Μελέτη μοντέλων πολλαπλής βηματικής πίεσης (multiple step-stress models) για λογοκριμένα δεδομένα
Αγγελοπούλου, Τρυφωνία Γ.
SubjectMultilevel models (Statistics) ; Accelerated life testing -- Statistical methods ; Failure time data analysis
The subject of this dissertation is to study step-stress testing, which is a special class of accelerated life-testing. The step-stress test allows the researcher to gradually increase the stress levels at some fixed time points during the experiment so as to ensure more failures. Moreover, in order to reduce the cost of the experiment and due to time constraints, censoring is unavoidable in practice. In this dissertation we present the step stress model under different types of censoring. Specifically, we discuss the simple step stress model, viz. the model with only with two stress levels, when the available data are Type-II censored, Type-I censored, Type-II hybrid censored, Type-I hybrid censored and Progressively Type-II censored. Furthermore, we present the multiple step-stress model under Type-I, Type-II and Progressive Type-I censoring. In addition, we describe a multi sample set-up for the case of a simple step-stress experiment under exponentially distributed lifetimes with Type-I and Type-II censoring. For all the above models, we present the maximum likelihood estimators (MLEs) of the parameters by assuming a cumulative exposure model with lifetimes being exponentially distributed.