The Cp Chart
The SPC Cp measurement shows how well the Six Sigma range fits into the specification range. This measurement is determined by dividing the specification limit (voice of the customer) by the control limit (voice of the process).
To calculate the Cp ratio, subtract the lower specification limit from the upper specification limit, then divide by six units.
The Cpk Chart
The SPC Cpk measurement shows the relationship of the Six Sigma spread to the specification limits. Cpk represents the lowest value of the capability against the upper or lower specification, showing where within the specification limits the process is producing.
To calculate the Cpk measurement, compare the average of the data to the specification limits.