Chi-square and Poissonian Data: Biases Even in the High-Count Regime and How to Avoid them

We demonstrate that two approximations to the chi^2 statistic as popularly employed by observational astronomers for fitting Poisson-distributed data can give rise to intrinsically biased model parameter estimates, even in the high counts regime, unless care is taken over the parameterization of the problem. For a small number of problems, previous studies have shown that the fractional bias introduced by these approximations is often small when the counts are high. However, we show that for a broad class of problem, unless the number of data bins is far smaller than \sqrt{N_c}, where N_c is the total number of counts in the dataset, the bias will still likely be comparable to, or even exceed, the statistical error. Conversely, we find that fits using Cash's C-statistic give comparatively unbiased parameter estimates when the counts are high. Taking into account their well-known problems in the low count regime, we conclude that these approximate chi^2 methods should not routinely be used for fitting an arbitrary, parameterized model to Poisson-distributed data, irrespective of the number of counts per bin, and instead the C-statistic should be adopted. We discuss several practical aspects of using the C-statistic in modelling real data. We illustrate the bias for two specific problems, measuring the count-rate from a lightcurve and obtaining the temperature of a thermal plasma from its X-ray spectrum measured with the Chandra X-ray observatory. In the context of X-ray astronomy, we argue the bias could give rise to systematically mis-calibrated satellites and a ~5-10% shift in galaxy cluster scaling relations.Comment: 9 pages, 2 figures. Accepted for publication in the Astrophysical Journa

A Broader Approach to Personalization

this article we describe an architecture which overcomes these problems by making use of standards. A log of user's interactions with his/her Web browser is kept in an XML file and a profile of the user that reflects the user's interests and preferences, is automatically and dynamically obtained from this log file. Privacy of the user is preserved through P3P. Any server that a user contacts is able to interpret the user profile and can also maintain like minded user groups again because the information that they receive is highly interoperable. When the meta data of the resources are expressed in RDF it will be a lot easier for agents to discover the resources on the Web that match user profiles. Currently meta data tags of HTML is used for this purpose

A Component-based Workflow System with Dynamic Modifications

. Adapting to changes in its environment dynamically is a very important aspect of workflow systems. In this paper, we propose a component-based workflow system architecture specifically designed for this purpose. To allow for easy modification of workflow instances, an instance is designed as an object that contains all the necessary data and control information as well as its execution history. This feature facilitates to dynamically modify the process definition on instance basis at run time. The system is designed to consist of functional components like, Basic Enactment Service, History Manager, Workflow Monitoring Tool, Dynamic Modification Tool, etc. The clients of the system are coded as network-transportable applets written in Java so that the end user can activate workflow system components by connecting to the Workflow Domain Manager over the Internet. In this paper we also present a workflow process definition language FLOWDL , its graphical representation FLOWGRAPH and a ..

An Adaptable Workflow System Architecture on the Internet for Electronic Commerce Applications

An electronic commerce (EC) process is a business process and defining it as a workflow provides all the advantages that come with this technology. Yet electronic commerce processes place certain demands on the workflow technology like the distribution of the load of the workflow engine to multiple servers, dynamic modification of workflows for adaptability, openness and availability. In this paper we propose a workflow system architecture to address these issues. The componentwise architecture of the system makes it possible to incorporate the functionality and thus the complexity only when it is actually needed. The infrastructure of the system is based on CORBA 2.0 where methods are invoked through XML. The clients of the system are coded as network transportable applets written in Java so that the end user can activate workflow components through the workflow domain manager over the network. The system provides high availability by replicating the component server repository and th..