Is the process unstable if we have a single value beyond the control limits
Process stability in SPC (Statistical Process Control) is the critical element to ensure the quality of products and services. It indicates that the process variation is “normal” and predictable, in other words we know with a high probability where the future values will fall.
The Control Chart is the fundamental tool used to monitor and analyze in real time the variation and location of production processes, providing a visual and statistical means of interpretation. The process control limits are determined using statistical methods, based on actual process data and indicate whether the process is statistically stable. They reflect the natural variation of the process, when it is stable, all points are between the control limits and no suspicious patterns appear, meaning the process is influenced only by common causes. As a rule, these limits are calculated at +-3 standard deviations from the process mean, the standard deviation being the statistical indicator that measures the variation of the values of a process.
A frequently asked question in practice is whether the occurrence of a single value outside the control limits is sufficient to consider whether the process is unstable? According to the principles and rules of SPC, the answer is “Yes”, a single observation located above or below the control limits represents a statistical signal of exit from control and instability, indicating the presence of a special cause of variation. Under these conditions, the process can no longer be considered in statistical control and requires investigation of the cause that generated the deviation. Widely used in industry, it considers as the first non-control criterion, known as “Rule 1” or “Beyond Control Limits” exactly the situation in which there is at least one point outside the control limits.
However, a single value outside the control limits does not necessarily mean that the process constantly produces non-conforming products or that a major intervention is necessary, it indicates that, at that time, there was a special cause that must be analyzed, understood and corrected. There are situations where the deviation may be the result of an isolated event, perhaps a setting error, a temporary change in the material used, an operator intervention or an equipment problem, but under no circumstances should it be ignored, even if "we only have one value beyond the control limits".
