1225089
Defining Explanation in Probabilistic Systems
As probabilistic systems gain popularity and are coming into wider use, the need for a mechanism that explains the system's findings and recommendations becomes more critical. The system will also need a mechanism for ordering competing explanations. We examine two representative approaches to explanation in the literature-- one due to Gardenfors and one due to Pearl--and show that both suffer from significant problems. We propose an approach to defining a notion of "better explanation" that combines some of the features of both together with more recent work by Pearl and others on causality.
8145017
Characterizing the Common Prior Assumption
Abstract Logical characterizations of the common prior assumption (CPA) are investigated. Two approaches are considered. The first is called frame distinguishability , and is similar in spirit to the approaches considered in the economics literature. Results similar to those obtained in the economics literature are proved here as well, namely, that we can distinguish finite spaces that satisfy the CPA from those that do not in terms of disagreements in expectation. However, it is shown that, for the language used here, no formulas can distinguish infinite spaces satisfying the CPA from those that do not. The second approach considered is that of finding a sound and complete axiomatization. Such an axiomatization is provided; again, the key axiom involves disagreements in expectation. The same axiom system is shown to be sound and complete both in the finite and the infinite case. Thus, the two approaches to characterizing the CPA behave quite differently in the case of infinite spaces. Journal of Economic Literature Classification Numbers: C70, D80.
23622807
Time to publication: a progress report
The process of submitting a paper to a journal can be incredibly frustrating for authors. Practically everyone who has been active in research for more than a few years has either first-or secondhand experience with some real horror stories. One of my goals when I took over JACM was to ease some of these frustrations, by speeding up the time to publication. I'd like to give you a sense of our progress in reaching that goal. Traditionally, a paper submitted to JACM went through many steps: the paper was submitted to the editor-in-chief; the editor-in-chief sent it to an editor; the editor sent it to referees; the paper went through one or more iterations between the referees, editor, editor-in-chief, and author(s); if the paper was accepted, it went on to the publication queue; authors received galleys and corrected them; and the paper finally appeared. As a result, the median time from submission to final acceptance for papers published in 1997 was 20 months. I would like to cut this down to 12 months. Often the most frustrating experience for authors is to submit a paper and then, after more than a year of waiting, to discover that it has been rejected. I don't have reliable statistics yet for how long such papers were in the system, but I would like to see significant progress here too. I would expect that the median time in the system for papers that are ultimately rejected will be 4 months. My predecessor, Tom Leighton, focused on what was the major cause of delay—the amount of time an accepted paper waited in the publication queue before it appeared. This wait used to be measured in years. The fact that the mean time to publication for 1997 was 20 months already shows how significant the improvement has been here. Currently (April 1998) the JACM web site shows that there are 19 papers in the queue, which means that the wait is approximately 3 issues, or 6 months. Since it is considered dangerous for an editor-in-chief to have too small a backlog, this is about the best that can be expected as far as the publication queue, at least until JACM goes completely electronic. Note, however, that final papers submitted in postscript are posted on the web site within a few days of submission; the papers on the queue are available to the world almost immediately. What …
16360228
Least expected cost query optimization: an exercise in utility
We identify two unreasonable, though standard, assumptions made by database query optimizers that can adversely affect the quality of the chosen evaluation plans. One assumption is that it is enough to optimize for the expected case---that is, the case where various parameters (like available memory) take on their expected value. The other assumption is that the parameters are constant throughout the execution of the query. We present an algorithm based on the ``System R''-style query optimization algorithm that does not rely on these assumptions. The algorithm we present chooses the plan of the least expected cost instead of the plan of least cost given some fixed value of the parameters. In execution environments that exhibit a high degree of variability, our techniques should result in better performance.
10886930
Belief Revision with Unreliable Observations
Research in belief revision has been dominated by work that lies firmly within the classic AGM paradigm, characterized by a well-known set of postulates governing the behavior of "rational" revision functions. A postulate that is rarely criticized is the success postulate: the result of revising by an observed proposition ? results in belief in ?. This postulate, however, is often undesirable in settings where an agent's observations may be imprecise or noisy. We propose a semantics that captures a new ontology for studying revision functions, which can handle noisy observations in a natural way while retaining the classical AGM model as a special case. We present a characterization theorem for our semantics, and describe a number of natural special cases that allow ease of specification and reasoning with revision functions. In particular, by making the Markov assumption, we can easily specify and reason about revision.
