ZüKoSt: Seminar on Applied Statistics

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Autumn Semester 2011

Date / Time

Speaker

Title

Location

27 October 2011
16:15-17:30

Tanja Stadler
ETH Zürich, Institute of Integrative Biology

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ZüKoSt Zürcher Kolloquium über Statistik

Title	Recovering macroevolutionary processes using phylogenetic methods
Speaker, Affiliation	Tanja Stadler, ETH Zürich, Institute of Integrative Biology
Date, Time	27 October 2011, 16:15-17:30
Location	HG G 19.1
Abstract	Phylogenetic trees of present-day species allow the inference of the rate of speciation and extinction which led to the present-day diversity. Classically, inference methods assume a constant rate of diversification. I will present a new inference methodology which can estimate changes in diversification rates through time, can detect mass extinction events, and can account for density-dependent speciation. The method is based on an in-depth analysis of a birth-death process with birth and death parameters being a function of time and / or the number of alive individuals. I use the method for testing the hypothesis of accelerated mammalian diversification following the extinction of the dinosaurs (65 Ma); none of the analyzed mammalian phylogenies showed a change in diversification rates at 65 Ma. Application of the method to bird data (Dendroica) reveals a density-dependent speciation process, agreeing with previous studies. The new method further allows to quantify the extinction rate which is estimated to be significantly larger than zero for these birds. The methods can easily be applied to other phylogenies using the R package TreePar available on CRAN.

Recovering macroevolutionary processes using phylogenetic methodsread_more

HG G 19.1

3 November 2011
16:15-17:30

Niel Hens
Universität Hasselt

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ZüKoSt Zürcher Kolloquium über Statistik

Title	The Statistical Analysis of Serial Seroprevalence and Final Size Data to Estimate Infectious Disease Parameters for Hepatitis A in Belgium
Speaker, Affiliation	Niel Hens, Universität Hasselt
Date, Time	3 November 2011, 16:15-17:30
Location	HG G 19.1
Abstract	Hepatitis A is one of the most common vaccine-preventable infectious diseases causing significant though usually self-limiting morbidity and mortality (especially in developing country settings). Vaccination of individuals implemented for more than ten years according to various, mostly targeted strategies, together with improved sanitary conditions, have contributed to a substantial reduction of the economic burden associated with disease management. We aim to document and analyse the evolving epidemiology in Belgium, and use it as an example to demonstrate novel methods for the estimation of infectious disease parameters, while accounting for vaccine-unrelated time heterogeneity and vaccine uptake. Using two age-specific seroprevalence datasets from 1993 and 2002, respectively, we show how to estimate important epidemiological parameters in a time heterogeneous setting. More specifically, using a semi-parametric proportional hazards model we show how the time heterogeneous transmission parameters and consequently the basic reproduction number can be estimated. We supplement the analysis of serial seroprevalence data with an analysis of final size data on a series of recent hepatitis A clusters. In the absence of knowledge about the number of initial cases, several authors inferred the effective reproduction number based on final size data. We extend these approaches taking into account data complexities such as truncation, censoring and heterogeneity. Moreover, using a spatial analysis we are able to link these results to the results obtained from analysing serial seroprevalence data. The basic reproduction number has been shown to decrease over the past few decades and is currently estimated at about 1. The effective reproduction number is estimated to be smaller than one, even in provinces where this number is relatively higher due to a greater presence of second-generation immigrants, who maintain strong links with HAV endemic countries. In conclusion, hepatitis A is no longer endemic in Belgium likely due to improved sanitary conditions. This changing situation also indicates that susceptible people, especially children, who travel to countries where hepatitis A is still endemic should be preferred recipients of the vaccine.

