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In this paper, step-stress partially accelerated life tests (SS-PALT) are considered when the lifetime of a product follows a two-parameter bathtub-shaped lifetime distribution (Chen distribution). Based on progressive Type-II censoring, the maximum likelihood estimates (MLEs) are obtained for the distribution parameters and acceleration factor. In addition, asymptotic variance and covariance matrix of the estimators are given. Approximate confidence intervals (CIs) for the parameters based on normal approximation to the asymptotic distribution of the MLEs and the bootstrap (CIs) are derived. An iterative procedure is used to obtain the estimators numerically using (Mathematica Package). A numerical example is presented to illustrate the method of estimation developed here. Finally, a Monte Carlo simulation study is performed to investigate the precision of the MLEs and to compare the CIs considered.

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