19,706 research outputs found
An approach to valuing ponds within farming systems for aquaculture
[PDF contains 37 pages
Block-Conditional Missing at Random Models for Missing Data
Two major ideas in the analysis of missing data are (a) the EM algorithm
[Dempster, Laird and Rubin, J. Roy. Statist. Soc. Ser. B 39 (1977) 1--38] for
maximum likelihood (ML) estimation, and (b) the formulation of models for the
joint distribution of the data and missing data indicators , and
associated "missing at random"; (MAR) condition under which a model for
is unnecessary [Rubin, Biometrika 63 (1976) 581--592]. Most previous work has
treated and as single blocks, yielding selection or pattern-mixture
models depending on how their joint distribution is factorized. This paper
explores "block-sequential"; models that interleave subsets of the variables
and their missing data indicators, and then make parameter restrictions based
on assumptions in each block. These include models that are not MAR. We examine
a subclass of block-sequential models we call block-conditional MAR (BCMAR)
models, and an associated block-monotone reduced likelihood strategy that
typically yields consistent estimates by selectively discarding some data.
Alternatively, full ML estimation can often be achieved via the EM algorithm.
We examine in some detail BCMAR models for the case of two multinomially
distributed categorical variables, and a two block structure where the first
block is categorical and the second block arises from a (possibly multivariate)
exponential family distribution.Comment: Published in at http://dx.doi.org/10.1214/10-STS344 the Statistical
Science (http://www.imstat.org/sts/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Comparing the Profitability of Beef Production Enterprises in North Dakota
Agricultural Finance, Production Economics,
Two-Method Planned Missing Designs for Longitudinal Research
We examine longitudinal extensions of the two-method measurement design, which uses planned missingness to optimize cost-efficiency and validity of hard-to-measure constructs. These designs use a combination of two measures: a âgold standardâ that is highly valid but expensive to administer, and an inexpensive (e.g., survey-based) measure that contains systematic measurement bias (e.g., response bias). Using simulated data on four measurement occasions, we compared the cost-efficiency and validity of longitudinal designs where the gold standard is measured at one or more measurement occasions. We manipulated the nature of the response bias over time (constant, increasing, fluctuating), the factorial structure of the response bias over time, and the constraints placed on the latent variable model. Our results showed that parameter bias is lowest when the gold standard is measured on at least two occasions. When a multifactorial structure was used to model response bias over time, it is necessary to have the âgold standardâ measures included at every time point, in which case most of the parameters showed low bias. Almost all parameters in all conditions displayed high relative efficiency, suggesting that the 2-method design is an effective way to reduce costs and improve both power and accuracy in longitudinal research
Dynamic FOV visible light communications receiver for dense optical networks
This study explores how the field-of-view (FOV) of a visible light communications (VLCs) receiver can be manipulated to realise the best signal-to-noise ratio (SNR) while supporting device mobility and optimal access point (AP) selection. The authors propose a dynamic FOV receiver that changes its aperture according to receiver velocity, location, and device orientation. The D-FOV technique is evaluated through modelling, analysis, and experimentation in an indoor environment comprised of 15 VLC APs. The proposed approach is also realised as an algorithm that is studied through analysis and simulation. The results of the study indicate the efficacy of the approach including a 3X increase in predicted SNR over static FOV approaches based on measured received signal strength in the testbed. Additionally, the collected data reveal that D-FOV increases effectiveness in the presence of noise. Finally, the study describes the tradeoffs among the number of VLC sources, FOV, user device velocity, and SNR as a performance metric.Accepted manuscrip
The shapes of the circumstellar silicate features
Around oxygen-rich stars the spectra of most long-period variables (LPV) show an excess infrared emission which is attributed to circumstellar silicate dust grains. It is known that the spectral energy distribution of the 10 micron emissions shows variations from star to star. With the availability of many Infrared Astronomy Satellite (IRAS) Low Resolution Spectra (LRS) in the 8 to 22 micron region, the 10 micron feature can be studied to determine its uniformity (or lack thereof). The excess silicate emissions (10 micron emission), divided into three groups characterized by similar spectral shapes, are discussed
Tensile film clamps and mounting block for the rheovibron and autovibron viscoelastometer
A set of film clamps and a mounting block for use in the determination of tensile modulus and damping properties of films in a manually operated or automated Rheovibron is diagrammed. These clamps and mounting block provide uniformity of sample gripping and alignment in the instrument. Operator dependence and data variability are greatly reduced
Further detections of OH masers in carbon stars with silicate features
A sample of J-type carbon stars was searched for OH maser emission. The new
detection of three OH lines towards two silicate carbon stars is reported. In
V778 Cyg, previously known as the main-lines (1665 and 1667 MHz) maser source,
the satellite 1612 MHz emission was discovered while in NSV 2814 the main OH
lines were detected. The presence of OH maser lines confirms the former
suggestion that oxygen-rich material is located in the vicinity (
cm) of silicate carbon stars.Comment: LaTeX2e, 4 pages with 2 figure
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