Use U charts to assess the number of defects per unit of measurement and determine whether or not the process is in control.

·    An in-control process exhibits only random variation in the number of defects per unit of measurement.

·    An out-of-control process exhibits unusual variation in the number of defects per unit due to the presence of special causes.

Defects are nonconformities (or flaws) in part of a product or service that do not render the product or service unusable. Examples of defects include air bubbles in a sheet of glass, weaknesses in insulated wire, or blemishes in cloth.

Use U charts when your sample sizes vary.

If your data exhibit overdispersion or underdispersion a Laney U' chart may more accurately distinguish between common cause variation and special cause variation.

A stenographic company wants to assess the quality of its transcription service. They sampled 25 samples (sets of pages) and counted the number of typographical errors for each sample. Differently sized sets of pages were sampled. Typographical errors are defects. Because the samples are not the same size, a U chart was used to analyze the data.

Data: Transcription.MTW (available in the Sample Data folder).