A comparison of two Hilbert spectral analyses of heart rate variability

The present paper compares the performance of two Hilbert spectral analyses when applied to a synthetic RR series from a nonstationary integral pulse frequency modulation model and to real RR series from a dataset of normal sinus arrhythmia. The Hilbert–Huang transformation based on empirical mode decomposition is compared to the presently introduced Hilbert–Olhede–Walden transformation based on stationary wavelet packet decomposition. The comparison gives consistent results pointing to a superior performance of the Hilbert–Olhede–Walden transformation showing 33–163 times smaller deviations when estimating the instantaneous frequency traces of the synthetic RR series. Artificial fluctuations caused by mode mixing in the Hilbert–Huang spectrum are seen in both the synthetic and real RR series. It can be concluded that the instantaneous frequencies and amplitudes estimated by the Hilbert–Huang transformation should be interpreted with caution when investigating heart rate variability.     

The Burt controversy: A comparison of Burt's data on IQ with data from other studies

A recent controversy has given reason to examine the place of Burt's data among other behavioral genetic data on IQ. Although Burt's kinship correlations are higher than the average of other studies, his results are generally consistent with other data.     

A comparison of physical image quality indices and observer performance in the radiographic detection of nylon beads

Although various models have been proposed in an attempt to predict the usefulness of a radiographic image in terms of its physical characteristics, no previous work has shown whether a single physical image quality index, such as a signal-to-noise ratio, can reliably predict the performance of a human observer over a broad range of image characteristics. We studied the relationship between physical and visual image quality for the task of detecting nylon beads in radiographs. Thirty-seven imaging cases with different combinations of physical image characteristics were considered; these included variations in object size and magnification, X-ray beam quality, screen-film system, screen-film contact, film density and illumination, and viewing distance. For each imaging case, visual image quality was quantified in terms of observer performance in a 2AFC visual detection experiment. Physical image quality indices were calculated according to eight different models of the detection process; these indices combined data regarding object size and attenuation, screen-film system MTF, film gradient, noise Wiener spectrum, and visual system response. The results of this work indicate that, for the conditions studied, human observer detection performance most closely resembles that of a sub-optimal statistical decision process.     

A comparison of three methods of setting prescribing budgets, using data derived from defined daily dose analyses of historic patterns of use.

BACKGROUND: Prescribing matters (particularly budget setting and research into prescribing variation between doctors) have been handicapped by the absence of credible measures of the volume of drugs prescribed. AIM: To use the defined daily dose (DDD) method to study variation in the volume and cost of drugs prescribed across the seven main British National Formulary (BNF) chapters with a view to comparing different methods of setting prescribing budgets. METHOD: Study of one year of prescribing statistics from all 129 general practices in Lothian, covering 808,059 patients: analyses of prescribing statistics for 1995 to define volume and cost/volume of prescribing for one year for 10 groups of practices defined by the age and deprivation status of their patients, for seven BNF chapters; creation of prescribing budgets for 1996 for each individual practice based on the use of target volume and cost statistics; comparison of 1996 DDD-based budgets with those set using the conventional historical approach; and comparison of DDD-based budgets with budgets set using a capitation-based formula derived from local cost/patient information. RESULTS: The volume of drugs prescribed was affected by the age structure of the practices in BNF Chapters 1 (gastrointestinal), 2 (cardiovascular), and 6 (endocrine), and by deprivation structure for BNF Chapters 3 (respiratory) and 4 (central nervous system). Costs per DDD in the major BNF chapters were largely independent of age, deprivation structure, or fundholding status. Capitation and DDD-based budgets were similar to each other, but both differed substantially from historic budgets. One practice in seven gained or lost more than 100,000 Pounds per annum using DDD or capitation budgets compared with historic budgets. The DDD-based budget, but not the capitation-based budget, can be used to set volume-specific prescribing targets. CONCLUSIONS: DDD-based and capitation-based prescribing budgets can be set using a simple explanatory model and generalizable methods. In this study, both differed substantially from historic budgets. DDD budgets could be created to accommodate new prescribing strategies and raised or lowered to reflect local intentions to alter overall prescribing volume or cost targets. We recommend that future work on setting budgets and researching prescribing variations should be based on DDD statistics.     

