Quality Control Questions
Questions
Q1
A quality inspector took six samples each of four observations, of the length of time
(minutes) it took to process loan applications at a credit union. Data for the exercise is
provided below:
Sample
Observation
1
12.11
12.14
12.10
12.11
2
12.11
12.10
12.11
12.08
3
12.15
12.12
12.10
12.11
4
12.09
12.09
12.11
12.15
5
12.12
12.10
12.08
12.10
6
12.09
12.14
12.13
12.12
Use the information to obtain two-sigma control limits for the means of future times. It is
known from previous experience that the standard deviation of the process is 0.02 minute.
Q2
A manager for an assembly department took data for twelve working days and found the
following number of defectives of mobile phone chargers over the period. Construct a control
chart for the fraction defective using three-sigma limits.
Sample No.
1
2
3
4
5
6
7
8
9
10
11
12
No. of Samples
250
251
249
249
252
246
247
255
246
248
253
255
No. of Defectives
30
40
34
32
20
33
55
21
48
31
36
32
Fraction Defective
0.120
0.159
0.137
Q3
The postmaster of a small town receives a certain number of complaints each day about mail
delivery. Construct a control chart to describe 98.5% of the variation in the following data
and determine whether the process is in control:
1
2
3
4
5
6
DAY
7
5a
10
14
8
9
6
5
a
Number of complaints
8
9
10
11
12
12
8
7
6
4
Quality Control Questions
Q4
A medical facility does MRIs for sports injuries. Occasionally a test yields inconclusive
results and must be repeated. Using the following sample data and n = 38, to construct a
control chart that will describe 95% percent of the chance variation in the process when the
process is in control. Is the process in control? If not, eliminate any values that are outside the
limits and compute revised limits.
1
2
3
4
5
6
1b
2
2
0
2
1
SAMPLE
7
8
2
0
9
10
11
12
13
2
7
3
2
1
Q5
Using the appropriate control chart, determine two-sigma control limits for each of the
following cases:
a. An inspector found an average of 3.7 scratches in the exterior paint of each of the cars
being shipped to dealers in a particular month.
b. Before shipping lawn mowers to dealers, an inspector attempts to start each mower
and notes any that do not start on the first try. The lot size is 100 mowers, and an
average of 4 did not start.
Q6
Surface defects have been counted on a number of rectangular steel plates, and the data is
shown below. The control chart for nonconformities is to be set up using this data.
a. Draw an appropriate control chart to describe 97.5% of the variation in the process.
b. Determine whether or not the process is in control.
c. Explain your response to (b) above.
Sample No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Non-conformities
1
0
2
1
1
2
0
0
2
1
1
5
8
0
b
Number of retests