When a new measurement is taken in a control chart, what should it be compared to?

In a control chart, a new measurement should be compared to past performance data to assess whether the process remains stable and within control limits. This practice is fundamental for quality control and process management, as it allows organizations to identify trends, shifts, or outliers that may indicate issues in the process.

By evaluating a new measurement alongside historical data, teams can ascertain if the current performance is consistent with established patterns. If the new data point falls within the control limits defined by past performance, it suggests that the process is operating normally. Conversely, if it falls outside these limits, it signals potential problems that may require investigation.

This comparison against historical data also helps teams understand the variability of their processes over time. It enables them to recognize when variations are due to common causes, which are inherent in the process, versus special causes that require attention.

While options like expected outcomes, market standards, and operational costs may provide useful context or benchmarks for overall performance, the primary focus of a control chart is to monitor and analyze data trends against historically collected measurements within the same process.