7572747
Set-theoretic completeness for epistemic and conditional logic
The standard approach to logic in the literature in philosophy and mathematics, which has also been adopted in computer science, is to define a language (the syntax), an appropriate class of models together with an interpretation of formulas in the language (the semantics), a collection of axioms and rules of inference characterizing reasoning (the proof theory), and then relate the proof theory to the semantics via soundness and completeness results. Here we consider an approach that is more common in the economics literature, which works purely at the semantic, set-theoretic level. We provide set-theoretic completeness results for a number of epistemic and conditional logics, and contrast the expressive power of the syntactic and set-theoretic approaches.
52151232
Knowledge and common knowledge in a distributed environment
Reasoning about knowledge seems to play a fundamental role in distributed systems. Indeed, such reasoning is a central part of the informal intuitive arguments used in the design of distributed protocols. Communication in a distributed system can be viewed as the act of transforming the system's state of knowledge. This paper presents a general framework for formalizing and reasoning about knowledge in distributed systems. It is shown that states of knowledge of groups of processors are useful concepts for the design and analysis of distributed protocols. In particular, distributed knowledge corresponds to knowledge that is “distributed” among the members of the group, while common knowledge corresponds to a fact being “publicly known.” The relationship between common knowledge and a variety of desirable actions in a distributed system is illustrated. Furthermore, it is shown that, formally speaking, in practical systems common knowledge cannot be attained. A number of weaker variants of common knowledge that are attainable in many cases of interest are introduced and investigated.
2160332
Decision Procedures and Expressiveness in the Temporal Logic of Branching Time
We consider the computation tree logic (CTL) proposed in (Sci. Comput. Programming 2 (1982), 241–260) which extends the unified branching time logic (UB) of (“Proc. Ann. ACM Sympos. Principles of Programming Languages, 1981,” pp. 164–176) by adding an until operator. It is established that CTL has the small model property by showing that any satisfiable CTL formulae is satisfiable in a small finite model obtained from the small “pseudomodel” resulting from the Fischer-Ladner quotient construction. Then an exponential time algorithm is given for deciding satisfiability in CTL, and the axiomatization of UB given in ibid. is extended to a complete axiomatization for CTL. Finally, the relative expressive power of a family of temporal logics obtained by extending or restricting the syntax of UB and CTL is studied.
26397523
Modelling knowledge and action in distributed systems
We present a formal model that captures the subtle interaction between knowledge and action in distributed systems. We view a distributed system as a set ofruns, where a run is a function from time toglobal states and a global state is a tuple consisting of anenvironment state and alocal state for earch process in the system. This model is a generalization of those used in many previous papers.Actions in this model are associated with functions from global states to global states. Aprotocol is a function from local states to actions. We extend the standard notion of a protocol by definingknowledge-based protocols, ones in which a process' actions may depend explicitly on its knowledge. Knowledge-based protocols provide a natural way of describing how actions should take place in a distributed system. Finally, we show how the notion of one protocolimplementing another can be captured in our model.
53236076
On the relationship between strand spaces and multi-agent systems
Strand spaces are a popular framework for the analysis of security protocols. Strand spaces have some similarities to a formalism used successfully to model protocols for distributed systems, namely multi-agent systems. We explore the exact relationship between these two frameworks here. It turns out that a key difference is the handling of agents, which are unspecified in strand spaces and explicit in multi-agent systems. We provide a family of translations from strand spaces to multi-agent systems parameterized by the choice of agents in the strand space. We also show that not every multi-agent system of interest can be expressed as a strand space. This reveals a lack of expressiveness in the strand-space framework that can be characterized by our translation. To highlight this lack of expressiveness, we show one simple way in which strand spaces can be extended to model more systems.
11366551
A Knowledge-Based Framework for Belief Change, Part II: Revision and Update
Abstract The study of belief change has been an active area in philosophy and AI. In recent years two special cases of belief change, belief revision and belief update, have been studied in detail. In a companion paper [FH94b] we introduced a new framework to model belief change. This framework combines temporal and epistemic modalities with a notion of plausibility, allowing us to examine the changes of beliefs over time. In this paper we show how belief revision and belief update can be captured in our framework. This allows us to compare the assumptions made by each method and to better understand the principles underlying them. In particular, it allows us to understand the source of Gardenfors' triviality result for belief revision [Gar86] and suggests a way of mitigating the problem. It also shows that Katsuno and Mendelzon's notion of belief update [KM91a] depends on several strong assumptions that may limit its applicability in AI.