I am interested to hear thoughts or examples of how some are using six sigma and/or other methods to capture and analyse data on service industries.

For example, by collecting data on "Costs of Sales", how do you determine your specification limits within control charts for data of this nature?

Your feedback would be appreciated.

The idea isn't to just develop control charts, the idea is to study your processes, flowchart them, measure them (understand your measurement process first though), and then use the data to assess their capability (are they adequate?).

If their capability needs to be improved, you can use brainstorming, simulations, experimental techniques, and other problem solving techniques to try to improve the process metrics.

Once your process is capable and stable, then, and only then, do you start to consider putting in systems to control the quality of the process. This may or may not include use of control charts. It may include use of controlled procedures & work instructions, checksheets, onging metrics, etc...

Don't limit yourself. Pick a process that is important to your business, make sure you have management sponsorship/buyin/support, then then start improving that process.