Worcester Polytechnic Institute

About: Worcester Polytechnic Institute is a education organization based out in Worcester, Massachusetts, United States. It is known for research contribution in the topics: Population & Data envelopment analysis. The organization has 6270 authors who have published 12704 publications receiving 332081 citations. The organization is also known as: WPI.

Dual-role factors in data envelopment analysis

Abstract: This paper presents a methodology for dealing with performance evaluation settings where factors can simultaneously play both input and output roles. Model structures are developed for classifying Decision-Making Units (DMUs) into three groups according to whether such a factor is behaving like an output, an input, or is in equilibrium, neither wanting to lose or gain any of the factors. We connect these ideas to those involving increasing, decreasing and constant returns to scale. Examples of factors that play this dual-role are: trainees in organizations, such as nurses, medical students, and doctoral students; awards to scholars or university departments; certain revenue—generating transactions in banks, and so on. We apply the model to the analysis of a set of university departments. In some settings, a dual-role factor may be one that can be reallocated, such as would be the case when DMUs are managed by a central authority. We develop the appropriate model structures to permit such a reallocation. W...

Piggyback server invalidation for proxy cache coherency

Abstract: We present a piggyback server invalidation (PSI) mechanism for maintaining stronger cache coherency in Web proxy caches while reducing overall costs. The basic idea is for servers to piggyback on a reply to a proxy client, the list of resources that have changed since the last access by the client. The proxy client invalidates cached entries on the list and can extend the lifetime of entries not on the list. This continues our prior work on piggyback cache validation (PCV) where we focused on piggybacking validation requests from the proxy cache to the server. Trace-driven simulation of PSI on two large, independent proxy log data sets, augmented with data from several server logs, shows PSI provides close to strong cache coherency while reducing the request traffic compared to existing cache coherency techniques. The best overall performance is obtained when the PSI and PCV techniques are combined. Compared to the best TTL-based policy, this hybrid policy reduces the average cost (considering response latency, request messages and bandwidth) by 7–9%, reduces the staleness ratio by 82–86%, yielding a staleness ratio of 0.001. © 1998 Published by Elsevier Science B.V. All rights reserved.

Data Envelopment Analysis with Nonhomogeneous DMUs

Abstract: Data envelopment analysis (DEA), as originally proposed, is a methodology for evaluating the relative efficiencies of a set of homogeneous decision-making units (DMUs) in the sense that each uses the same input and output measures (in varying amounts from one DMU to another) In some situations, however, the assumption of homogeneity among DMUs may not apply As an example, consider the case where the DMUs are plants in the same industry that may not all produce the same products Evaluating efficiencies in the absence of homogeneity gives rise to the issue of how to fairly compare a DMU to other units, some of which may not be exactly in the same “business” A related problem, and one that has been examined extensively in the literature, is the missing data problem; a DMU produces a certain output, but its value is not known One approach taken to address this problem is to “create” a value for the missing output (eg, substituting zero, or by taking the average of known values), and use it to fill in t

Theory of a two-photon laser amplifier

Abstract: We discuss the propagation of an electromagnetic pulse through an active medium prepared in a state of inversion between two levels of the same parity. Since no electric dipole transition is possible between the chosen atomic levels, we investigate the possibility of amplification of an injected signal having a carrier frequency equal to one-half the atomic-transition frequency. We show that under suitable conditions a nonlinear atomic polarization can be generated which oscillates at the same frequency as the incident electromagnetic pulse. The coupled atom-field evolution is described by the usual self-consistent approach. When atomic relaxation effects are negligible, we derive an equation describing the spatial evolution of the energy of the propagating pulse. From this equation we characterize the threshold condition for power amplification and classify the multiple steady-state solutions of the propagation problem. The evolution of the pulse envelope through the amplifier is analyzed with the help of a hybrid computer simulation. Pulse-envelope modulation and multiple-pulse formation even in the asymptotic limit of long amplifiers are displayed.