Binary and multi-category ratings in a laboratory observer performance study: a comparison

The authors investigated radiologists, performances during retrospective interpretation of screening mammograms when using a binary decision whether to recall a woman for additional procedures or not and compared it with their receiver operating characteristic (ROC) type performance curves using a semi-continuous rating scale. Under an Institutional Review Board approved protocol nine experienced radiologists independently rated an enriched set of 155 examinations that they had not personally read in the clinic, mixed with other enriched sets of examinations that they had individually read in the clinic, using both a screening BI-RADS rating scale (recall/not recall) and a semi-continuous ROC type rating scale (0 to 100). The vertical distance, namely the difference in sensitivity levels at the same specificity levels, between the empirical ROC curve and the binary operating point were computed for each reader. The vertical distance averaged over all readers was used to assess the proximity of the performance levels under the binary and ROC-type rating scale. There does not appear to be any systematic tendency of the readers towards a better performance when using either of the two rating approaches, namely four readers performed better using the semi-continuous rating scale, four readers performed better with the binary scale, and one reader had the point exactly on the empirical ROC curve. Only one of the nine readers had a binary "operating point" that was statistically distant from the same reader's empirical ROC curve. Reader-specific differences ranged from -0.046 to 0.128 with an average width of the corresponding 95% confidence intervals of 0.2 and p-values ranging for individual readers from 0.050 to 0.966. On average, radiologists performed similarly when using the two rating scales in that the average distance between the run in individual reader's binary operating point and their ROC curve was close to zero. The 95% confidence interval for the fixed-reader average (0.016) was (-0.0206, 0.0631) (two-sided p-value 0.35). In conclusion the authors found that in retrospective observer performance studies the use of a binary response or a semi-continuous rating scale led to consistent results in terms of performance as measured by sensitivity-specificity operating points.     

A side-by-side comparison of the performance and time-and-motion data of VITEK MS

European Journal of Clinical Microbiology & Infectious Diseases - Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry systems are designed for rapid and...     

Cytotoxicity of precious and nonprecious alloys--experimental comparison of in vitro data from two laboratories

The aim of this investigation was to evaluate and compare the reproducibility of cytotoxicity data generated in two different laboratories using the same testing protocols. A series of dental alloys that are widely used in both countries were chosen. These alloys (five precious, two nonprecious) were wet ground up to 1200 grit SiC, sterilized in 70% ethanol, and extracted in sterile culture medium for 7 days. Pure copper was used as a positive control and Teflonreg and media only were used as negative controls. Test and control samples were randomized and blinded to each laboratory. Cells, primary human gingival fibroblasts, and immortalized 3T3 fibroblasts, were exposed to the extracts for 24 h. Extract cytotoxicity was evaluated spectrophotometrically with the use of a mitochondrial enzyme activity assay. Data were collected from both laboratories, combined, and subjected to a mixed-model analysis of variance. No statistical difference was obtained for the immortalized 3T3 cells, except for two extracts in which differences between the two labs were significant but were still not cytotoxic. Furthermore, no statistical differences were found for the primary cells. These data strongly suggest that cytotoxicity tests performed in different laboratories with the use of the same test materials may lead to comparable results if sample preparation, cells, test procedures, and data analyses are carefully considered.     

A comparison of the validity of two psychiatric screening questionnaires (GHQ-12 and SRQ-20) in Brazil, using Relative Operating Characteristic (ROC) analysis

The General Health Questionnaire (GHQ-12) (Goldberg, 1972) and the Self Report Questionnaire (SRQ-20) (Harding et al. 1980) were simultaneously validated against the criterion of the Clinical Interview Schedule (CIS) (Goldberg et al. 1970) in three primary care clinics in the city of Sao Paulo, Brazil. A comparison between the two screening instruments was carried out. The product-moment correlation between the two sets of scores was +0.72, and the validation coefficients for the GHQ-12 and the SRQ-20 were respectively: sensitivity 85% and 83%; specificity 79% and 80%; overall misclassification rate 18% and 19%. The two screening instruments were further compared by the application of Relative Operating Characteristic (ROC) analysis and, again, were found to be very similar in performance. The area under the ROC curve was 0.87 for the GHQ and 0.90 for the SRQ-20 (Z = 0.92, NS). Both psychiatric screening instruments were therefore found to be acceptably valid when assessed against the CIS in three primary care settings in Brazil. The application of ROC analysis to studies of this kind was feasible and straightforward. It was found to be superior to the conventional method of presentation of validity data. It is recommended that the application of ROC analysis to psychiatric screening studies be further utilized and explored.     

