Individual control charts in paperboard industry

The stiffness of paperboard is an important continuous variable measured in the labs of paperboard industry to evaluate the quality of the final product. The variable is approximately normal distributed, in this case study, and individual control charts to monitor the stiffness are obtained based on average moving range. Since the available sample size is small we decide to estimate robust control limits using a non-parametric method based on empirical quantiles (that performs also well under the normality of the observations), with the bootstrap procedure. The comparison of the control limits should be made based on the required accuracy. We also compare the output of the stable process (i.e., in statistical control) with the process specifications and we conclude that it is not a capable process.