A "hugging the center line" pattern in a control chart can be the result of taking each sample from a different process but not mixed together.

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A "hugging the center line" pattern usually indicates that the process is in control and is consistent. If each sample is taken from a different process, that would result in a more erratic pattern with greater variation. A consistent pattern could be an indication of tampering or manipulation of the data.

A "Hugging the Center Line" Pattern in a Control Chart