Block 4 - lecture 3

Question 1

what’s Rbar?

Answer 1

the average range across subgroups

Question 2

how to use Rbar?

Answer 2

estimate standard deviation

sigma-hat = Rbar /d2

Question 3

Positives of using Rbar for standard deviation?

Answer 3

• ranges and quicker than averages

Question 4

Negatives of using Rbar for standard deviation?

Answer 4

• not true process capability
• needs more than 25 subgroups for accuracy
• assumes process is in control

Question 5

what’s sbar

Answer 5

the average standard deviation across subgroups

Question 6

how to find process capability from standard deviation?

Answer 6

sigma-hat = sbar / C4

Question 7

Positives of using sbar for standard deviation?

Answer 7

more accurate

Question 8

Negatives of using sbar for standard deviation?

Answer 8

takes much more time

Question 9

What does a histogram need in order to be representative?

Answer 9

at least 50 measurements

Question 10

Cp?

Answer 10

capability index = tolerance / process capability

Question 11

what is a good capability index?

Answer 11

> 1 is good, as the tolerance is larger than the capability to allow for shift

Question 12

sigma subscript 0 vs sigma-hat?

Answer 12

6 sigma subscript 0 is the true process capability, sigma hat is an estimate

Question 13

standard minimum process capability index?

Answer 13

1.33

Question 14

How to calculate Cpk?

Answer 14

(Xbar to closest spec) / 3 sigma

Question 15

How to calculate Cpk?

Answer 15

(Xbar to closest spec) / 3 sigma

Question 16

what happens to Cp with an uncentered process?

Answer 16

process capability is overestimated

Question 17

Cpk < 0?

Answer 17

process mean is out on spec limits

Question 18

Cpk = 1?

Answer 18

process limit is at a spec limi

Question 19

Cpk = 0?

Answer 19

process mean is at a spec limit

Question 20

Cp = Cpk?

Answer 20

the process is centred

Question 21

Cpk >Cp?

Answer 21

impossible

Question 22

Cpk < 1?

Answer 22

process produces some parts out of spec

Question 23

Cp < 1?

Answer 23

the process isn’t capable, even if centered