Abstract
Organizational context is now accepted as a central concept in attempts to understand error in human-machine systems. However, accounts which emphasize the processes of everyday organizing, such as accountability and work activity, are needed in order to establish organizational requirements for design. In this article, we provide a framework for the consideration of organizational contexts of human error in high-consequence work systems, with a view to integrating empirical insights and supporting practical design work. We draw on computer-supported cooperative work conceptualizations of the process of everyday organizing, particularly the notion of "accountability for work activity" which is pivotal to our organizational account of error. The conceptual framework is characterized here as a set of dimensions which are expressive concerning the relationship between accountability and work activity in different contexts: (1) explicit-implicit; (2) global-local; (3) stable-transient and (4) dependent-independent. The framework is demonstrated with respect to everyday work practices in a radiology department and its analytical utility validated with respect to two documented aviation system failures. Applying the framework has enabled us to identify and define, in terms of the dimensions, a number of contexts for vulnerability in high-consequence systems: contexts for collusion, violation, deference, loss of control, buck passing and complacency. These are discussed in terms of requirements for error-tolerant design. In the final section of the article, links between the various contexts for vulnerability and the design process are explored. © 1997 Academic Press Limited.

Abstract
This paper investigates the use of formal mathematical models in the design of interactive systems and argues for the development of generic models that describe the behaviour of a class of interactive systems. In recent years a number of authors have suggested methods for modelling interactive systems using notations and frameworks drawn from software engineering mathematics. We argue that these models tend to be either: so abstract as to limit their ability to express important interaction concerns for specific systems, and limited in the degree to which they support the construction of software that conforms to the designer's intention; or so specific to an individual system that they provide only limited re-use across development projects and are therefore likely to be too expensive to develop except in a few special applications such as safety-critical systems. We argue that it is possible to construct a generic model of a class of interactive systems at an intermediate level of abstraction. Such a model would offer wider reusability than detailed specifications of a single system, but greater expressiveness and support for software development than fully general abstract models. To support our argument we review a number of existing models in the literature and present a generic model of interactive case memories, a class of systems used in case-based reasoning. © 1997 Academic Press Limited.

Abstract
The control of manufacturing processes requires the use of a communication infrastructure. Since the object-oriented model is well adapted for the modelling of industrial processes, an ORE seems the logical choice for the communication layer. This paper presents an ORE developed for this purpose, based on DCE services.

Abstract
An efficient algorithm for dynamically multiplexing MPEG2 encoded video sources is presented. Sources are grouped into classes regarding different combined levels of spatial detail and amount of movement. Simulations were performed using different associations of sources belonging to distinct classes, different bit rates and GOP structures. The implications associated to a real implementation are analysed and a modular architecture is proposed. Simulation results are presented and discussed, showing that sequences with higher spatial detail and motion are those which exhibit the higher quality improvements.

Abstract

Abstract