Block 6 - lecture 1

Question 1

SPC?

Answer 1

Statistical Process Control

ie. Xbar, R, np, p, c and u charts

Question 2

techniques to determine the sample size needed?

Answer 2

• acceptance sampling
• fixed percentage
• occasional random checks
• 100% inspection

Question 3

sample?

Answer 3

a predetermined number of units that are inspected in each lot?

Question 4

when is a lot accepted?

Answer 4

when the number of nonconforming units is measured to be below the acceptance number

Question 5

N, n and c. What do they stand for in acceptance sampling?

Answer 5

N = lot size
n= sample size
c = acceptance number

Question 6

when is sampling used?

Answer 6

1. destructive test
2. 100% inspection expensive vs passing a nonconforming unit
3. many similar units
4. no quality charts from suppliers
5. automation not possible

Question 7

advantages of sampling?

Answer 7

• gives responsibility
• less expensive
• less repetitive job
• applies for destructive testing
• lots not accepted encourages improvement

Question 8

disadvantages of sampling?

Answer 8

• risk of accepting nonconforming units
• needs planning and documentation
• less data

Question 9

sampling plan?

Answer 9

plan specifying lot size, sample size and acceptance requirement

Question 10

types of sampling plans?

Answer 10

• single sampling (ie. 1 random sample taken)

- double sampling (another sample is sometimes taken)

Question 11

What are the symbols for the values in a double sampling plan?

Answer 11

N = lot number
n1 = first sample no.
c1 = first acceptance no.
r1 = first nonacceptance no.
n2 = second sample no.
c2 = combined acceptance no.
r2 = combined nonacceptance no.

Question 12

aspects of lot formation that can affect sampling accuracy?

Answer 12

• different machine used
• different material
• different operator

Question 13

how lot formation can affect sampling costs?

Answer 13

larger lot sizes require smaller sample size per unit

Question 14

how to implement random sampling?

Answer 14

• assign numbers to units

- use a random number generator

Question 15

disadvantages of random sampling?

Answer 15

• requires allocated numbers
• time consuming
• larger sample needed
• more errors
• high cost

Question 16

stratified sampling?

Answer 16

• divide the lot into strata (eg. layers) and divide further

Question 17

disadvantages of stratified sampling?

Answer 17

• high cost, especially if strata must be created

- error is subgroups are selected at different rates

Question 18

advantages of stratified sampling?

Answer 18

• statistical efficiency

- different methods in strata

Question 19

systematic sampling?

Answer 19

• first sample is picked
• units picked from a list at certain intervals
• the starting number and interval must allow the desired sample size

Question 20

advantages of systematic sampling?

Answer 20

• simple to design

- simple to use

Question 21

disadvantages of systematic sampling?

Answer 21

• periodic trends might skew results

- moderate cost

Question 22

uses for non accepted lots?

Answer 22

• reworked in customer production facilities (slow)
• reworked on customer site (may save shipping, supplier image damage if staff see)
• returned to supplier (forces improvement)