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Optimum times for step-stress cumulative exposure model using log-logistic distribution for loss of glucocorticoid fast feedback in depression

S. Lakshmi, I. Christy Raj

Abstract


We assume that the life time of a test group follows a log-logistic distribution with known scale parameter. The optimum times of changing stress level for simple step-stress plans under a cumulative exposure model are obtained by minimizing the asymptotic variance of the maximum likelihood estimator of the model parameters at the design stress with respect to the change time. In the application part this study compared fast-feedback inhibition of β-endorphin secretion hydrocortisone in 16 control subjects and 16 depressed patients. Here the data suggested the decreased sensitivity to glucocorticoid fast feed-back in depressed patients and complement existing studies demonstrating decreased sensitivity to proportional feedback by dexamethasone in depressed patients. The log-logistic distribution is utilized for the corresponding medical data, and the feedback is compared with the mathematical model.

Keywords


Maximum Likelihood Estimation, Simple Step-Stress, Fisher Information Matrix, β-endorphin, cortisol, hydrocortisone.

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