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 ${Z}$ and missing data indicators ${M}$, and associated "missing at random"; (MAR) condition under which a model for ${M}$ is unnecessary [Rubin, Biometrika 63 (1976) 581--592]. Most previous work has treated ${Z}$ and ${M}$ 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

Linear Support Vector Machines for Error Correction in Optical Data Transmission

Reduction of bit error rates in optical transmission systems is an important task that is difficult to achieve. As speeds increase, the difficulty in reducing bit error rates also increases. Channels have differing characteristics, which may change over time, and any error correction employed must be capable of operating at extremely high speeds. In this paper, a linear support vector machine is used to classify large-scale data sets of simulated optical transmission data in order to demonstrate their effectiveness at reducing bit error rates and their adaptability to the specifics of each channel. For the classification, LIBLINEAR is used, which is related to the popular LIBSVM classifier. It is found that is possible to reduce the error rate on a very noisy channel to about 3 bits in a thousand. This is done by a linear separator that can be built in hardware and can operate at the high speed required of an operationally useful decode

Application of Suction-cup-attached VHF Transmitters to the Study of Beluga, Delphinapterus leucas, Surfacing Behavior in Cook Inlet, Alaska

Suction-cup-attached VHF radio transmittes were deployed on belugas, Delphinapterus leucas, in Cook Inlet, Alaska, in 1994 and 1995 to characterize the whales' surfacing behavior. Data from video recordings were also used to characterize behavior of undisturbed whales and whales actively pursued for tagging. Statistics for dive intervals (time between the midpoints of contiguous surfacings) and surfacing intevals (time at the surface per surfacing) were estimated. Operations took place on the tidal delta of the Susitna and Little Susitna Rivers. During the 2-yr study, eight whales were successfully tagged, five tags remained attached for >60 min, and data from these were used in the analyses. Mean dive interval was 24.1 sec (interwhale SD=6.4 sec, n=5). The mean surfacing interval, as determined from the duration of signals received from the radio transmitters, was 1.8 sec (SD=0.3 sec, n=125) for one of the whales. Videotaped behaviors were categorized as "head-lifts" or "slow-rolls." Belugas were more likely to head-lift than to slow-roll during vessel approaches and tagging attempts when compared to undisturbed whales. In undisturbed groups, surfacing intervals determined from video records were significantly different between head-lifting (average = 1.02 sect, SD=0.38 sed, n=28) and slow-rolling whales (average = 2.45 sec, SD=0.37 sec, n=106). Undisturbed juveniles exhibited shorter slow-roll surfacing intervals (average = 2.25 sec, SD=0.32 sec, n=36) than adults (average = 2.55 sec, SD=0.36 sec, n=70). We did not observe strong reactions by the belugas to the suction-cup tags. This tagging method shows promise for obtaining surfacing data for durations of several days

La nature, le rôle et l'influence de la doctrine universitaire en droit administratif québécois

In view of the increasingly divergent tendencies in legal scholarship, many authors are reconsidering whether « la doctrine » can really serve as a formal source of law. Since university professors are primarly responsible for producing this scholarly commentary, at least in the field of administrative law, it is worth asking two related questions : first, does university scholarship in law have a specific purpose or logic ? and second, does the nature of administrative law require that this scholarship have a particular direction ? In this essay, both questions are answered in the affirmative : University legal scholarship has a synthetic function not shared by ordinary legal writing, and given the case law foundations of the administrative law, this scholarship must also reveal the underlying conceptual structure of the subject

Accessibilité pour qui ? Selon quelles conceptions de la justice ?

Le rapport du Groupe de travail sur l’accessibilité à la justice du Québec, intitulé Jalons pour une plus grande accessibilité à la justice, a été présenté en septembre 1991. Ses diverses recommandations ont visé plusieurs matières : l'aide juridique, les programmes d'assurance frais juridiques, la Cour des petites créances, les modes non judiciaires de règlement de conflits, l'information et l'éducation juridique, et les besoins particuliers de certains groupes cibles tels que les communautés culturelles et les autochtones. Le présent essai a pour objet de situer les recommandations du Groupe de travail dans le contexte du débat actuel parmi les juristes québécois sur la « déjuridicisation », la déjudiciarisation et le droit préventif. Il soulève également la critique de notre système de justice évoquée par les tenants de la philosophie postmoderne. Le texte offre en conclusion quelques suggestions pour une reconceptualisation de l'accessibilité à la justice discutées au sein du Groupe de travail mais qui n'ont pas été traduites en recommandations formelles.The Report of the Quebec Task Force on Accessibility to Justice entitled Steps Toward a Greater Accessibility to Justice was submitted in September 1991. Its various recommendations treated subjects as disparate as legal aid, pre-paid legal insurance, the small claims court, alternative dispute resolution, legal information and education, and the special needs of non-mainstream groups such as cultural communities and aboriginal peoples. This essay situates the various recommendations of the Task Force within the larger debate about dejuridicization, dejudi-cialization and preventive law which is animating Quebec jurists today. It also raises certain challenges to our current conception of legal justice flowing from the post-modern critique of society. The essay concludes with a number of suggestions for broadening our conception of access to justice which were discussed by the Task Force but which were not the object of any formal recommendations