1 Six Sigma : Statistical View Dedy Sugiarto

2 What is Variation? Less Variation = Higher Quality

3 Definition Quality is inversely proportional to variability. Quality improvement is the reduction of variability in processes and products.

4 Percentage Thickness Histogram : A barchart showing the distribution of the process.

5 The world tends to be bell-shaped or Normal Distribution Even very rare outcomes are possible (probability > 0) Even very rare outcomes are possible (probability > 0) Fewer in the “tails” (lower) Fewer in the “tails” (upper) Most outcomes occur in the middle

6 “Normal” bell shaped curve Normal distributions are divide up into 3 standard deviations on each side of the mean

7 Types of variation Common Causes of variaton Special Causes of variaton

8 Reducing Variation 3 Structured problem solving 3 Planned experiments Reducing Variation in a Stable Process Make Permanent Changes Changes are based on the scientific approach Examples: new equipment, equipment upgrade, new procedure, new machine settings, better raw material

9 Capability Analysis (The Concept) v If the process mean is in control then virtually all points will remain within control limits. v Staying within control limits does not necessarily mean that specification limits are satisfied. v Specification limits usually dictated by customers.

10 Capability Analysis (The Indices) Potential Capability Index This index show how well a process is able to meet specifications. The higher the value of the index mean the more capable is the process. C p < 1 (Process is unsatisfactory). 1 < C p < 1,33 (Process is of medium relative capability). C p > 1,33 (Process show high relative capability).

11 Capability Analysis (The Indices) Actual Capability Index This index takes process centering into account! If C p = C pk (Process is centered at the midpoint of the specifications). C pk < C p (Process is off- center).

12 Capability Analysis (The Conclusion) v The message from process capability studies: –First reduce the variation in the process. –Then shift the mean of the process towards the target.

13 Sigma level and Capability process index Six Sigma = Cp = Cpk = 2