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A. Trautsamwieser:
"Estimation of time series models with Lasso type algorithms";
Betreuer/in(nen): W. Scherrer; Institut für Wirtschaftsmathematik, 2008; Abschlussprüfung: 10/2008.

This thesis deals mainly with the theory of variable selection in regression which is particularly important for model interpretation. Nowadays we are confronted with an enormous amount of data due to modern computer science. This is one reason for neglecting the common Ordinary Least Squares solution, which usually provides
a good prediction mean squared error, but is not interpretable as it includes all available predictors.
We considered some methods which offer a good interpretation on the one hand, and a satisfiable prediction accuracy on the other hand. All of them solve constrained regression problems. We examined the different methods both theoretically and practically, the latter by comparing the methods on times series data.
This work is organised as follows. In Chapter 1 some preliminaries are stated. Then, in Chapter 2 the Lasso estimator is introduced and discussed. We propose an effcient algorithm and show some asymptotic properties which point out that the Lasso is inconsistent in variable selection if the tuning parameter λ is chosen such
that the estimation is consistent. We further present some cases in which consistent variable selection is possible and mention some other problems of the Lasso. In Chapter 3, relatives of the Lasso are described. We discuss the Adaptive Lasso, which puts data-dependent weights on the absolut size of the parameters and prove that this method is able to combine estimation consistency and variable selection consistency. Then we describe the Elastic Net procedure which solves a regression problem with a linear combination of the Ridge penalty and the Lasso penalty as constraint. This method combines the advantages of the Lasso estimator and the Ridge estimator. Afterwards we introduce the Huberised Lasso method, a robustified version of the Lasso and then we present the Marginal Bridge estimator, which uses a non convex penalty term. Thereupon, in Chapter 4 we compare the methods for the orthonormal case. Accordingly we discuss the optimal choice of the tuning parameters in Chapter 5. Last but not least we provide some empirical results in Chapter 6 and give discussions/conclusions in Chapter 7.