monitor non-compliance (measurement error) during visual field assessments.Methods: Forty-two healthy adults (42 eyes) and seven glaucoma patients (14 eyes) underwent two same-day visual field assessments. An ordinary webcam was used to compute seven potential biomarkers of task compliance, based primarily on eye gaze, head pose, and facial expression. We quantified the association between each biomarker and measurement error, as defined by (1) test-retest differences in overall test scores (mean sensitivity), and (2) failures to respond to visible stimuli on individual trials (stimuli -3 dB or more brighter than threshold).Results: In healthy eyes, three of the seven biomarkers were significantly associated with overall (test-retest) measurement error (P = 0.003-0.007), and at least two others exhibited possible trends (P = 0.052-0.060). The weighted linear sum of all seven biomarkers was associated with overall measurement error, in both healthy eyes (r = 0.51, P < 0.001) and patients (r = 0.65, P < 0.001). Five biomarkers were each associated with failures to respond to visible stimuli on individual trials (all P < 0.001).Conclusions: Inexpensive, autonomous measures of task compliance are associated with measurement error in visual field assessments, in terms of both the overall reliability of a test and failures to respond on particular trials ("lapses"). This could be helpful for identifying low-quality assessments and for improving assessment techniques (e.g., by discounting suspect responses or by automatically triggering comfort breaks or encouragement).Translational Relevance: This study explores a potential way of improving the reliability of visual field assessments, a crucial but notoriously unreliable clinical measure