Process capability indices are widely used to check quality standards both at the production level and for business activity. They consider the location and the deviation from specification limits and targets. The literature contains many contributions on this topic both in the univariate and the multivariate context. Motivated by a real semiconductor case study, we discuss the role of rational subgroups and the challenge they present in the computation of capability indices, especially when data refer to lots of products. In addition, our context involves a mix of problems: unilateral specification limit, nonsymmetric distribution of the data, evidence of data from a mixture of distributions, and the need to filter one component of the mixture. After solving the previous issues and because of the peculiar characteristics of semiconductor processes based on the so called "wafers," we contribute to the literature a proposal on how to compute capability indices in the case of heteroscedastic spatial processes. With a generalized additive model, we show that it is possible to estimate a capability surface that allows the identification of regions expected to not be fully compliant with the desired quality standards. K E Y W O R D S dry-etching semiconductor processes, mixture distributions, process capability indices, sampling network 1 | INTRODUCTION Process capability indices (PCIs) are well-known tools to estimate the mean-deviance performance of key product characteristics with respect to both targets and specification limits. The main aim of PCIs was originally to evaluate the instant quality of a process given specification limits and prior to manufacturing. Next, because of their simple interpretation, they have also become a fundamental tool for commercial activity. Nowadays, PCIs are unavoidable instruments for managers and engineers; hence, when assumptions upon which these indices are grounded can be violated, much effort has been made to extend and adapt them to such situations (see Montgomery 1). A flow of contributions is available in the literature for both univariate and multivariate cases. The capability indices that researchers have proposed are so many that, since the masterful work of Kotz and Johnson, 2 the word "avalanche" has been coined. A detailed and somewhat critical review on PCIs is available in Kotz and Lovelace, 3 whereas Vännman 4 proposes a unified approach. A thorough review of contributions is reported in de-Felipe and Benedito 5 and Bong-Jin and Kwan-Woo 6 among others). Most of the papers focus on the assumption of normality of the random components, but how capability indices could be efficiently estimated when non normal distributions are involved remains