You only live up to the standards you set: An evaluation of different approaches to standard setting

Interpretation of performance in reference to a standard can provide nuanced, finely-tuned information regarding examinee abilities beyond that of just a total score. However, there is a multitude of ways to set performance standards yet little guidance regarding which method operates best and under what circumstances. Traditional methods are the most common approach adopted in practice and heavily involve subject matter experts (SMEs). Two other approaches have been suggested in the literature as alternative ways to set performance standards, although they have yet to be implemented in practice. Data-driven approaches do not involve SMEs but rather rely solely upon statistical techniques to classify examinees into groups. Integrated approaches are a newer standard setting method that combines judgments provided by SMEs with statistical techniques to inform the creation of performance standards. The primary purpose of this dissertation was to describe and illustrate the traditional, data-driven, and integrated approaches used to establish performance standards on tests. A traditional standard setting was conducted using the modified Angoff procedure. Latent class analysis (LCA)—a data-driven classification technique—was performed in which model parameters were first freely estimated to assess the fit of various general LCA models and later constrained to create ordered groups for various ordinal LCA models. The traditional and data-driven standard setting methods were combined to form an “integrated” approach. SMEs’ ratings of expected examinee performance (derived from the modified Angoff standard setting) were used as item difficulty constraints in an integrated LCA model, the Angoff LCA. The results were used to compare examinee classifications from all three approaches and model-data fit amongst the statistically-oriented methods. Although classifications were planned for comparison across all three approaches, issues were encountered with the Angoff LCA. Therefore, the comparisons of primary interest were between the modified Angoff and championed LCA model. The results did not offer a clear-cut decision about which approach to champion. Ultimately, the modified Angoff was selected as the most appropriate standard setting approach for the test administered. Important considerations are offered for researchers who wish to use data-driven models to set standards and ideas are proposed for future research

The Multi-Component Nature of the Vela Pulsar Nonthermal X-ray Spectrum

We report on our analysis of a 274 ks observation of the Vela pulsar with the Rossi X-Ray Timing Explorer (RXTE). The double-peaked, pulsed emission at 2 - 30 keV, which we had previously detected during a 93 ks observation, is confirmed with much improved statistics. There is now clear evidence, both in the spectrum and the light curve, that the emission in the RXTE band is a blend of two separate non-thermal components. The spectrum of the harder component connects smoothly with the OSSE, COMPTEL and EGRET spectrum and the peaks in the light curve are in phase coincidence with those of the high-energy light curve. The spectrum of the softer component is consistent with an extrapolation to the pulsed optical flux, and the second RXTE pulse is in phase coincidence with the second optical peak. In addition, we see a peak in the 2-8 keV RXTE pulse profile at the radio phase.Comment: 12 pages, 3 figures, accepted for publication in Astrophysical Journa

Gamma-Ray Observations of GRO J1655-40

The bright transient X-ray source GRO J1655-40 = XN Sco 1994 was observed by the OSSE instrument on the Compton Gamma Ray Observatory (GRO). Preliminary results are reported here. The initial outburst from GRO J1655-40 was detected by BATSE on 27 Jul 1994. OSSE observations were made in five separate viewing periods starting between 4 Aug 1994 and 4 Apr 1995. The first, third, and fifth observations are near the peak luminosity. In the second observation, the source flux had dropped by several orders of magnitude and we can only set an upper limit. The fourth observation is a weak detection after the period of maximum outburst. In contrast with other X-ray novae such as GRO J0422+32, the spectrum determined by OSSE is consistent with a simple power law over the full range of detection, about 50 - 600 keV. The photon spectral index is in the range of -2.5 to 2.8 in all of the observations. We set an upper limit on fractional rms variation \u3c5% in the frequency range 0.01 – 60 Hz. No significant narrow or broad line features are observed at any energy

Correlation between X-ray Lightcurve Shape and Radio Arrival Time in the Vela Pulsar

We report the results of simultaneous observations of the Vela pulsar in X-rays and radio from the RXTE satellite and the Mount Pleasant Radio Observatory in Tasmania. We sought correlations between the Vela's X-ray emission and radio arrival times on a pulse by pulse basis. At a confidence level of 99.8% we have found significantly higher flux density in Vela's main X-ray peak during radio pulses that arrived early. This excess flux shifts to the 'trough' following the 2nd X-ray peak during radio pulses that arrive later. Our results suggest that the mechanism producing the radio pulses is intimately connected to the mechanism producing X-rays. Current models using resonant absorption of radio emission in the outer magnetosphere as a cause of the X-ray emission are explored as a possible explanation for the correlation.Comment: 6 pages, 5 figures, accepted by Ap

The `bare' strange stars might not be bare

It is proposed that the `bare' strange matter stars might not be bare, and radio pulsars might be in fact `bare' strange stars. As strange matter stars being intensely magnetized rotate, the induced unipolar electric fields would be large enough to construct magnetospheres. This situation is very similar to that discussed by many authors for rotating neutron stars. Also, the strange stars with accretion crusts in binaries could act as X-ray pulsars or X-ray bursters. There are some advantages if radio pulsars are `bare' strange stars.Comment: 11 pages, 1 Postscript figures, LaTeX, Chin. Phys. Lett. 1998, Vol.15, Nov.12, p.93