Hugging the centerline describes a chart with far too little scatter, such as 15 or more points in a row inside the 1 sigma zone. It almost never means the process got better. The usual causes are subgroups that mix several streams, such as different chambers or cavities, which inflates the within-subgroup range and the limits, or limits left over from a noisier period. Extended run rules flag it as stratification.
This chart's been hugging the centerline for a month. The subgroups mix all four heads, so the limits are three times too wide.
Also heard as
- hugging
- stratification
Related terms
Rational subgroup
Group of measurements chosen so that only routine, short-term variation can occur within it.
Western Electric rules
Standard set of run rules that flag nonrandom patterns on a control chart, not just single points past the limits.
Control limits
Boundaries on a control chart computed from the process's own variation, marking how far routine noise normally reaches.
X-bar and R chart
Paired control charts for subgroup averages and subgroup ranges, the workhorse for measured data sampled in small groups.