Statistical Quality Control (ISS (Statistical Services) Statistics Paper IV): Questions 24  26 of 34
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Question number: 24
» Statistical Quality Control » Control Charts » Variable
Describe in Detail
When should RChart be Analyzed? Six samples which contain 20 observations per sample have been collected and the sample means and sample ranges are computed as shown below. Test whether the sample means are within control limits of ?
Sample  Mean  Range 
1  3.06  0.42 
2  3.15  0.5 
3  3.11  0.41 
4  3.13  0.46 
5  3.06  0.46 
6  3.09  0.45 
Explanation

Range chart should be analyzed before the mean chart since variability of the process must be in control before the mean chart can be analyzed.

If the R chart indicates that the dispersion of the quality by the process is out of control, generally it is better not to construct chart, until the quality dispersion is brought under control
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Question number: 25
» Statistical Quality Control » Control Charts » Variable
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: 26
» Statistical Quality Control » Control Charts » Attributes
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 3σ 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 threesigma 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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