R. Clifford Blair

A computerized method of visual acuity testing: adaptation of the early treatment of diabetic retinopathy study testing protocol.

PURPOSE To develop a computerized method of visual acuity testing for clinical research as an alternative to the standard Early Treatment for Diabetic Retinopathy Study (ETDRS) testing protocol, and to evaluate its test-retest reliability and concordance with standard ETDRS testing. DESIGN Test-retest reliability study. METHODS Multicenter setting of a study population of 265 patients at three clinical sites. Visual acuity was measured with both the electronic visual acuity testing algorithm (E-ETDRS) and standard ETDRS protocol (S-ETDRS) twice on one eye of each patient. E-ETDRS testing was conducted using the electronic visual acuity tester (EVA), which utilizes a programmed Palm (Palm, Inc, Santa Clara, California, USA) hand-held device communicating with a personal computer and 17-inch monitor at a test distance of 3 meters. RESULTS For the E-ETDRS protocol, test-retest reliability was high (r = 0.99; with 89% and 98% of retests within 0.1 logMAR and 0.2 logMAR of initial tests, respectively) and comparable with that of S-ETDRS testing (r = 0.99; with 87% and 98% of retests within 0.1 logMAR and 0.2 logMAR of initial test, respectively). The E-ETDRS and S-ETDRS scores were highly correlated (r = 0.96 for initial tests and r = 0.97 for repeat tests). Based on estimates of 95% confidence intervals, a change in visual acuity of 0.2 logMAR (10 letters) from a baseline level is unlikely to be related to measurement variability using either the E-ETDRS or the S-ETDRS visual acuity testing protocol. CONCLUSIONS The E-ETDRS protocol has high test-retest reliability and good concordance with S-ETDRS testing. The computerized method has advantages over the S-ETDRS testing in electronically capturing the data for each tested letter, requiring only a single distance for testing from 20/12 to 20/800, potentially reducing testing time, and potentially decreasing technician-related bias.

A more realistic look at the robustness and type II error properties of the t test to departures from population normality

The Type I and II error properties of the t test were evaluated by means of a Monte Carlo study that sampled 8 real distribution shapes identified by Micceri (1986, 1989) as being representative of types encountered in psychology and education research. Results showed the independent-samples t tests to be reasonably robust to Type I error when (a) sample sizes are equal, (b) sample sizes are fairly large, and (c) tests are two-tailed rather than one-tailed. Nonrobust results were obtained primarily under distributions with extreme skew. The / test was robust to Type II error under these nonnormal distributions, but researchers should not overlook robust nonparametric competitors that are often more powerful than the t test when its underlying assumptions are violated. Along with Pearsons chi-squared test, the independent-samples t test must be counted among the best-known statistical procedures in current use. Given its familiarity and utility, it is not surprising that over the years, this test has received an inor

An exact statistical method for comparing topographic maps, with any number of subjects and electrodes

SummaryStatistical methods for testing differences between neural images (e.g., PET, MRI or EEG maps) are problematic because they require (1) an untenable assumption of data sphericity and (2) a high subject to electrode ratio. We propose and demonstrate an exact and distribution-free method of significance testing which avoids the sphericity assumption and may be computed for any combination of electrode and subject numbers. While this procedure is rigorously rooted in permutation test theory, it is intuitively comprehensible. The sensitivity of the permutation test to graded changes in dipole location for systematically varying levels of signal/noise ratio, intersubject variability and number of subjects was demonstrated through a simulation of 70 different conditions, generating 5,000 different data sets for each condition. Data sets were simulated from a homogenous single-shell dipole model. For noise levels commonly encountered in evoked potential studies and for situations where the number of subjects was less than the number of electrodes, the permutation test was very sensitive to a change in dipole location of less than 0.75 cm. This method is especially sensitive to localized changes that would be “washed-out‘ by more traditional methods of analysis. It is superior to all previous methods of statistical analysis for comparing topographical maps, because the test is exact, there is no assumption of a multivariate normal distribution or of the correlation structure of the data requiring correction, the test can be tailored to the specific experimental hypotheses rather than allowing the statistical tests to limit the experimental design, and there is no limitation on the number of electrodes that can be simultaneously analyzed.

Limitations of the rank transform statistic in tests for interactions

Conover and Iman (1976), Iman (1974), and Iman and Conover (1976) have found the rank transform test to be robust and powerful when testing for interaction in experimental designs.The current study shows that, insofar as tests for interactions are concerned, the rank transform test is robust and powerful in some circumstances but is dramatically nonrobust and manifests power significantly below that of the usual F test in some cases. Therefore, this procedure should be used only withcaution when employed in designs suchas those examined here.

Topographical and temporal stability of the P300.

