Interpretation & Calculations
Our thought was to begin using SPC for one month and then look at the data and decide what the control limits should be from there. For the initial month, the SPC software will automatically calculate the control limits. Is this a recommended approach?
Christy W., Quality Specialist
Your overall strategy is generally sound: Let the process run for a period of time, and calculate control limits for the process. However, if your process is initially out of control (as yours is), you need to understand and remove the causes of instability to apply the limits to the future. It would be worthwhile to take a look at the process during the times of instability and try to determine what is different. You can start with brainstorming, and you may even stumble on something that can be corrected immediately, but the best approach usually leads towards some designed experiments. Did I lose you? Some people get nervous as soon as they hear experimental design. DOE does not have to be difficult, but it should be planned.
Learn more about the SPC principles and tools for process improvement in Statistical Process Control Demystified (2011, McGraw-Hill) by Paul Keller, in his online SPC Concepts short course (only $39), or his online SPC certification course ($350) or online Green Belt certification course ($499).