Browsing by Subject "long-term capability"

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Assessment of a long-term and short-term process capability in the approach of analysis of variance (ANOVA)
(2009) Czarski, Andrzej
One of the fundamental tasks of the Statistical Process Control (SPC) is the assessment of capability of a process relating to client's expectations. Relating to a client, all process capability indices are, first of all, information on technological potential of a process, so they can be a criterion respecting selection of a producer. Relating to a producer, the indices can be a proof for process improvement or an alarm signal regarding a process performance. In the industrial practice long-term and short-term capability indices are determined very often (marked adequately as Pp, Ppk and Cp, Cpk). This method enables a detailed analysis of the process performance relating to a variability. The point is, first of all, to identify a presence of a so-called variability dependent on time caused e.g. by changes in a lot of raw material, successive process settings, a service of different operators etc. In other words we are interested if a process position for the sake of an analyzed parameter undergoes statistically significant variations in time. The study is mainly of a methodological character. There is a deepened interpretation of indices of a long-term and short-term process capability based on a one-way analysis of variance (ANOVA) presented. The performed considerations are illustrated with a computational example based on data concerning heat treatment of drop forgings. The better, more complete understanding the method of the assessment of a long-term and short-term process capability should facilitate the Users a correct interpretation of results and enable to avoid mistakes connected with possible not meeting the method assumptions necessary for use.