The exponentially weighted moving average (EWMA) ̅ chart with the variable-samplinginterval (VSI) feature is usually scrutinized under the assumption of known process parameters. However, in practice, process parameters are usually unknown and they need to be estimated from the in-control Phase-I dataset. With this in mind, this paper proposes the VSI EWMA ̅ chart where the process parameters are estimated. A Markov Chain approach is adopted to derive the run-length properties of the VSI EWMA ̅ chart with estimated process parameters.The standard deviation of the average time to signal (SDATS) is employed to measure the practitioner-to-practitioner variation in the control chart's performance. This variation occurs because different Phase-I datasets are used among practitioners to estimate the process parameters. Based on the SDATS criterion, this paper provides recommendations regarding the minimum number of required Phase-I samples. For an optimum implementation, this paper develops two optimization algorithms for the VSI EWMA ̅ chart with estimated process parameters, i.e. by minimizing the (i) out-of-control AATS (expected value of the average time to signal) and (ii) out-of-control EAATS (expected value of the AATS), for the cases of deterministic and unknown shift sizes, respectively. With the implementation of these new design procedures, the VSI EWMA ̅ chart with estimated process parameters is not only able to achieve a desirable in-control performance, but it is also able to quickly detect changes in the process. Keywords: expected value of the average time to signal, known and unknown shift sizes, optimization design, parameter estimation, standard deviation of the average time to signal, standard deviation of the time to signal