*** Introduction ***
If you noticed there was not Golden Nugget last week, it is because I was on vacation. Please remember you can always visit www.JimRaffel.com to review an archive of previous Golden Nuggets. There is still time to register for our User’s Group Conference coming up August 21-23 at the Mirage Hotel in Las Vegas, NV. You will need to call us directly at 262-820-1131 to make all arrangements at this late date.
*** Jim Raffel’s Week 8 Golden Nugget…Is it a Specification, a Guideline or a Process Capability? ***
We are often asked how to set both reference values (sometimes called target values, or gold standards), and the tolerance window associated with the reference values that have been set. I will start by saying my choice with all other factors being equal is to set both the reference and tolerance based upon process capabilities. Process capabilities are the most difficult of the three to explain so I will begin with the other two.
We define a specification as a reference value and tolerance range provided by the print buyer. For example, consumer product company K writes into the print contract that all special colors must be maintained within 2.5 Delta E of the L*a*b* values they provide for the color. If you are thinking a specification is something like SNAP, GRACoL, or SWOP (not a breakfast cereal by the way), please read on to see the way we define them.
We consider SNAP (Specifications for Newsprint Advertising Production), GRACoL (General Requirements for Applications in Commercial Offset Lithography), and SWOP (Specifications for Web Offset Publications) to be industry guidelines. While I am POSITIVE I will get several e-mails letting me know how wrong I am on this, let me explain. I look at it this way. Unless one of the above guidelines has been written into a contract, it is just that a guideline. Once a print buyer adds a clause to the contract that says print will conform to SWOP, it becomes a specification. What’s the difference? Well, you loose money and customers if you do not achieve specifications written into contracts. Guidelines on the other hand are a great aim point for jobs and customers who do not provide written specifications.
Process capabilities are a whole different ball game. I will not be able to do the topic justice here, but we are devoting almost 1/2 day at the User’s Group Conference in less than 2 weeks to this topic. Basically process capabilities are determined by operating under normal conditions for an extend period of time. Over this period of time samples are collected and measured. For example, in an ink jet proofing environment you would run for an entire month to collect samples on different shifts and under different temperature and humidity conditions. The samples are then analyzed using statistical methods to determine mean (average) L*a*b* values, and the accompanying standard deviation of the aforementioned values. Your reference (or target) is then set to the mean L*a*b* values, and the tolerance is set to a factor of the standard deviation. In ink jet proofing we have found a plus tolerance of three standard deviations to be achievable 99% of the time. When speaking of Delta E, the minus tolerance is not a factor. If your mean is Delta E over time is say 1.5, and you then run proofs with a mean Delta E of .1, they will be visually acceptable.
So, for my week 8 Golden Nugget, I have once again chosen to enter controversial waters. I look forward to the e-mails questioning my statements above and offering alternative views. Also, remember you can simply add comments to each golden nugget at www.JimRaffel,com, and all readers will seem them even if they are not included in a later golden nugget.