31,845 research outputs found
Two-Method Planned Missing Designs for Longitudinal Research
We examine longitudinal extensions of the two-method measurement design, which uses planned missingness to optimize cost-efficiency and validity of hard-to-measure constructs. These designs use a combination of two measures: a “gold standard” that is highly valid but expensive to administer, and an inexpensive (e.g., survey-based) measure that contains systematic measurement bias (e.g., response bias). Using simulated data on four measurement occasions, we compared the cost-efficiency and validity of longitudinal designs where the gold standard is measured at one or more measurement occasions. We manipulated the nature of the response bias over time (constant, increasing, fluctuating), the factorial structure of the response bias over time, and the constraints placed on the latent variable model. Our results showed that parameter bias is lowest when the gold standard is measured on at least two occasions. When a multifactorial structure was used to model response bias over time, it is necessary to have the “gold standard” measures included at every time point, in which case most of the parameters showed low bias. Almost all parameters in all conditions displayed high relative efficiency, suggesting that the 2-method design is an effective way to reduce costs and improve both power and accuracy in longitudinal research
Global Trajectory Optimisation : Can We Prune the Solution Space When Considering Deep Space Manoeuvres? [Final Report]
This document contains a report on the work done under the ESA/Ariadna study 06/4101 on the global optimization of space trajectories with multiple gravity assist (GA) and deep space manoeuvres (DSM). The study was performed by a joint team of scientists from the University of Reading and the University of Glasgow
Cost-Bounded Active Classification Using Partially Observable Markov Decision Processes
Active classification, i.e., the sequential decision-making process aimed at
data acquisition for classification purposes, arises naturally in many
applications, including medical diagnosis, intrusion detection, and object
tracking. In this work, we study the problem of actively classifying dynamical
systems with a finite set of Markov decision process (MDP) models. We are
interested in finding strategies that actively interact with the dynamical
system, and observe its reactions so that the true model is determined
efficiently with high confidence. To this end, we present a decision-theoretic
framework based on partially observable Markov decision processes (POMDPs). The
proposed framework relies on assigning a classification belief (a probability
distribution) to each candidate MDP model. Given an initial belief, some
misclassification probabilities, a cost bound, and a finite time horizon, we
design POMDP strategies leading to classification decisions. We present two
different approaches to find such strategies. The first approach computes the
optimal strategy "exactly" using value iteration. To overcome the computational
complexity of finding exact solutions, the second approach is based on adaptive
sampling to approximate the optimal probability of reaching a classification
decision. We illustrate the proposed methodology using two examples from
medical diagnosis and intruder detection
Subspace procrustes analysis
Postprint (author's final draft
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