In: Statistics and Probability
The following data represent samples that were taken on 10 separate days. Each day has a varying sample size and the number of defects for the items sampled is listed. We want to see if this process is consistent and in control.
| 
 Day  | 
 Sample Size  | 
 Defects  | 
| 
 1  | 
 110  | 
 6  | 
| 
 2  | 
 112  | 
 3  | 
| 
 3  | 
 195  | 
 8  | 
| 
 4  | 
 185  | 
 6  | 
| 
 5  | 
 245  | 
 10  | 
| 
 6  | 
 250  | 
 7  | 
| 
 7  | 
 100  | 
 5  | 
| 
 8  | 
 170  | 
 7  | 
| 
 9  | 
 250  | 
 20  | 
| 
 10  | 
 270  | 
 25  | 
a. Find the UCL.
b. Find the LCL.
c. Is the process in control? Why/why not?
From the given data
| Size (ni) | Number of defects (ui) | ||||
| Day | ui = ci/ni | LCL | UCL | ||
| 1 | 110 | 6 | 0.0545 | -0.01345 | 0.116257 | 
| 2 | 112 | 3 | 0.0268 | -0.01287 | 0.115675 | 
| 3 | 195 | 8 | 0.041 | 0.002696 | 0.100113 | 
| 4 | 185 | 6 | 0.0324 | 0.001397 | 0.101412 | 
| 5 | 245 | 10 | 0.0408 | 0.00795 | 0.094859 | 
| 6 | 250 | 7 | 0.028 | 0.008386 | 0.094422 | 
| 7 | 100 | 5 | 0.05 | -0.01661 | 0.119422 | 
| 8 | 170 | 7 | 0.0412 | -0.00076 | 0.103571 | 
| 9 | 250 | 20 | 0.08 | 0.008386 | 0.094422 | 
| 10 | 270 | 25 | 0.0926 | 0.01001 | 0.092799 | 
| Total : | 1887 | 97 | 

c) NControl Chart:

Interpretation: The process is under control since all points lies between LCL and UCL.
The number of defects per unit is stable. No subgroups are out of control