Testing equality of variances in the analysis of repeated measurements

The problem of comparing the precisions of two instruments using repeated measurements can be cast as an extension of the Pitman-Morgan problem of testing equality of variances of a bivariate normal distribution. Hawkins (1981) decomposes the hypothesis of equal variances in this model into two subhypotheses for which simple tests exist. For the overall hypothesis he proposes to combine the tests of the subhypotheses using Fisher's method and empirically compares the component tests and their combination with the likelihood ratio test. In this paper an attempt is made to resolve some discrepancies and puzzling conclusions in Hawkins's study and to propose simple modifications.\ud \ud The new tests are compared to the tests discussed by Hawkins and to each other both in terms of the finite sample power (estimated by Monte Carlo simulation) and theoretically in terms of asymptotic relative efficiencies

With a little help from my friends: Developing an assisted automated peritoneal dialysis program in Western Australia

Background: Nurse-assisted automated peritoneal dialysis (AAPD) offers a model of care that has been successfully used in frail dialysis populations internationally. AAPD offers cost savings over hospitalisation on peritoneal dialysis (PD) or in-centre haemodialysis (HD). Method: A pilot AAPD model of care was developed in Western Australia (WA). Patient evaluation was measured utilising a perceptions of dialysis survey, clinical events, hospitalisation and peritonitis rates, Charlson Comorbidity Index (CCI), KDQoL-SF 36 and a survey. Staff opinions and perceived competency were measured by an online survey. Economic analysis was undertaken. Results: A successful collaborative model was developed. 40 staff were trained and competency significantly improved during program delivery (p < 0.0001). 15 patients with an average CCI score of 8.7 used the service for 18 periods of care over 18 months (mean 33 days SD 47). Two non-renal cause deaths and two episodes of peritonitis occurred. Patient opinions were extremely positive. Cost savings were estimated at $620,000. Conclusion: In WA, an AAPD pilot program has been successfully developed and delivered. A sustainable model has overcome initial hurdles. Staff have gained new skills and delivered effective care, demonstrated by high patient acceptance. The program was cost-effective compared to staying in hospital or transferring to HD

M Dwarfs from Hubble Space Telescope Star Counts. IV

We study a sample of about 1400 disk M dwarfs that are found in 148 fields observed with the Wide Field Camera 2 (WFC2) on the Hubble Space Telescope and 162 fields observed with pre-repair Planetary Camera 1 (PC1), of which 95 of the WFC2 fields are newly analyzed. The method of maximum likelihood is applied to derive the luminosity function and the Galactic disk parameters. At first, we use a local color-magnitude relation and a locally determined mass-luminosity relation in our analysis. The results are consistent with those of previous work but with considerably reduced statistical errors. These small statistical errors motivate us to investigate the systematic uncertainties. Considering the metallicity gradient above the Galactic plane, we introduce a modified color-magnitude relation that is a function of Galactic height. The resultant M dwarf luminosity function has a shape similar to that derived using the local color-magnitude relation but with a higher peak value. The peak occurs at $M_V \sim 12$ and the luminosity function drops sharply toward $M_V \sim 14$. We then apply a height-dependent mass-luminosity function interpolated from theoretical models with different metallicities to calculate the mass function. Unlike the mass function obtained using local relations, which has a power-law index $\alpha = 0.47$, the one derived from the height-dependent relations tends to be flat ($\alpha = -0.10$). The resultant local surface density of disk M dwarfs (12.2 +/- 1.6 M_sun pc^{-2}) is somewhat smaller than the one obtained using local relations (14.3 +/- 1.3 M_sun pc^{-2}). Our measurement favors a short disk scale length, H = 2.75 +/- 0.16 (statistical) +/- 0.25 (systematic) kpc.Comment: 20 pages, 10 ps figures, accepted for publication in Ap

Americans’ Explanations for Racial Disparities Across Three Decades (1985-2016): Are the Views of African Americans and Whites Converging?

