Statistical Quality Control (ISS (Statistical Services) Statistics Paper IV): Questions 18  20 of 34
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Question number: 18
» Statistical Quality Control » Control Charts » Variable
Describe in Detail
A machine is manufacturing mica discs. Samples of size 5 are drawn every hour and their thickness in units are recorded as follows. On the basis of chart, Check whether the production is under control?
Sample No.

Observations
 
1

8

9

15

4

11

2

7

10

7

6

8

3

11

12

10

9

10

4

12

8

6

9

12

5

11

10

6

14

11

6

7

7

10

4

11

7

10

7

4

10

10

8

8

11

11

7

7

9

8

11

8

14

12

10

12

9

12

17

11

11

7

7

9

17

13

12

9

9

4

4

11

13

10

12

12

12

12

14

8

11

9

6

8

15

10

13

9

4

9

16

9

11

8

5

11 
Explanation
Sample No.

Observations

Total

Sample Mean

Sample Range
 
1

8

9

15

4

11

47

9.4

11

2

7

10

7

6

8

38

7.6

4

3

11 
Question number: 19
» Statistical Quality Control » Control Charts » Variable
Describe in Detail
a) If the average range for the 4 samples is 0.2 millimeters and the average mean of the observations is 12.5 millimeters, obtain the threesigma control limits for the sample mean.
b) 5 observations have been taken from the10 samples in order to test the volume in the bags. The average sample range is 0.5 ounces. Obtain the 3σ limits for the sample range.
Explanation
a) It is given that n = 5 and
For n = 5, The value of
Control limits for :
And
b) We are given that, number of observations, n = 5,
For n = 5, from the tables
Question number: 20
» Statistical Quality Control » Control Charts » Variable
Describe in Detail
A new oven has been installed in a bakery and to test the efficiency of this, the oven temperature readings have been taken at four different places inside the oven every 30 min. The following table gives readings. Determine the control limits for the mean temperature.
Time

Readings
 
8: 00

400

500

550

390

8: 30

440

420

380

380

9: 00

410

450

470

430

9: 30

390

390

410

410

10: 00

370

420

460

410

10: 30

390

400

390

400 
Explanation
Readings

Total

Sample mean

Sample Range
 
400

500

550

390

1840

460

160

440

420

380

380

1620

405

60

410

450

470

430

1760

440

60

390

390

410 