Sixteen normal adults listened to a standard oddball auditory paradigm 3 times, each time separated by 15 min, and this protocol was then repeated 1-2 months later. Stability of the wave forms was measured between 250 and 500 msec for each subject and for each of 16 electrodes by the cross-correlation technique described by Glaser and Ruchkin (1976) and Gevins (1987) in which points on 2 digitized wave forms are paired to produce a correlation coefficient. For each subject, the correlation coefficient was generated for each electrode and then averaged across the 2 parietal and 2 central electrodes to produce a single stability measure for the central electrodes, while the cross-correlation coefficients for the 12 remaining electrodes were averaged for a peripheral stability measure. Three-way repeated measures ANOVAs were performed to determine the significance of cross-correlation coefficient differences. The stability over 15 min for the central electrodes was 0.80, indicating that the P300 was very stable over a short time. The peripheral electrodes were significantly less stable than the central electrodes (P = 0.001). The stability of the wave forms was virtually unchanged when assessed over 1 month (P = 0.9). The target wave forms were significantly more stable than the difference waves (target minus non-target) for both the central and peripheral electrodes (P = 0.04 and 0.01). When the 3 blocks within each session were averaged, there was a significant increase in stability (P less than 0.0005). The wave form cross-correlation coefficients can be used as a measure of the stability of a topographical map over time. The map of the target P300 wave form is very stable, showing no loss in stability from 15 min to 1 month, is more stable than the difference wave form and significantly increases in stability when separate trial blocks are averaged together.

An Investigation of the Type I Error and Power Properties of the Rank Transform Procedure in Factorial ANOVA

This study examined the Type I error and power properties of the rank transform test when employed in the context of a balanced 2x2x2 fixed effects ANOVA. The results showed the rank transform procedure to be erratic with respect to both Type I error and power. Under some circumstances the test was both robust and powerful, whereas in other circumstances it was decidedly nonrobust and manifested power considerably below that of the usual ANOVA F test. It is recommended that researchers avoid this test except in those specific circumstances where its properties are well understood.

A Comparison of the Power of Wilcoxon's Rank-Sum Statistic to that of Student'st Statistic Under Various Nonnormal Distributions

Computer generated Monte Carlo techniques were used to compare the power of Wilcoxons rank-sum test to the power of the two independent meanst test for situations in which samples were drawn from (1) uniform, (2) Laplace, (3) half-normal, (4) exponential, (5) mixed-normal, and (6) mixed-uniform distributions. Sample sizes studied were (n 1,n 2) = (3,9), (6,6), (9,27), (18,18), (27,81), and (54,54). It was concluded that (1) generally speaking, the Wilcoxon statistic held very large power advantages over thet statistic, (2) asymptotic relative efficiencies were reasonably good indicators of the relative power of the two statistics, (3) results obtained from smaller samples were often markedly different from the results obtained from larger samples, and (4) because of the narrow ranges of population shapes and sample sizes investigated in some widely cited previous studies of this type, the conclusions reached in those studies must now be deemed questionable.

An Investigation of the Robustness of the t Test to Unit of Analysis Violations

In the first part of this study, sampling experiments were used to assess the Type I error rates of the t test in situations where classes or schools were randomly assigned to groups but analyses were carried out on individual student scores. It was found that Type I error rates were grossly inflated under all conditions investigated. In the second part of the study, mathematical analysis was used to demonstrate the fact that even small amounts of between class (or school) variation can cause large inflations in the Type I error rate of the t test when analyses are incorrectly carried out on individual student scores.

The late somatosensory evoked potential in premature and term infants. II. Topography and latency development

The maturation of latency and scalp voltage topography of the simultaneously bilateral somatosensory evoked potential was studied in 53 neurologically intact pre-term and term infants, from 31 to 40 weeks post-menstrual age. Four peaks (N1, P1, N2 and P2) were reliably identified in all infants. The latency of each peak decreased as the infants matured. Each peak had a unique voltage scalp topography that remained stable as infants matured, even though the maps changed in amplitude intensity. N2 was large, easily identifiable with a central peak, and extremely stable in topography, suggesting that it might be used to evaluate the functional status of the somatosensory cortex in pre-term and term infants who are at high risk for developing intracranial hemorrhage leading to abnormalities of tone and delays in motor development.

Psychiatric Issues in Bariatric Surgery

In this study, 100 patients were evaluated prior to surgery to assess psychiatric status. The demographic anthropometric and psychological characteristics are described. Seventeen patients developed severe psychiatric complications and required hospitalization; the diagnoses which precipitated hospital admission were most commonly affective disorders (especially major depression with suicidal ideation). The only deaths in the sample of 100 occurred among the patients who required post-surgical psychiatric hospitalization. The hospitalized group was compared to a matched group drawn from the original 100 patients. Factors associated with post-surgical psychiatric hospitalization were: presurgical psychiatric hospitalization, presence of multiple pre-surgical Axis I psychiatric diagnoses, and untreated Axis I diagnosis at the time of pre-surgical assessment. Psychiatric screening criteria were revised and 31 subsequent patients were evaluated; less than half of this group were found suitable for surgery at the time of preliminary assessment.