Page 255 - Lean six sigma demystified

Chapter 7 ReduCing VaRiation with Six Sigm a 233

machines, materials, and measurement. I call them a more politically correct
people, process, machines, materials, and measurement (P M ).
3
2
? still struggling

ever try to put a product together only to discover that all of the holes for screws
don’t line up? that’s variation. ever had to wait at your doctor’s office? the wait
time varies doesn’t it? that’s variation.

Goal Posts and Targets

The goal for all solutions to problems associated with variation is to center the
distribution over the ideal target value and minimize the amount of variation
around that target value. Sounds easy, doesn’t it?
For most products, customers have a target value and some tolerance for prod-
ucts around the target value. Your ability to produce products centered around
the target value with a minimum amount of variation will determine the qual-
ity of your product.
For parts to fit together properly, the bolt cannot be bigger or smaller than
its nut it screws into; the cap cannot be bigger or smaller than its bottle. In
many ways, this is like the goal posts in a U.S. football game: There’s a left and
a right post, and the kicker’s job is to kick the ball between the two posts. Any-
thing outside of the posts results in no score (or in Six Sigma terms, a noncon-
forming part). The left and right post might be considered to be the game’s
specification limits.
Customers specify their requirements for targets and tolerances in one of
two ways.

• Target and tolerance (e.g., 74 plus or minus 0.05)
• Upper (USL) and lower (LSL) specification limits (e.g., USL=74.05,
LSL=73.95).

tiP Don’t confuse specification limits (i.e., USL and LSL) with control limits (UCL
and LCL). Customers set specification limits; control charts use your data to cal-
culate control limits.