The paper deals with attributes control charts, especially with the control charts for fraction nonconforming, and their use in retrospective analysis of a process. Several forms of these charts are applied in a case study. The study concerns the occurrence of defects, especially the deformations on hot-rolled wire manufactured on the wire rod mill at Arcelor Mittal Ostrava a.s. Number of wire coils with defects is of interest. P charts are used to identify subgroups with an inflated fraction nonconforming that could distort results of evaluating differences in fraction nonconforming between steel grades, hot-rolling temperatures, and wire diameters. In addition, it is shown how to modify the traditional p chart in the case when a substantial part of the points fall outside control limits due to large subgroup sizes.Keywords: Outliers, p chart, Laney’s chart, hot-rolled wire, deformations
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