The Statistical Analysis of Serial Seroprevalence and Final Size Data to Estimate Infectious Disease Parameters for Hepatitis A in Belgiumread_more

HG G 19.1

17 November 2011
16:15-17:30

Erik van Zwet
Universität Leiden

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ZüKoSt Zürcher Kolloquium über Statistik

Title	Introduction to Causal Inference
Speaker, Affiliation	Erik van Zwet, Universität Leiden
Date, Time	17 November 2011, 16:15-17:30
Location	HG G 19.1
Abstract	Researchers often want to know if one thing causes another. Statisticians respond that it is possible to test for association, but that association does not imply causation -- at least, not without further assumptions. Causal inference is about exploring what happens if we are willing to make such additional assumptions. Unfortunately, with its particular notation and terminology, causal inference seems very different from standard statistics. Judea Pearl, who wrote a book on causality, even states: "Almost by definition, causal and statistical concepts do not mix". I respectfully disagree, and in this talk I will argue that causal and statistical concepts mix very well.

Introduction to Causal Inferenceread_more

HG G 19.1

1 December 2011
16:15-17:30

Steffen Unkel
Open University, UK

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ZüKoSt Zürcher Kolloquium über Statistik

Title	On assessing time-varying association for shared frailty models with bivariate current status data
Speaker, Affiliation	Steffen Unkel, Open University, UK
Date, Time	1 December 2011, 16:15-17:30
Location	HG G 19.1
Abstract	The relative frailty variance among survivors provides a readily interpretable measure of how the heterogeneity of a population, as represented by a frailty model, evolves over time. In the first part of this talk, a new measure for assessing the temporal variation in the strength of association in bivariate current status data is proposed. This novel measure is relevant for shared frailty models. We show that this measure is particularly convenient,owing to its connection with the relative frailty variance and its interpretability in suggesting appropriate frailty models. We introduce a method of estimation and standard errors for this measure. We discuss its properties and compare it to two existing measures of association applicable to current status data. Small sample performance of the measure in realistic scenarios is investigated using simulations. In the second part of this talk, we investigate the possible shapes of the relative frailty variance function for the purpose of model selection, and review available frailty distribution families in this context. Several new families of frailty distributions are introduced, including simple but flexible time-varying frailty models. The methods are illustrated with bivariate serological survey data on different pairs of infections.

On assessing time-varying association for shared frailty models with bivariate current status dataread_more

HG G 19.1

8 December 2011
16:15-17:30

Marcel Dettling
Zürcher Hochschule für angewandte Wissenschaften, Winterthur

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ZüKoSt Zürcher Kolloquium über Statistik

Title	Modellierung von Schadstofffrachten in Gewässern um den Sempachersee
Speaker, Affiliation	Marcel Dettling, Zürcher Hochschule für angewandte Wissenschaften, Winterthur
Date, Time	8 December 2011, 16:15-17:30
Location	HG G 19.1
Abstract	Im Kanton Luzern wird der Schadstoffeintrag in den Sempachersee überwacht. Dazu werden ständige Durchflussmessungen vorgenommen, während die Schadstoffkonzentrationen nur periodisch gemessen werden. Die nicht beobachteten Konzentrationen werden nach statistischer Analyse mit einem Regressionsansatz geschätzt, und schliesslich zu einer totalen Fracht aggregiert. Dabei machte das von Gewässer- und Bodenfachleuten ursprünglich verwendete nichtlineare Regressionsmodell "Probleme". Es musste in einem ersten Schritt durch Startwertschätzung, Einschränkung von Parameterbereichen und einer Entkoppelung durch Reparametrisierung einsatzfähig getrimmt werden. Ebenso wurde darauf ein neuartiger zweiter Ansatz entwickelt, welcher mit einer einfachen Heuristik das vorliegende Anwendungsproblem "linearisiert". Dies mindert einerseits die technisch-mathematischen Herausforderungen, und erlaubt andererseits das Einbeziehen von zusätzlichen Einflussgrössen, sowie eine auf Bootstrap basierende Fehlerrechnung. Im Vortrag werden die beiden Ansätze einander gegenübergestellt und verglichen. Ebenso werden die Herausforderungen in der praktischen Umsetzung aufgezeigt und die vorgenommenen Verbesserungen in kurzen Theorieblöcken erläutert.

Modellierung von Schadstofffrachten in Gewässern um den Sempacherseeread_more

HG G 19.1

Notes: if you want you can subscribe to the iCal/ics Calender.

Organisers: Peter Bühlmann, Leonhard Held, Markus Kalisch, Hansruedi Künsch, Martin Mächler, Werner Stahel, Sara van de Geer, Michael Wolf

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