**Conflict of interest**

limits are of equal width, only 0.27% of the expected observations will be out of

The traditional x-chart and moving range chart represent the first option to monitor the analytical method or process over a long time. The EWAMA and Cusum are two good alternatives in those situations where it is important to detect small process shifts. The capability indices are calculated to evaluate whether the process or analytical method under study is able to provide sufficient conforming results when it is operating under statistical control. To calculate the PCIs, it is necessary to know the actual performance of the process or analytical method (using the control chart) and the customer requirements in terms of specification limits. The specification limits should be determined externally from previous knowledge of inherent process variability. However, different criteria have been proposed to fix these limits. The Cpm and Cpk indices were used as part of the control strategy. Cpk is the best option because it is not dependent on the process or method being centered. However, Cpm is more sensitive to departure from the method target than Cpk is. Independent of the criteria used to establish the specification limits, computation of the capability indices depends on the analyzed response, and their application is

The authors wish to thank Dr. José B. Fariña from University of La Laguna, Tenerife, Spain, for providing the compaction process data. This research was financed by Instituto de Salud Carlos III, Ministerio de Ciencia, and Innovación y

limited to each particular situation and is not general.

Universidades y FEDER as part of Project PI18/01380.

The results obtained showed that we cannot use the control limits as specification limits, since the method is considered "inadequate" according to the criteria proposed by Pearn and Shu [24]. However, the control limits can be used in the development of the specification limits. In this example, a level of variability 2.5% of T can be enough to declare the method capable (1.0 < Cpm < 1.33), as seen in **Table 2**. All these aspects were also analyzed using the Cpk index. The results are summarized in **Table 2**. To determine if the method meets the capability requirement, we must calculate the critical value Co for Cpk based on α risk, sample size, and C value (i.e., the required method capability) [25]. We find the critical value Co = 1.081, based on C = 1.0, α = 0.05, and sample size n = 200. When the specification limits are set at 2.5% of T value, the estimated Cpk value is 1.30 and exceeds the critical value of 1.081, demonstrating that the method meets the capability requirements. Furthermore, if the limits are increased to 5% of the T value, the Cpk increases to 2.65 with a critical value of 2.18 (based on C = 2.00; α = 0.05; n = 200), indicating that the capability is "super." The obtained quality requirements were similar in both cases. The use of Cpk is clearly preferable when the limits are not equidistant, whereas the Cpm index can be overly conservative in this scenario. In our case, the target is at the center of the specification range, and if the aim of our method is to achieve a measure close to the target value with minimum variation, then Cpm is the most sensitive capability index. Given two analytical methods with different performances (i.e., precision and accuracy) and the same method departure, a simple comparison between both Cpm is sufficient to select the better, although similar results were obtained with the Cpk index. This fact was analyzed by Oliva and Llabrés [29].

specification in the long term, and the process yield is 100%.

*Quality Control - Intelligent Manufacturing, Robust Design and Charts*

**4. Conclusions**

**Acknowledgements**

**226**

The authors declare no conflict of interest.
