Let the width of the garage (D) and that of the car (d) is measured in some units called as sigma or σ, further
Width of the garage is sacrosanct = 12σ (assume for the time being), then following three cases can occur depending on the ratio of D/d = Cp.
The process sigma level is 6σ when the Cp =2, this is represented by case ‘C’ given below. Point to be noted is that there is a margin of safety (=3σ) on both side of the car before car touches the garage. The ideal width of car is taken as 6σ (don’t ask why, right now!)
Process capability Cpk: is measured in the terms of the σ distance between the center of the of the car (C1) and the wall of the garage. Cpk tell us “how far is the car from left wall or the right wall of the garage (or customer’s specifications).
You may argue that what’s the big deal if my existing car doesn’t fit into the garage, I will rebuild the garage that can fit my bigger car.
You can always do it with garage but not with customer’s specifications as you can’t dictate it or simply customer won’t change his processes to fit your defective products. The only way is to improve your existing process or else lose your business to your competitors.
We can easily understand the concept of six-sigma using the analogy of parking a car into the garage and if you want to understand it more seriously, just imagine that you are entering a tunnel (instead of garage) at the speed of 150-200 Km/Hr!
When ever a car is bigger than the garage, it’s not possible to enter the garage. Take another car whose width is slightly less than the width of the car. This time it is possible to enter the garage but we will get frequent scratches unless you are an expert driver! Take a third car whose width is just half of the width of the garage , it’s is now possible for me to enter the garage even if I am drunk.
Car width represents the “current process capability“
Garage width represents the “Customer’s specifications“
We should remember that
“We need to buy a car according to the dimensions of the garage”
“We need to change our manufacturing processes to meet the customer’s specification”