# Statistical Quality Control-Control Charts (ISS (Statistical Services) Statistics Paper IV): Questions 23 - 25 of 31

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## Question number: 23

» Statistical Quality Control » Control Charts » Variable

Essay Question▾

### Describe in Detail

a) What is major difference between X bar Chart and R chart?

b) What is OC plot?

c) What is Average outgoing Quality (AOQ)?

d) What is Average Total Inspection (ATI)?

### Explanation

a) is used to control the process mean, R chart is used to control process variation. reveals undesirable variations between samples as far as their averages are concerned while the R chart reveals any undesirable variations within samples.

b) Operating Characteristic (O. C. ) curve is a graphic representation of the relations

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## Question number: 24

» Statistical Quality Control » Control Charts » Attributes

Essay Question▾

### Describe in Detail

a) In welding of seams, defects included pinholes, cracks, cold laps, etc. A record was made of the number of defects found in one seam each hour and given below. Obtain the 3-σ limits for number of defects.

 1/12/1983 8 A. M. 2 12 A. M. 6 9 A. M. 4 1 P. M. 4 10 A. M. 7 2 P. M. 9 11 A. M. 3 3 P. M. 9 12 A. M. 1 3/12/1983 8 A. M. 6 1 P. M. 4 9 A. M. 4 2 P. M. 8 10 A. M. 3 3 P. M. 9 11 A. M. 9 2/12/1983 8 A. M. 5 12 A. M. 7 9 A. M. 3 1 P. M. 4 10 A. M. 7 2 P. M. 7 11 A. M. 11 3 P. M. 12

b) The number of daily customer complaints are monitored at a telephone exchange. Complaints have been recorded over the past twenty days. Develop three-sigma control limits for the following data.

 Days No. of complaints Days No. of complaints 1 2 11 1 2 3 12 3 3 1 13 2 4 3 14 1 5 3 15 1 6 2 16 1 7 3 17 3 8 1 18 2 9 1 19 2 10 3 20 4

### Explanation

a) Average number of defects per sample is

=6+ =13.35

=6-

Since number of defects cannot be negative, can be considered as zero.

b) Average number of defects per sample is

=2.2+ =6.65

=2.2- →0

Since number of defects

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## Question number: 25

» Statistical Quality Control » Control Charts » Variable

Essay Question▾

### 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. Based on chart, Check whether the production is under control?

 Sample No. Observstions 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 Sample Range 1 8 9 15 4 11 11 2 7 10 7 6 8 4 3 11 12 10 9 10 3 4 12 8 6 9 12 6 5 11 10 6 14 11 8 6 7 7 10 4 11 7 7 10 7 4 10 10 6 8 8 11 11 7 7 4 9 8 11 8 14 12 6 10 12 9 12 17 11 8 11 7 7 9 17 13 10 12 9 9 4 4 11 7 13 10 12 12 12 12 2 14 8 11 9 6 8 5 15 10 13 9 4 9 9 16 9 11 8 5 11 6 Total 102

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