Interpretation & Calculations
Short Run analysis combines data from several runs into a single analysis.
Short Run SPC is typically used to analyze processes with an insufficient amount of data available from a given product or service classification to adequately define the characteristics of the process. In manufacturing, for instance, you may only produce thirty units of a given part number, then reset the machine for a different part number. Although the process is fundamentally the same (if it is acted upon by the same causal system), the first part number may be one inch in diameter, plus or minus 1/8 inch, and the second part number five inches in diameter, plus or minus 1/8 inch. This difference in nominal size prevents you from charting the raw measurements from the different part numbers on the same chart. Likewise, in a service application, the amount of time to resolve a customer complaint may be influenced by the type of complaint, such as one day for incorrect item shipped versus five days for incorrect billing. In either case, we are interested in statistically significant changes to our system, relative to either a nominal value (which we define) or an average value (which the system defines).
Thus, if we assume that the process is influenced by a common set of causes, regardless of the run (i.e. part number, complaint type, etc.), then we could use a single control chart to define the operating level for all runs. In order to do this, we must standardize each observation for short run SPC based on its properties. Standardization may be performed a number of ways, as explained below (Montgomery, 1991; Pyzdek, 1992a).
Nominal Control Charts: These charts are created by simply subtracting the Nominal value of the run from the observation. Here, the nominal value is usually the midpoint of the specification limits, the target value, or the historical mean (average) value observed from past studies. However, the Nominal charting method MUST only be used for short run SPC if it can be safely assumed that each run has the same amount of variation. This method of standardization is a useful short run technique for any subgroup size, and subgroup size may vary.
Stabilized Control Charts: As mentioned above, the Nominal control chart is only valid when each run has the same amount of variation. In manufacturing, even though two parts are produced by the same process, the effects of different raw material may increase the process variation for one part. Likewise, the variation in time to resolve a billing complaint may be much larger than the incorrect shipment, since more departments may be involved. When the level of variation is not similar for all runs, then we must also standardize relative to the variance. This is done in much the same manner as standardizing the mean.
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).