A measured value hits a floor where it can no longer decrease. This typically occurs when a measurement reaches zero, drops below the detection limit of a sensor, or encounters a physical hard stop. On a control chart, a metric that bottoms out will show an artificial flatline, which distorts standard deviation calculations and violates normal distribution assumptions.
The defect rate bottomed out at zero on Tuesday, so the lower control limit on the p chart is completely meaningless right now.
When a process bottoms out against a zero boundary, you usually need to switch to a non-normal distribution or a one-sided control limit to avoid triggering false out-of-control alarms.
Related terms
Control chart
Time-ordered plot of a process statistic with a centerline and limits that flag when the process has changed.
p chart
Control chart for the fraction of nonconforming units in each sample, built on the binomial distribution.
Standard deviation
Typical distance of individual values from their mean, the basic measure of spread, written as sigma or s.