This research investigates the statistical efficiency of variable sampling size & sampling interval(VSSI) $ar{X}$ charts under two assignable causes. Algorithms for calculating the average run length(ARL) and average time to signal(ATS) of the VSSI $ar{X}$ chart are proposed by employing Markov chain method. States of the process are defined according to the process characteristics after the occurrence of an assignable cause. Transition probabilities are carefully derived from the state definition. Statistical properties of the proposed chart are also investigated. A simple procedure for designing the proposed chart is presented based on the properties. Extensive sensitivity analyses show that the VSSI $ar{X}$ chart is superior to the VSS or VSI $ar{X}$ chart as well as to the Shewhart $ar{X}$ chart in statistical sense, even tinder two assignable causes.