PUMA Footprints: linking theory and craft skill in usability evaluation

‘Footprints’ are marks or features of a design that alert the analyst to the possible existence of usability difficulties caused by violations of design principles. PUMA Footprints make an explicit link between the theory underlying a Programmable User Model and the design principles that can be derived from that theory. While principles are widely presented as being intuitively obvious, it is desirable that they should have a theoretical basis. However, working directly with theory tends to be time-consuming, and demands a high level of skill. PUMA footprints offer a theory-based justification for various usability principles, with guidelines on detecting violations of those principles

Reasoning about order errors in interaction

Reliability of an interactive system depends on users as well as the device implementation. User errors can result in catastrophic system failure. However, work from the field of cognitive science shows that systems can be designed so as to completely eliminate whole classes of user errors. This means that user errors should also fall within the remit of verification methods. In this paper we demonstrate how the HOL theorem prover [7] can be used to detect and prove the absence of the family of errors known as order errors. This is done by taking account of the goals and knowledge of users. We provide an explicit generic user model which embodies theory from the cognitive sciences about the way people are known to act. The user model describes action based on user communication goals. These are goals that a user adopts based on their knowledge of the task they must perform to achieve their goals. We use a simple example of a vending machine to demonstrate the approach. We prove that a user does achieve their goal for a particular design of machine. In doing so we demonstrate that communication goal based errors cannot occur

Modelling rational user behaviour as games between an angel and a demon

Formal models of rational user behavior are essential for user-centered reasoning about interactive systems. At an abstract level, planned behavior and reactive behavior are two important aspects of the rational behavior of users for which existing cognitive modeling approaches are too detailed. In this paper, we propose a novel treatment of these aspects within our formal framework of cognitively plausible behavior. We develop an abstract, formal model of rational behavior as a game between two opponents. Intuitively, an Angel abstractly represents the planning aspects, whereas a Demon represents the reactive aspects of user behavior. The formalization is carried out within the MOCHA framework and is illustrated by simple examples of interactive tasks

Fiscal Decentralisation: The Swiss Case

Switzerland provides a potential laboratory for testing various hypotheses connected with tax competition because of its extremely decentralized fiscal system. Twenty-six cantons (some of them extremely small) have retained the ultimate power of deciding tax questions, and hence not only to limit the Federal level of the State to about 1/3 of total public expenditure, but also to retain absolute power to set their own levels of taxation. Furthermore, citizens can launch referenda on tax issues at any level of government.Fiscal competition between cantons should therefore be easy to spot, resulting in lower tax cantons enjoying higher economic growth (the classical hypothesis), or in a race to the bottom with lower tax cantons offering a lower level of public services (the harmful competition hypothesis). In fact, neither of these hypotheses is confirmed. This leads the author to suggest a third hypothesis based on public choice theory (politicians in high-income cantons are led to raise high taxes). The author suggests that the classical and the public choice hypotheses cancel each other out, leaving a mixed picture. The article ends with some reflexions on the implications of the Swiss experience for fiscal harmonization at a European level.La Suisse offre un véritable laboratoire pour tester plusieurs hypothèses de concurrence fiscale en raison de son système fiscal extrêmement décentralisé. Vingt six cantons (dont quelques uns très petits) ont conservé le pouvoir suprême de décider les questions fiscales et, donc, non seulement de limiter le niveau fédéral de lEtat à 1/3 des dépenses publiques totales, mais aussi de conserver le pouvoir absolu pour fixer leurs propres niveaux de taxation. De plus, les habitants peuvent lancer des référendums en matière fiscale à tout niveau du gouvernement.La concurrence fiscale entre les cantons devrait être par conséquent plus facile à repérer, les cantons les moins imposés ayant une croissance économique supérieure (hypothèse classique), ou en un > face aux cantons les moins imposés offrant un niveau inférieur de services publics (hypothèse de concurrence néfaste). En fait, aucune de ces hypothèses nest confirmée. Cela conduit lauteur à suggérer une troisième hypothèse basée sur la théorie des décisions publiques (les politiciens des cantons à revenus élevés sont conduits à lever des impôts élevés). Lauteur suggère que les hypothèses classiques et celles des décisions publiques se neutralisent lune lautre, laissant place à un modèle mixte. Larticle se termine par quelques réflexions quant aux implications de lexpérience Suisse pour une harmonisation fiscale au niveau europée

The British Tax System: Opposing Trends

This article points to the highly centralized nature of the British tax system. A first section shows how all tax law derives from Parliament, the onlie begetter of legally enforceable instruments. It is suggested that this system is not democratically accountable at sub-national levels of government. Reforms of the Thatcher era have resulted in the privatization of many public services, leading to the stabilization of State expenditure as a proportion of GDP. However, at the same time, both tax revenue and expenditure have become increasingly centralized. Public service tasks are determined and financed by central government, and local government is for the most part merely the agent through which government policy is realized. The creation of the Scottish and Welsh Assemblies does not alter this overall assessment. Given the high level of centralization, taxes are uniform throughout the country and there is no scope for meaningful regional fiscal competition. Since regional economic development in Britain is notoriously uneven, the author notes that fiscal uniformity does not lead to economic convergence. A final section is devoted to showing why this is not surprising and why fiscal harmonisation at a European level would be a source of growing, not reduced, disparities.Cet article met laccent sur la nature hautement centralisée du système fiscal britannique. Une première section montre comment toutes les lois fiscales proviennent du Parlement, la source unique des instruments légaux exécutoires. Nous considérons que ce système nest pas démocrat- iquement defendable aux niveaux subnationaux de gouvernement. Les réformes de lère Thatcher ont conduit à une privatisation de beaucoup de services publics, entrainant une stabilisation des dépenses de lEtat en pourcentage du PIB. Cependant, dans le meme temps, à la fois les recettes fiscales et les dépenses sont devenues de plus en plus centralisées. Les taches du service public sont définies et financées par le gouvernement central et le gouvernement local est dans la majeure partie des cas lagent à travers lequel la politique du gouvernement sexerce. La creation des Assemblées Ecossaises et galloises ne modifie pas cette evaluation densemble. Etant donné le niveau élevé de centralisation, les impôts sont uniformes à travers le pays et il ny a pas de possibilité pour une concurrence fiscale régionale significative. Alors que le développement économique regional en Grande Bretagne est notoirement inégal, lauteur remarque que luniformité fiscale ne conduit pas à uneconvergence économique. Une section finale est consacrée à montrer pourquoi cela nest en rien surprenant et pourquoi lharmonisation fiscale à un niveau européen serait une source de disparités croissantes, et non décroissante