sigmagrenzen

Sigmagrenzen, also known as sigma limits or sigma bands, are statistical concepts used in quality control and process monitoring. They represent the upper and lower boundaries that define an acceptable range of variation for a particular process or measurement. These limits are typically calculated based on the standard deviation, a statistical measure of the dispersion of data points around the mean.

The most common application of sigmagrenzen is in control charts, such as X-bar and R charts or p-charts. These charts visually display process data over time and include the calculated sigmagrenzen. Data points that fall outside these limits are flagged as potential signals of a process that is out of statistical control, indicating that a special cause of variation may be present.

The number of standard deviations used to define the sigmagrenzen can vary. A common practice is to use three standard deviations (3-sigma limits). This means that if a process is operating under a normal distribution and is in statistical control, approximately 99.73% of the data points are expected to fall within these 3-sigma limits. Other limits, such as 2-sigma or 6-sigma, may be used depending on the desired level of sensitivity and the criticality of the process.

The use of sigmagrenzen helps to distinguish between common cause variation, which is inherent in any process, and special cause variation, which arises from specific, identifiable factors that can be addressed and corrected. By monitoring data points relative to these limits, organizations can proactively identify and manage deviations, leading to improved product quality, reduced waste, and enhanced process efficiency.