Six Sigma u Chart - Statistical process control charts are line graphs that show how a process performs over time. Is it predictable or "out of control"? How much variation is there? Are there any special causes of variation (e.g., the point above the upper control limit shown below)? Purpose: A statistical control chart for Excel to determine stability of "counted" data (e.g., errors per widget, inquiries per month, etc.) when the sample size varies. The u chart will help evaluate process stability when there can be more than one defect per unit. Examples might include: the number of defective elements on a circuit board, the number of defects in a dining experience--order wrong, food too cold, check wrong, or the number of defects in bank statement, invoice, or bill. This chart is especially useful when you want to know how many defects there are not just how many defective items there are. It's one thing to know how many defective circuit boards, meals, statements, invoices, or bills there are; it is another thing to know how many defects were found in these defective items. The u chart is used when the sample size varies: the number of circuit boards, meals, or bills delivered each day varies.