A comparison of multiple imputation methods for missing data in longitudinal studies

Background

Multiple imputation (MI) is now widely used to handle missing data in longitudinal studies. Several MI techniques have been proposed to impute incomplete longitudinal covariates, including standard fully conditional specification (FCS-Standard) and joint multivariate normal imputation (JM-MVN), which treat repeated measurements as distinct variables, and various extensions based on generalized linear mixed models. Although these MI approaches have been implemented in various software packages, there has not been a comprehensive evaluation of the relative performance of these methods in the context of longitudinal data.

Method

Using both empirical data and a simulation study based on data from the six waves of the Longitudinal Study of Australian Children (N?=?4661), we investigated the performance of a wide range of MI methods available in standard software packages for investigating the association between child body mass index (BMI) and quality of life using both a linear regression and a linear mixed-effects model.

Results

In this paper, we have identified and compared 12 different MI methods for imputing missing data in longitudinal studies. Analysis of simulated data under missing at random (MAR) mechanisms showed that the generally available MI methods provided less biased estimates with better coverage for the linear regression model and around half of these methods performed well for the estimation of regression parameters for a linear mixed model with random intercept. With the observed data, we observed an inverse association between child BMI and quality of life, with available data as well as multiple imputation.

Conclusion

Both FCS-Standard and JM-MVN performed well for the estimation of regression parameters in both analysis models. More complex methods that explicitly reflect the longitudinal structure for these analysis models may only be needed in specific circumstances such as irregularly spaced data.

     

Some statistical issues in the analyses of data from longitudinal studies of elderly chronic care populations

OBJECTIVE: This article discusses broad statistical issues common to much medical research: intent-to-treat analysis vs. completers analysis; clustered hierarchical and repeated-measures data; missing data and dropouts; and assessment of direct, indirect, and total effects. Traditional approaches and statistical techniques are reviewed and contrasted with modern methods for analysis of medical studies. METHOD: The concepts are introduced and discussed in general terms; they are illustrated with an example. The example comes from a study of the effect of residence in special care units (SCUs) for demented elderly on the daily function of nursing homes residents. More than 700 residents from 22 nursing facilities, residing in either an SCU or a non-SCU were assessed three times at approximate 6-month intervals. RESULTS: Results from both the application of traditional statistical techniques and modern methods for the analysis of repeated-measures of hierarchical multicenter data are presented, interpreted, and compared. Advantages and shortcomings of these approaches are discussed. CONCLUSION: This article advocates the use of mixed models and proper causal reasoning and terminology in the analysis and publication of results from studies on aging and life course.     

A comparison of two skin test methodologies and allergens from two different manufacturers.

BACKGROUND: Skin prick tests (SPTs) are a frequently used method for evaluation of atopy. A variety of standard allergen preparations are available, together with a number of different methods of application. OBJECTIVE: The objective of this study was to compare SPT reactivity 1) using Soluprick SQ allergens (ALK Allergologisk Laboratorium A/S, H?rsholm, Denmark) and Bayer allergens [Bayer Corporation, West Haven, CT], and 2) using two common methods of application, a standard prick lancet and a Quintest, both produced by Bayer. METHODS: SPTs were undertaken on 22 adult volunteers (mean age 40 years, 17 female, 5 male). Wheal size was recorded as mean diameters (mm) and area (mm2). RESULTS: Bayer allergens produced larger mean diameters and areas to dust mites than ALK allergens, with the differences significant when allergens were applied with the lancet. There was a tendency for the ALK cat allergen to produce larger reactions than the Bayer product. The method of application also influenced the wheal size, with the lancet producing significantly larger mean diameters than the Quintest for the histamine and allergens from both manufacturers, except the ALK cat allergen. There were similar differences between methods of application for reactions measured as an area. CONCLUSIONS: SPTs that use different allergens or different methods of application will not provide comparable assessments of atopy.     

