kontrolllimits

Kontrollimits, or control limits, are a fundamental concept in statistical process control (SPC). They are lines on a control chart that represent the upper and lower boundaries of acceptable variation for a process. These limits are not arbitrary specifications but are calculated directly from the data generated by the process itself.

The primary purpose of control limits is to distinguish between common cause variation and special cause variation. Common cause variation, also known as random variation, is inherent in any process and is typically due to many small, unpredictable factors. Special cause variation, on the other hand, arises from specific, identifiable events or assignable causes that are outside the normal functioning of the process.

When a process is in statistical control, all data points fall within the control limits, and there are no discernible patterns indicating special causes. If a data point falls outside the control limits, or if there are non-random patterns within the limits, it signals that a special cause is likely present and requires investigation. Identifying and eliminating special causes is crucial for improving process stability and predictability.

Control limits are typically calculated at three standard deviations above and below the process average. This range encompasses approximately 99.73% of the data if the process is stable and normally distributed. The calculation method can vary depending on the type of data (e.g., variable data or attribute data) and the specific control chart used.