Block 4 - lecture 2

Question 1

central limit theorem?

Answer 1

the average of non-normal individual values tends towards normal for n=4 and as the sample size increases

Question 2

who proposed central limit theorem and how?

Answer 2

Stewart (1931) found the distributions of averages (n=4) for uniform and triangular distributions

Question 3

how does central limit theorem help Xbar charts?

Answer 3

means the individual values don’t need to have a normal distribution.

Question 4

what values do control limits apply to?

Answer 4

averages of subgroups

Question 5

what values do spec limits apply to?

Answer 5

individual values

Question 6

what’s the tolerance, in relation to process capability?

Answer 6

tolerance is the difference between spec limits

Question 7

3 situations for capability and tolerance?

Answer 7

tolerance>capability - desirable, out of control without out of spec

Tolerance=capability - any shift means out of spec

tolerance

Question 8

how to eliminate scrap by changing the process center?

Answer 8

if the centre is 6 standard deviations above lower spec limit (increases rework but removes scrap)
HOLES/SLOTS USE UPPER

Question 9

how to find the percentage in a range of a normal distribution?

Answer 9

find the Z value
Z=(Zi-mu)/sd
look for ratio in table eg. 0.01=1%