Despite the promises of the Civil Rights Movement last century, African Americans continue to experience less socioeconomic success than their White peers. Research has found that explanations for this racial gap differ by race, with Whites more likely to adhere to individualistic explanations and African Americans more likely to agree with structural causes. We use color-blind racism, standpoint theory, and a presidential administration timeline to frame an examination of three decades (1985-2016) of General Social Survey data on explanations for racial differences. We find that among all Americans regardless of race, agreement with both person-blame and system-blame explanations has declined over time. We also find that the gap between African Americans and Whites in agreement with these explanations is closing over time, including trends that suggest African Americans may now be more supportive of some person-blame explanations for racial disparities than are Whites

A Fast Algorithm for Robust Regression with Penalised Trimmed Squares

The presence of groups containing high leverage outliers makes linear regression a difficult problem due to the masking effect. The available high breakdown estimators based on Least Trimmed Squares often do not succeed in detecting masked high leverage outliers in finite samples. An alternative to the LTS estimator, called Penalised Trimmed Squares (PTS) estimator, was introduced by the authors in \cite{ZiouAv:05,ZiAvPi:07} and it appears to be less sensitive to the masking problem. This estimator is defined by a Quadratic Mixed Integer Programming (QMIP) problem, where in the objective function a penalty cost for each observation is included which serves as an upper bound on the residual error for any feasible regression line. Since the PTS does not require presetting the number of outliers to delete from the data set, it has better efficiency with respect to other estimators. However, due to the high computational complexity of the resulting QMIP problem, exact solutions for moderately large regression problems is infeasible. In this paper we further establish the theoretical properties of the PTS estimator, such as high breakdown and efficiency, and propose an approximate algorithm called Fast-PTS to compute the PTS estimator for large data sets efficiently. Extensive computational experiments on sets of benchmark instances with varying degrees of outlier contamination, indicate that the proposed algorithm performs well in identifying groups of high leverage outliers in reasonable computational time.Comment: 27 page

A Study of CO Emission in High Redshift QSOs Using the Owens Valley Millimeter Array

Searches for CO emission in high-redshift objects have traditionally suffered from the accuracy of optically-derived redshifts due to lack of bandwidth in correlators at radio observatories. This problem has motivated the creation of the new COBRA continuum correlator, with 4 GHz available bandwidth, at the Owens Valley Radio Observatory Millimeter Array. Presented here are the first scientific results from COBRA. We report detections of redshifted CO(J=3-2) emission in the QSOs SMM J04135+10277 and VCV J140955.5+562827, as well as a probable detection in RX J0911.4+0551. At redshifts of z=2.846, z=2.585, and z=2.796, we find integrated CO flux densities of 5.4 Jy km/s, 2.4 Jy km/s, and 2.9 Jy km/s for SMM J04135+10277, VCV J140955.5+562827, and RX J0911.4+0551, respectively, over linewidths of Delta(V_{FWHM}) ~ 350 km/s. These measurements, when corrected for gravitational lensing, correspond to molecular gas masses of order M(H_2) ~ 10^{9.6-11.1} solar masses, and are consistent with previous CO observations of high-redshift QSOs. We also report 3-sigma upper limits on CO(3-2) emission in the QSO LBQS 0018-0220 of 1.3 Jy km/s. We do not detect significant 3mm continuum emission from any of the QSOs, with the exception of a tentative (3-sigma) detection in RX J0911.4+0551 of S_{3mm}=0.92 mJy/beam.Comment: 18 pages, 5 figures, 2 tables, accepted to ApJ. Changes made for version 2: citations added, 2 objects added to Table 2 and Figure

A spectroscopic search for faint secondaries in cataclysmic variables

The secondary in cataclysmic variables (CV's) is usually detected by cross-correlation of the CV spectrum with that of a K or M dwarf template, to produce a radial velocity curve. Although this method has demonstrated its power, it has its limits in the case of noisy spectra, such as are found when the secondary is faint. A method of co-adding spectra, called skew mapping, has been proposed in the past. Gradually, examples of its application are being published. Nonetheless, so far no journal article has described the technique in detail. To answer this need, this paper explores in detail the capabilities of skew mapping when determining the amplitude of the radial velocity for faint secondaries. It demonstrates the method's power over techniques that are more conventional, when the signal-to-noise (s/n) ratio is poor. The paper suggests an approach to assessing the quality of results. This leads in the case of the investigated objects to a first tier of results, where we find K2=127+-23 km/s for SY Cnc, K2=144+-18 km/s for RW Sex, and K2=262+-14 km/s for UX UMa. These we believe to be the first direct determinations of K2 for these objects. Furthermore, we also obtain K2=263+-30 km/s for RW Tri, close to a skew mapping result obtained elsewhere. In the first three cases, we use these results to derive the mass of the white dwarf companion. A second tier of results includes UU Aqr, EX Hya, and LX Ser, for which we propose more tentative values of K2. Clear failures of the method are also discussed (EF Eri, VV Pup, SW Sex).Comment: 23 pages, 22 figures, 10 tables; translated to pdf from a Word file - no TeX version available. Accepted for publication in MNRA