We have seen that variation is a part of life, we need to learn to live with it. At most we can make an effort to reduce it by using 6sigma tools.
This happens because you can’t control everything involved in any process. There are some uncontrollable factors known as “common causes” in six-sigma. For example
You are producing some part to be used in automobiles, there will be a variation in product specification as there will be wear and tear of machines, change of operators etc.
If we repeat any process 100 times, all product/output of the process would not have same specifications, it might happen all 100 are within the desired specification. If we plot a histogram of the product specification from a stabilized process, it would look like
We can see that maximum products would be clustered around the mean and as we move away from the mean, number of products decreases.
Width of the customer’s specification is analogous to garage’s width and the process variation is analogous to car’s width. If you don’t have proper control on your process (driving) you are going to crash your process (car) against the customer’s specifications (garage walls).
Now I feel that everyone agrees that variation is a part of life and we need to learn to cope with it. The only thing we can do is to minimize it by using some proven methodology so that whatever we are producing (product or services) should always meets customer’s specifications or should have enough safety margin. This proven methodology of reducing variability is called as 6sigma.
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”