A comparison of two methods for facilitating clinical data integration by medical students

Two important charting strategies to help students organize patients' data are Weed's problem-oriented medical record (POMR) and Russell's condition diagram (CD). The authors conducted the present study in 1987 to determine whether either was superior for clinical data integration. Sophomore medical students at The University of Texas Medical School at San Antonio indicated whether they preferred the POMR, the CD, or neither. They were then divided into three study groups according to their preferences, with the POMR and CD groups receiving 80 hours of training and the control group receiving only the standard preclinical training. Each student then examined a standardized patient and wrote an open-ended report about the patient's medical problem. After examining a second patient, students were asked to write a structured report providing information about each of ten components of diagnosis. Both the CD and the POMR groups scored numerically higher on the structured type of report than did the controls, but only the CD group scored significantly higher. The CD group also scored higher than did the POMR group on both types of report, but the differences were not statistically significant. This study indicates that the clinical reasoning of medical students can be enhanced by focused training in either the CD or the POMR methods. It suggests that the CD format may be particularly helpful for students with lower academic achievement.     

A data management approach to quality assurance using colorectal carcinoma reports from two institutions as a model

We used data management software to compare pathology report data concerning regional lymph node sampling for colorectal carcinoma from 2 institutions using different dissection methods. Data were retrieved from 2 disparate anatomic pathology information systems for all cases of colorectal carcinoma in 2003 involving the ascending and descending colon. Initial sorting of the data included overall lymph node recovery to assess differences between the dissection methods at the 2 institutions. Additional segregation of the data was used to challenge the application's capability of accurately addressing the complexity of the process. This software approach can be used to evaluate data from disparate computer systems, and we demonstrate how an automated function can enable institutions to compare internal pathologic assessment processes and the results of those comparisons. The use of this process has future implications for pathology quality assurance in other areas.     

Microsatellite evolution in modern humans: a comparison of two data sets from the same populations

We genotyped 64 dinucleotide microsatellite repeats in individuals from populations that represent all inhabited continents. Microsatellite summary statistics are reported for these data, as well as for a data set that includes 28 out of 30 loci studied by Bowcock et al . (1994) in the same individuals. For both data sets, diversity statistics such as heterozygosity, number of alleles per locus, and number of private alleles per locus produced the highest values in Africans, intermediate values in Europeans and Asians, and low values in Americans. Evolutionary trees of populations based on genetic distances separated groups from different continents. Corresponding trees were topologically similar for the two data sets, with the exception that the (δμ)² genetic distance reliably distinguished groups from different continents for the larger data set, but not for the smaller one. Consistent with our results from diversity statistics and from evolutionary trees, population growth statistics S _k and β, which seem particularly useful for indicating recent and ancient population size changes, confirm a model of human evolution in which human populations expand in size and through space following the departure of a small group from Africa.     

A comparison of two ulcerogenic techniques

Long Evans rats 45, 130, or 280 days old were exposed either to a supine restraint or a water restraint treatment for two hr. This was followed by a 2-hr rest period. Core body temperature was recorded every 30 min. Stress-induced hypothermia was greatest in the water restraint treatment and in younger rats irrespective in both restraint conditions. The water restraint procedure as compared to the supine restraint produced significantly more ulcers in 130- and 280-day-old rats.     

Validity of the Gaussian assumption in the analysis of ROC data obtained from scintigraphic-like images

The Gaussian assumption (GA) has been found experimentally to provide an acceptable fit to ROC data elicited from visual experiments. When the GA is satisfied the two-parameter index D(delta m, s) determines the ROC completely. It is however not known whether the GA is an intrinsic property of the eye-brain system, or whether it is a consequence of the type of visual stimuli selected for the experiments. Using scintigraphic-like images to test the validity of the GA, it is found that (i) series of visual stimuli with as few variables as possible produce ROC data compatible with the GA, but (ii) it is possible to find stimuli for which the GA provides a very poor account of experimental data. These results lead us to doubt that the GA is an intrinsic property of the human visual system.