An &Xmacr; chart with variable sample sizes; a new adaptive method

In the last years a new class of control charts which parameters change during production was developed. In this paper, we present a new procedure that consists in a very simple and attractive method to vary the samples size. In this procedure, which is based in the idea presented by Rodrigues Dias to obtain different sampling intervals, the size of the following sample depends on the information in the last instant of inspection, assuming different values, in a distinct form that it happens when we use the technique usually known by VSS, which considers only two values. Some statistical properties are studied using, in particular, Markov chains. Using a control chart for the sample mean, we present and analyse, in a critical way, some results obtained for different changes of the quality. A comparative study with some previously developed procedures is also presented.