Process Capability and Performance: Process Performance (Pp, Ppk)

Question 1

Process Capability

Answer 1

• How do you know if your process is capable?
• Process Capability Pp measures the process spread vs the specification spread
  
  • In other words, how distributed the outcomes of your process is vs what the requirements are

Question 2

Specification Spread/Voice of the Client

Answer 2

• Let’s imagine that your process has 2 specifications; a Lower Specification Limit (LSL) which is the lowest value allowed and an Upper Specification Limit (USL), the highest value allowed. The difference between the two is the specification limit aka Voice of the Client

Question 3

Process Spread/Voice of the Process

Answer 3

• The distance between the highest value generated and the lowest

Question 4

Process Spread vs Specification Spread

Answer 4

• Think of the Specification Spread as the sides of your garage - those are static, they are not moving, and it is important that your process puts values inside those bounds
• The Process Spread is the size of the car you are trying to fit in

Question 5

Can a Process meet Specifications?

Answer 5

• The answer is in the amount of variation in your process.
• If your process spread is greater than the specification spread, then the answer is no.
  
  • However, if the process spread is less than the specification spread, then process variation is low enough for it to fit
    *

Question 6

Process Meets Specifications: Good Potential Performance

Answer 6

Question 7

Process Does NOT Meet Specifications: Bad Potential Performance

Answer 7

Question 8

Calculating Process Capability (Pp)

Answer 8

Pp = (USL – LSL) / 6* s : where s the standard deviation, or the ‘fatness’ or dispersion of the bell curve.

Question 9

What is a ‘Good’ Process Capability (Pp) Number?

How do we come to that?

Answer 9

According to Six Sigma, we want a Pp of above 1.5 because that would reflect a process with less than 3.4 DPMO – the definition of 6 Sigma quality.

How do we come to that?

Well, we want to have 6 sigmas (standard deviations) between the mean of the process and the LSL. Since a normal distribution is symmetric, that means we also want 6 sigmas between the mean and the USL. That’s a total of 12 sigmas between the USL and LSL.

In other words, USL – LSL should = 12 for us to reach 6 σ quality standards of 3.4 DPMO.

See how that is reflected in the equation Pp = (USL – LSL) / 6* s ?

Let’s replace (USL – LSL) with 12: Pp = (USL – LSL) / 6* s = 12 σ / 6 * s = 2 σ / s

Question 10

Process Capability Index

Is the process acceptable? Ppk (Capability)

Answer 10

• Ppk is another performance index that measures how close the current process mean’s proximity is to the specification limits.
  
  • In other words, does the process deliver acceptable results?
  • The way we tell this is trying to see how centered the process is. If the process is not centered well, it is deemed not acceptable

Question 11

Calculating Ppk

Answer 11

• There are 2 ways to calculate Ppk, depending on how your process is aligning
  
  • Process Mean close to USL
    
    • If your Process Mean (central tendency) is closer to the UDL, use:
      
      • Ppk = [USL- x(bar)] / 3 s
        
        • Where x(bar) is the Process Mean
  • Process Mean close to LSL
    
    • If your Process Mean (central tendency) is closer to the LSL, use:
      
      • Ppk= [x(bar) - LSL] / 3 s
      • Where x(bar) is the Process Mean

Question 12

Interpreting Ppk Scores

Answer 12

• A Ppk of 1 means that there is “half of a bell curve” between the center of the process and the nearest specification limit. That means your process is completely centered

Question 13

Pp, Ppk in relation to Z Scores

Answer 13

• Ppkcan be determined by dividing the Z score by 3.
• A Z score is the same as a standard score; the number of a standard deviations above the mean
• Z = x - mean of the population / standard deviation
• Ppk = (USL - µ) / 3σ = z / 3