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After a brief discussion of the threshold concept in toxicology, we consider models for the estimation of thresholds in the case where the observed response is binary or quantal. A generalization of the four-parameter Tukey-lambda family of distributions is proposed as a useful class of models for threshold estimation. Properties of these models are(More)
To test the hypothesis that cognitive impairment may be present early in the course of HTLV-III/LAV infection, intravenous drug abusers (IDVAs) without overt symptoms of AIDS related illness were tested with standard neuropsychological and psychosocial measures. This study is the baseline for a prospective longitudinal study of the natural history of(More)
Platelet-activating factor (PAF), a potent endogenous phospholipid released by a variety of mammalian cells, induces platelet activation in vivo and in vitro. Little is known, however, about the physiological modulation of its actions. We have examined the ability of two naturally occurring compounds which stimulate cAMP production, vasoactive intestinal(More)
The ACS solar blind channel (SBC) is a photon-counting MAMA detector capable of producing two-dimensional imaging in the UV at wavelengths 1150– 1700Å, with a field of view (FOV) of 31 ×35. We describe the on-orbit performance of the ACS/SBC from an analysis of data obtained from the service mission observatory verification (SMOV) programs. These data show(More)
The off-axis location of the Advanced Camera for Surveys causes strong geometric distortion in all detectors – the Wide Field Camera (WFC), High Resolution Camera (HRC), and Solar Blind Camera (SBC). Dithered observations of rich star cluster fields are used to calibrate the dirtortion. We describe the observations obtained, the algorithms used to perform(More)
We consider a regression to the mean problem with a very large sample for the first measurement and relatively small subsample for the second measurement, selected on the basis of the initial measurement. This is a situation that often occurs in screening trials. We propose to estimate the unselected population mean and variance from the first measurement(More)
We provide some background on pointing ACS, and present the dither and mosaic patterns that will be provided as a convenience for HST Cycle 11 Phase II proposal writers. This report supersedes any previous documents which describe specific ACS dither and mosaic patterns-namely, ACS Instrument Science Report 98-02 by Stiavelli, and the engineering versions(More)
The dark count rates in the STIS MAMA detectors have been monitored. This report covers the period since the Servicing Mission 4 of May 2009. We find both long-term and short-term variations which for the NUV side we express as a function of date and temperature. The NUV dark rate has declined significantly from its surprisingly high initial rate of 0.014(More)
1. ABSTRACT Astrometric calibrations of JWST will use observations of a reference field in the Large Magellanic Cloud. This field will itself be astrometrically calibrated using observations with ACS/WFC on HST. The higher-order geometric distortions of ACS/WFC have previously been calibrated to high accuracy. However, the absolute scale and orientation of(More)