Scale-space theory: a basic tool for analyzing structures at different scales

An inherent property of objects in the world is that they only exist as meaningful entities over certain ranges of scale. If one aims to describe the structure of unknown real-world signals, then a multi-scale representation of data is of crucial importance. This paper gives a tutorial review of a special type of multi-scale representation—linear scale-space representation—which has been developed by the computer vision community to handle image structures at different scales in a consistent manner. The basic idea is to embed the original signal into a one-parameter family of gradually smoothed signals in which the fine-scale details are successively suppressed. Under rather general conditions on the type of computations that are to be performed at the first stages of visual processing, in what can be termed ‘the visual front-end’, it can be shown that the Gaussian kernel and its derivatives are singled out as the only possible smoothing kernels. The conditions that specify the Gaussian kernel are, basically, linearity and shift invariance, combined with different ways of formalizing the notion that structures at coarse scales should correspond to simplifications of corresponding structures at fine scales-they should not be accidental phenomena created by the smoothing method. Notably, several different ways of choosing scale-space axioms give rise to the same conclusion. The output from the scale-space representation can be used for a variety of early visual tasks; operations such as feature detection, feature classification and shape computation can be expressed directly in terms of (possibly non-linear) combinations of Gaussian derivatives at multiple scales. In this sense the scale-space representation canserve as a basis for early vision. During the last few decades, a number of other approaches to multiscale representations have been developed, which are more or less related to scale-space theory, notably the theories of pyramids, wavelets and multi grid methods.Despite their qualitative differences, the increasing propularity of each of these approaches indicates that the crucial notion of scale is increasingly appreciated by the computer.vision community and by researchers in other related fields. An interesting similarity to biological vision is that the scale-space operators closely resemble receptive field profiles registered in neurophysiological studies of the mam- malian retina and visual cortex.     

‘Something sensational…* The sexual diary as a tool for mapping detailed sexual behaviour

Reliable information about the detail and context of sexual behaviour is crucial for describing high-risk behaviours for the transmission of Aids/HIV, and for estimating parameters of epidemiological models. It is possible to adapt the method of systematic diary-keeping to this end and develop a tool which is also close to the subject's conceptions of the ‘chunks’ of sexual behaviour. The structure of reports of sexual behaviour is viewed as akin to linguistic structure, with a basic unit of the sexual act built into a ‘session’. Such structure leads naturally to a coding system which allows natural language accounts to be represented symbolically, and parsed and analysed formally. The coded version is easily learnt and used, and is incomprehensible to outsiders. Examples are given of its use to collect systematic data on homosexual and bisexual behaviour.     

A case of mistaken identity? Debating the dilemmas of street-level bureaucracy with Musil et al.

Lipsky's Street-level Bureaucracy (Russell Sage Foundation, New York, 1980 Lipsky, M. 1980. Street-level Bureaucracy: The Dilemmas of Individuals in Public Service, New York: Russell Sage Foundation.  [Google Scholar]) has exerted a strong influence on the study of public service organisations. There has been a growing interest in using this perspective to understand the organisational context of social work and Musil et al.'s article in the EJSW (2004, ‘Do social workers avoid the dilemmas of work with clients?’, European Journal of Social Work, vol. 7, no. 3, pp. 305–319) reflects this interest. Musil et al. argue that it is possible to identify two forms of practitioner response to the constraints of street-level bureaucracies: adapting working practices in ways that disadvantage service users or challenging working conditions in order to achieve more professionally acceptable practice. Their contribution to the debate is helpful, particularly with regard to their identification of responses by practitioners that seek to advance the interests of service users. However, we view their approach as constrained by lack of consideration of the construction of social work roles within particular street-level bureaucracies. We suggest that Lipsky's work should be approached as a tentative analytic framework, rather than as a fixed model and we argue that a productive approach to research on social workers as street-level bureaucrats is conjunctural analysis. Such analysis examines the contexts, circumstances and statuses of practitioners and how these factors shape the specific forms of street-level practice that operate in particular organisational settings. We illustrate this in our discussion of the factors that are likely to have had a bearing on the two practice settings used as case studies by Musil et al.     

Staffing a Multiskilled Workforce with Varying Levels of Productivity: An Analysis of Cross-training Policies*

Service operations that utilize cross-trained employees face complex workforce staffing decisions that have important implications for both cost and productivity. These decisions are further complicated when cross-trained employees have different productivity levels in multiple work activity categories. A method for policy analysis in such environments can be beneficial in determining low-cost staffing plans with appropriate cross-training configurations. In this paper, we present an integer linear programming model for evaluating cross-training configurations at the policy level. The objective of the model is to minimize workforce staffing costs subject to the satisfaction of minimum labor requirements across a planning horizon of a single work shift. The model was used to evaluate eight cross-training structures (consisting of 36 unique cross-training configurations) across 512 labor requirement patterns. These structures, as well as the labor requirement patterns, were established based on data collected from maintenance operations at a large paper mill in the United States. The results indicate that asymmetric cross-training structures that permit chaining of employee skill classes across work activity categories are particularly useful.     

Statistical considerations for design and analysis of stability,comparability and formulation tests

This article considers designed experiments for stability, comparability, and formulation testing that are analyzed with regression models in which the degradation rate is a fixed effect. In this setting, we investigate how the number of lots, the number of time points and their locations affect the precision of the entities of interest, leverages of the time points, detection of non-linearity and interim analyses. This investigation shows that modifying time point locations suggested by ICH for stability studies can significantly improve these objectives. In addition, we show that estimates of precision can be biased when a regression model that assumes independent measurements is used in the presence of within-assay session correlation. This bias can lead to longer shelf life estimates in stability studies and loss of power in comparability studies. Mixed-effect models that take into account within-assay session correlation are shown to reduce this bias. The findings in this article are obtained from well known statistical theory but provide valuable practical advice to scientists and statisticians designing and interpreting these types of experiments.