1,556 research outputs found
Morphological stability and fluid dynamics of vapor crystal growth
Research on morphological stability and fluid dynamics of crystal growth is discussed. Interfacial heat and mass transfer research is discussed. The finding of surface roughening is a precursor to a solid-solid phase transition was further quantified. Progress was obtained with the mass spectroscopic characterization of GeSe-Ge I sub 4
Sequential monitoring with conditional randomization tests
Sequential monitoring in clinical trials is often employed to allow for early
stopping and other interim decisions, while maintaining the type I error rate.
However, sequential monitoring is typically described only in the context of a
population model. We describe a computational method to implement sequential
monitoring in a randomization-based context. In particular, we discuss a new
technique for the computation of approximate conditional tests following
restricted randomization procedures and then apply this technique to
approximate the joint distribution of sequentially computed conditional
randomization tests. We also describe the computation of a randomization-based
analog of the information fraction. We apply these techniques to a restricted
randomization procedure, Efron's [Biometrika 58 (1971) 403--417] biased coin
design. These techniques require derivation of certain conditional
probabilities and conditional covariances of the randomization procedure. We
employ combinatoric techniques to derive these for the biased coin design.Comment: Published in at http://dx.doi.org/10.1214/11-AOS941 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Handling Covariates in the Design of Clinical Trials
There has been a split in the statistics community about the need for taking
covariates into account in the design phase of a clinical trial. There are many
advocates of using stratification and covariate-adaptive randomization to
promote balance on certain known covariates. However, balance does not always
promote efficiency or ensure more patients are assigned to the better
treatment. We describe these procedures, including model-based procedures, for
incorporating covariates into the design of clinical trials, and give examples
where balance, efficiency and ethical considerations may be in conflict. We
advocate a new class of procedures, covariate-adjusted response-adaptive (CARA)
randomization procedures that attempt to optimize both efficiency and ethical
considerations, while maintaining randomization. We review all these
procedures, present a few new simulation studies, and conclude with our
philosophy.Comment: Published in at http://dx.doi.org/10.1214/08-STS269 the Statistical
Science (http://www.imstat.org/sts/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Exact properties of Efron's biased coin randomization procedure
Efron [Biometrika 58 (1971) 403--417] developed a restricted randomization
procedure to promote balance between two treatment groups in a sequential
clinical trial. He called this the biased coin design. He also introduced the
concept of accidental bias, and investigated properties of the procedure with
respect to both accidental and selection bias, balance, and randomization-based
inference using the steady-state properties of the induced Markov chain. In
this paper we revisit this procedure, and derive closed-form expressions for
the exact properties of the measures derived asymptotically in Efron's paper.
In particular, we derive the exact distribution of the treatment imbalance and
the variance-covariance matrix of the treatment assignments. These results have
application in the design and analysis of clinical trials, by providing exact
formulas to determine the role of the coin's bias probability in the context of
selection and accidental bias, balancing properties and randomization-based
inference.Comment: Published in at http://dx.doi.org/10.1214/09-AOS758 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
A proposed non-intrusive method for finding coefficients of slip and molecular reflectivity in microgravity
A proposed experimental program to look at a series of vapor transport properties measured along solid and liquid surfaces is described. The research objectives proposed are: (1) with accuracy otherwise unobtainable on ground, to determine the coefficient of slip measured between gases and the surfaces of liquids and solids; (2) for the first time, to classify and tabulate dominant surface effects found for a variety of solids, particularly those crystalized by vapor transport; and (3) to extend understanding of settling rates predicted for cosmic dust and condensed vapor falling through planetary atmospheres. The method used to obtain these objectives, has aided, to an order of magnitude, understanding of various liquid-gas interfaces such as oil and water. But to date, no similar characterization has proved successful for solids or liquids of uncertain densities. Likewise, no data exist in either ground-based research or as part of a microgravity program that, when collected with the high accuracy expected in low gravity, could definitely settle outstanding questions in kinetic theory, molecular dynamics, and cosmic physics
X-ray transmission microscope development
We are developing a hard x-ray microscope for direct observation of solidification dynamics in metal alloys and metal matrix composites. The Fein-Focus Inc. x-ray source was delivered in September and found to perform better than expected. Confirmed resolution of better than 2 micrometers was obtained and magnifications up to 800X were measured. Nickel beads of 30 micrometer diameter were easily detected through 6mm of aluminum. X-ray metallography was performed on several specimens showing high resolution and clear definition of 3-dimensional structures. Prototype furnace installed and tested
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