170,099 research outputs found
Continuous-time Mallows processes
No full textIn this article, we introduce \textit{Mallows processes}, defined to be
continuous-time c\`adl\`ag processes with Mallows distributed marginals. We
show that such processes exist and that they can be restricted to have certain
natural properties. In particular, we prove that there exists \textit{regular}
Mallows processes, defined to have their inversions numbers
be independent
increasing stochastic processes with jumps of size . We further show that
there exists a unique Markov process which is a regular Mallows process.
Finally, we study properties of regular Mallows processes and show various
results on the structure of these objects. Among others, we prove that the
graph structure related to regular Mallows processes looks like an
\textit{expanded hypercube} where we stacked hypercubes on the dimension
; we also prove that the first jumping times of regular Mallows
processes converge to a Poisson point process.Comment: 23 pages, 1 figur
An R package for permutations, Mallows and Generalized Mallows models
No full text[EN]Probability models on permutations associate a probability value to each of the permutations on n items. This paper considers two popular probability models, the Mallows model and the Generalized Mallows model. We describe methods for making inference, sampling and learning such distributions, some of which are novel in the literature. This paper also describes operations for permutations, with special attention in those related with the Kendall and Cayley distances and the random generation of permutations. These operations are of key importance for the efficient computation of the operations on distributions. These algorithms are implemented in the associated R package. Moreover, the internal code is written in C++
An R package for permutations, Mallows and Generalized Mallows models
No full text[EN]Probability models on permutations associate a probability value to each of the permutations on n items. This paper considers two popular probability models, the Mallows model and the Generalized Mallows model. We describe methods for making inference, sampling and learning such distributions, some of which are novel in the literature. This paper also describes operations for permutations, with special attention in those related with the Kendall and Cayley distances and the random generation of permutations. These operations are of key importance for the efficient computation of the operations on distributions. These algorithms are implemented in the associated R package. Moreover, the internal code is written in C++
Clustered Mallows Model
No full textRankings are a type of preference elicitation that arise in experiments where
assessors arrange items, for example, in decreasing order of utility. Orderings
of n items labelled {1,...,n} denoted are permutations that reflect strict
preferences. For a number of reasons, strict preferences can be unrealistic
assumptions for real data. For example, when items share common traits it may
be reasonable to attribute them equal ranks. Also, there can be different
importance attributions to decisions that form the ranking. In a situation
with, for example, a large number of items, an assessor may wish to rank at top
a certain number items; to rank other items at the bottom and to express
indifference to all others. In addition, when aggregating opinions, a judging
body might be decisive about some parts of the rank but ambiguous for others.
In this paper we extend the well-known Mallows (Mallows, 1957) model (MM) to
accommodate item indifference, a phenomenon that can be in place for a variety
of reasons, such as those above mentioned.The underlying grouping of similar
items motivates the proposed Clustered Mallows Model (CMM). The CMM can be
interpreted as a Mallows distribution for tied ranks where ties are learned
from the data. The CMM provides the flexibility to combine strict and
indifferent relations, achieving a simpler and robust representation of rank
collections in the form of ordered clusters. Bayesian inference for the CMM is
in the class of doubly-intractable problems since the model's normalisation
constant is not available in closed form. We overcome this challenge by
sampling from the posterior with a version of the exchange algorithm
\citep{murray2006}. Real data analysis of food preferences and results of
Formula 1 races are presented, illustrating the CMM in practical situations.Comment: Paper submitted for publicatio
PEMILIHAN VARIABEL PADA REGRESI LINIER DENGAN METODE STATISTIK C,MALLOWS
No full textTulisan ini bertujuan untuk mengetahui keidentikan statisik Tp yang didefinisikan Tp WI' -K + 2p dengan statistik Cp Mallows yang didefinisikan
Cp =n + 2p dengan K menyatakan jumlah parameter pada model penuh dan
p menyatakan j umlah parameter pada submodel (model yang telah disederhanakan). Pada estimasi kuadrat terkecil, kedua metode ini akan identik dengan nilai CI- P dan nilai Tp :s; p
A geometric interpretation of Mallows' C-p statistic and an alternative plot in variable selection
No full textMallows' C-p plot is a useful tool for variable selection in linear regression. Though not as popular as the C-p plot, Spjotvoll's F-p and P-p plots are also used in the variable selection procedure. The C-p, F-p and P-p plots are useful in their own right. If the interest is the direct measure of the amount of bias of the submodels and a distributional assumption is not made about the error term, a C-p or F-p plot is used. If a formal testing procedure is to be performed, then a P-p plot is employed. A geometrical approach is used in order to propose an alternative plot that unifies all the information in these three plots, and that has some advantages over them. A Mathematica package has been written to implement the approach. (c) 2007 Elsevier B.V. All rights reserved
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Linear programming bounds for doubly-even self-dual codes
No full textUsing a variant of linear programming method we
derive a new upper bound on the minimum distance d of doubly-even self-dual codes of length n. Asymptotically, for n growing, it gives d/n <=166315 + o(1), thus improving on the Mallows–
Odlyzko–Sloane bound of 1/6. To establish this, we prove that in any doubly even-self-dual code the distance distribution is asymptotically upper-bounded by the corresponding normalized binomial distribution in a certain interval
Mitomycin C in highly myopic eyes - Author reply
No full textOphthalmology. 2005 Feb;112(2):208-18; discussion 219.
Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes.
Gambato C, Ghirlando A, Moretto E, Busato F, Midena E.
SourceRefractive Surgery Service and Antimetabolite Therapy Research Unit, Department of Ophthalmology, University of Padova, Padova, Italy.
Abstract
PURPOSE: To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.
DESIGN: Prospective, double-masked, randomized clinical trial.
PARTICIPANTS: Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.
METHODS: In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).
MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.
RESULTS: Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).
CONCLUSION: Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.
Comment in
Ophthalmology. 2006 Feb;113(2):357; author reply 357-8
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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