Kapasitetsindekser

Kapasitetsindekser, often translated as capacity indices or capability indices, are a set of statistical tools used in quality control to assess the ability of a manufacturing process to produce output that meets specified requirements. These indices quantify how well a process's natural variation fits within the tolerance limits set for a particular product characteristic.

The most common capacity indices include Cp, Cpk, Pp, and Ppk. Cp (Process Capability) measures the potential capability of a process, assuming it is centered within the tolerance limits. It compares the width of the tolerance interval to the width of the process's natural variation, often represented by six standard deviations. A higher Cp value indicates a more capable process.

Cpk (Process Capability Index) is a more practical measure as it accounts for process centering. It is the minimum of the difference between the upper tolerance limit and the process mean, and the process mean and the lower tolerance limit, all divided by three standard deviations. Cpk reflects the actual capability of the process relative to the specification limits, even if the process is not perfectly centered.

Pp (Process Performance) and Ppk (Process Performance Index) are similar to Cp and Cpk, respectively, but they are calculated using the overall process variation (often estimated using the standard deviation of all data collected over a period) rather than short-term or within-subgroup variation. This makes Pp and Ppk useful for assessing long-term performance and consistency.

Interpreting these indices involves comparing their values against established benchmarks. Generally, higher index values signify a more robust and capable process. For instance, a Cpk of 1.33 is often considered a minimum acceptable level in many industries, indicating that the process is capable of producing output within specifications with a low probability of defects. These indices are crucial for identifying areas for process improvement and ensuring product quality.