What signals indicate an unusual shift in a U-chart or C-chart?

• A. Points outside control limits, runs, trends or patterns
• B. A single inside-control-point
• C. Random scatter within limits
• D. A steady increase within limits

Answer: (A)

In U-charts and C-charts, you’re looking for signals that the process variation is no longer just random, or common-cause variation. The strongest indicators of an unusual shift are points that fall outside the control limits, plus nonrandom sequences within the chart such as runs, trends, or other patterns. A point outside the limits is a clear sign that something has changed beyond expected variation and needs investigation. Nonrandom patterns inside the limits, like several points in a row on one side or a steady upward or downward trend, suggest a shift in the process that warrants attention even though all points may still appear to be within limits.

The other scenarios reflect normal or non-shift behavior: a single point inside the limits isn’t evidence of a shift, random scatter within limits represents typical variation, and a steady increase within limits could be concerning only if it forms a detectable nonrandom pattern or eventually exceeds limits.