Process Capability is a function of which of these?

Process Capability is a statistical measure of a process's ability to produce output within customer-defined specification limits. It is fundamentally a comparison between the "Voice of the Customer" (customer requirements, which set the specification limits) and the "Voice of the Process" (the actual process performance, measured by its variation and centering). To accurately assess this, data on the process output must be collected over a period of time to ensure the process is stable (in statistical control) and its inherent variability is correctly understood. Therefore, process capability is a function of all three elements combined.

A. This is incomplete. Customer requirements alone only define the target; they do not measure the process's ability to hit that target.
B. This is incomplete. Process performance measures how the process behaves, but without comparison to customer requirements, its capability cannot be determined.
C. This is incomplete. Output over time provides the necessary data, but it is the analysis of this data against requirements that defines capability.

1. Montgomery, D. C. (2013). Introduction to statistical quality control (7th ed.). John Wiley & Sons. In Chapter 8, "Process Capability Analysis," it is explained that process capability ratios (like Cp and Cpk) are calculated by comparing the specification width (derived from customer requirements) to the natural tolerance limits of the process (a measure of process performance based on its inherent variation). The entire analysis is predicated on data from output over time to ensure the process is in a state of statistical control (p. 369-371).

2. Pyzdek, T., & Keller, P. A. (2018). The Six Sigma Handbook (5th ed.). McGraw-Hill Education. Chapter 16, "Process Capability," states: "Process capability compares the output of an in-control process to the specification limits... The comparison is made by forming the ratio of the spread between the process specifications [customer requirements] to the spread of the process values, as measured by 6 process standard deviation units [process performance]." The prerequisite of an "in-control process" necessitates analyzing output over time.

3. Antony, J., & Banuelas, R. (2002). Key ingredients for the effective implementation of Six Sigma program. Measuring Business Excellence, 6(4), 20-27. https://doi.org/10.1108/13683040210451679. The article emphasizes that a core Six Sigma activity is determining process capability, which involves understanding process variation (process performance) and comparing it against customer specifications (customer requirements) using data collected from the process (output over time).