University of Piraeus

About: University of Piraeus is a education organization based out in Piraeus, Attiki, Greece. It is known for research contribution in the topics: Context (language use) & Computer science. The organization has 1731 authors who have published 6209 publications receiving 106699 citations.

Order Restricted Inference for Exponential Step-Stress Models

Abstract: In the context of multiple step-stress models, which is a special type of accelerated life-testing model, interest lies on the expected lifetimes of the experimental units under different stress levels. Although the expected lifetime is shortened as the stress level increases, this information has not been incorporated so far into the associated inferential procedures. For this reason, we develop here the order restricted maximum likelihood estimation (MLE) for multiple step-stress models with exponentially distributed lifetimes under Type-I, and Type-II censored sampling situations. Moreover, the existence of the unrestricted MLE for a certain stress level is conditional on observing failures at that particular stress level. Under the order restriction, MLE exist even for stress levels without observed failures, provided that these stress levels are internal. We also discuss hypothesis testing problems under order restrictions.

Measuring the Market Risk of Freight Rates: A Value-at-Risk Approach

Abstract: The fluctuation of shipping freight rates (freight rate risk) is an important source of market risk for all participants in the freight markets including hedge funds, commodity and energy producers. We measure the freight rate risk by the Value-at-Risk (VaR) approach. A range of parametric and non-parametric VaR methods is applied to various popular freight markets for dry and wet cargoes. Backtesting is conducted in two stages by means of statistical tests and a subjective loss function that uses the Expected Shortfall, respectively. We find that the simplest non-parametric methods should be used to measure freight rate risk. In addition, freight rate risk is greater in the wet cargoes markets. The margins in the growing freight derivatives markets should be set accordingly.

Human Plausible Reasoning for Intelligent Help

Abstract: This paper is about providing intelligent help to users interacting with an operating system. Its main focus is an investigation of Human Plausible Reasoning Theory (Collins & Michalski, 1989) to infer the commands the user should have typed, given what they did type. The theory has been adapted and incorporated into a prototype Intelligent Help System (IHS) for UNIX users, called RESCUER, and has been used for the generation and evaluation of hypotheses about users‘ beliefs underlying the observed users‘ actions on the UNIX file store. The hypotheses generated by RESCUER were compared to those made by human experts on the sample scripts from UNIX user sessions. The potential for Human Plausible Reasoning as a mechanism to reason about slips and misconceptions is discussed.