1,554 research outputs found
Using Prior Knowledge and Learning from Experience in Estimation of Distribution Algorithms
Estimation of distribution algorithms (EDAs) are stochastic optimization techniques that explore the space of potential solutions by building and sampling explicit probabilistic models of promising candidate solutions. One of the primary advantages of EDAs over many other stochastic optimization techniques is that after each run they leave behind a sequence of probabilistic models describing useful decompositions of the problem. This sequence of models can be seen as a roadmap of how the EDA solves the problem. While this roadmap holds a great deal of information about the problem, until recently this information has largely been ignored. My thesis is that it is possible to exploit this information to speed up problem solving in EDAs in a principled way. The main contribution of this dissertation will be to show that there are multiple ways to exploit this problem-specific knowledge. Most importantly, it can be done in a principled way such that these methods lead to substantial speedups without requiring parameter tuning or hand-inspection of models
Topics in social network analysis and network science
This chapter introduces statistical methods used in the analysis of social
networks and in the rapidly evolving parallel-field of network science.
Although several instances of social network analysis in health services
research have appeared recently, the majority involve only the most basic
methods and thus scratch the surface of what might be accomplished.
Cutting-edge methods using relevant examples and illustrations in health
services research are provided
Chance and Necessity in Evolution: Lessons from RNA
The relationship between sequences and secondary structures or shapes in RNA
exhibits robust statistical properties summarized by three notions: (1) the
notion of a typical shape (that among all sequences of fixed length certain
shapes are realized much more frequently than others), (2) the notion of shape
space covering (that all typical shapes are realized in a small neighborhood of
any random sequence), and (3) the notion of a neutral network (that sequences
folding into the same typical shape form networks that percolate through
sequence space). Neutral networks loosen the requirements on the mutation rate
for selection to remain effective. The original (genotypic) error threshold has
to be reformulated in terms of a phenotypic error threshold. With regard to
adaptation, neutrality has two seemingly contradictory effects: It acts as a
buffer against mutations ensuring that a phenotype is preserved. Yet it is
deeply enabling, because it permits evolutionary change to occur by allowing
the sequence context to vary silently until a single point mutation can become
phenotypically consequential. Neutrality also influences predictability of
adaptive trajectories in seemingly contradictory ways. On the one hand it
increases the uncertainty of their genotypic trace. At the same time neutrality
structures the access from one shape to another, thereby inducing a topology
among RNA shapes which permits a distinction between continuous and
discontinuous shape transformations. To the extent that adaptive trajectories
must undergo such transformations, their phenotypic trace becomes more
predictable.Comment: 37 pages, 14 figures; 1998 CNLS conference; high quality figures at
http://www.santafe.edu/~walte
Space is the machine: a configurational theory of architecture
Since The social logic of space was published in 1984, Bill Hillier and his colleagues at University College London have been conducting research on how space features in the form and functioning of buildings and cities. A key outcome is the concept of ââŹËspatial configurationââŹâ˘ ââŹâ meaning relations which take account of other relations in a complex. New techniques have been developed and applied to a wide range of architectural and urban problems. The aim of this book is to assemble some of this work and show how it leads the way to a new type of theory of architecture: an ââŹËanalyticââŹâ˘ theory in which understanding and design advance together. The success of configurational ideas in bringing to light the spatial logic of buildings and cities suggests that it might be possible to extend these ideas to other areas of the human sciences where problems of configuration and pattern are critical
Space is the machine, part one: theoretical preliminaries
âTheoretical Preliminariesâ deals with the most basic of all questions which architectural theory tries to answer: what is architecture, and what are theories, that they can be needed in architecture? In the first chapter, âWhat architecture adds to buildingâ, the key concepts of the book are set out on the way to a definition of architecture. The argument is that in addition to functioning as bodily protection, buildings operate socially in two ways: they constitute the social organisation of everyday life as the spatial configurations of space in which we live and move, and represent social organisation as physical configurations of forms and elements that we see. Both social dimensions of building are therefore configurational in nature, and it is the habit of the human mind to handle configuration unconsciously and intuitively, in much the same way as we handle the grammatical and semantic structures of a language intuitively. Our minds are very effective in handling configuration in this way, but because we do work this way, we find it very difficult to analyse and talk rationally about the configurational aspects of things. Configuration is in general ânon-discursiveâ, meaning that we do not know how to talk about it and do not in general talk about it even when we are most actively using it. In vernacular buildings, the configurational, or non-discursive, aspects of space and form are handled exactly like the grammar of language, that is, as an implication of the manipulation of the surface elements, or words and groups of words in the language case, building elements and geometrical coordinations in building. In the vernacular the act of building reproduces cultural given spatial and formal patterns. This is why it seldom seems âwrongâ. Architecture, in contrast, is the taking into conscious, reflective thought of these non-discursive and configurational aspects of space and form, leading to the exercise of choice within a wide field of possibility, rather than the reduplication of the patterns specific to a culture. Architecture is, in essence, the application of speculative and abstract thought to the non-discursive aspects of building, and because it is so, it is also its application to the social and cultural contents of building.
Chapter 2, âThe need for an analytic theory of architectureâ, then takes this argument into architectural theory. Architectural theories are essentially attempts to subject the non-discursive aspects of space and form to rational analysis, and to establish principles to guide design in the field of choice, principles which are now needed as cultural guidance is no longer automatic as it is in a vernacular tradition. Architectural theories are both analytic in that they always depend on conjectures about what human beings are like, but they are also normative, and say how the world should be rather more strongly than they say how it is. This means that architecture can be innovative and experimental through the agency of theories, but it can also be wrong. Because theories can be wrong, architects need to be able to evaluate how good their theories are in practice, since the repetition of theoretical error - as in much of the modernist housing programme - will inevitably lead to the curtailment of architectural freedom. The consequence of this is the need for a truly analytic theory of architecture, that is, one which permits the investigation of the non-discursive without bias towards one or other specific non-discursive style.
Chapter 3, âNon-discursive techniqueâ, outlines the prime requirement for permitting architects to begin this theoretical learning: the need for neutral techniques for the description and analysis of the non-discursive aspects of space and form, that is, techniques that are not simply expressions of partisanship for a particular type of configuration, as most architectural theories have been in the past. The chapter notes a critical difference between regularities and theories. Regularities are repeated phenomena, either in the form of apparent typing or apparent consistencies in the time order in which events occur. Regularities are patterns in surface phenomena. Theories are attempts to model the underlying processes that produce regularities. Every science theorises on the basis of its regularities. Social sciences tend to be weak not because they lack theories but because they lack regularities which theories can seek to explain and which therefore offer the prime test of theories. The first task in the quest for an analytic theory of architecture is therefore to seek regularities. The first purpose of ânon-discursive techniqueâ is to pursue this task
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