In: Operations Management
You are considering relaxing your control requirements that determine what is acceptable quality; you have been using a 99.0% confidence interval but want to begin using a 97.5% confidence interval.
Your team has collected the following data from 4 samples of 7 observations each. The calculated standard deviation is 13.981.
| Sample 1 | Sample 2 | Sample 3 | Sample 4 | |
| Obs 1 | 392.2 | 415.1 | 413.6 | 399.7 | 
| Obs 2 | 392.3 | 408.1 | 394.9 | 402.3 | 
| Obs 3 | 405.4 | 428.6 | 410.1 | 400.6 | 
| Obs 4 | 410.3 | 398.2 | 410.8 | 427.6 | 
| Obs 5 | 423.3 | 403.3 | 423.2 | 385.8 | 
| Obs 6 | 413.9 | 421.1 | 402.7 | 431.0 | 
| Obs 7 | 426.7 | 433.5 | 385.8 | 405.9 | 
What is the UCL for the mean given the new confidence interval of 97.5%? (Keep one decimal point in your answer)
Following table highlights average values of each sample consisting of 4 samples.
Average value of each sample = Sum of 7 observations / 7
| 
 Sample 1  | 
 Sample 2  | 
 Sample 3  | 
 Sample 4  | 
|
| 
 Observation 1  | 
 392.2  | 
 415.1  | 
 413.6  | 
 399.7  | 
| 
 Observation 2  | 
 392.3  | 
 408.1  | 
 394.9  | 
 402.3  | 
| 
 Observation 3  | 
 405.4  | 
 428.6  | 
 410.1  | 
 400.6  | 
| 
 Observation 4  | 
 410.3  | 
 398.2  | 
 410.8  | 
 427.6  | 
| 
 Observation 5  | 
 423.3  | 
 403.3  | 
 423.2  | 
 385.8  | 
| 
 Observation 6  | 
 413.9  | 
 421.1  | 
 402.7  | 
 431  | 
| 
 Observation 7  | 
 426.7  | 
 433.5  | 
 385.8  | 
 405.9  | 
| 
 Average =  | 
 409.1571  | 
 415.4143  | 
 405.8714  | 
 407.5571  | 
Thus , Xbar = Average of averages = ( 409.1571 + 415.4143 + 405.8714 + 407.5571) /4 = 409.5
Z value for 97.5% confidence interval = NORMSINV ( 0.9875 ) = 2.2414
Given are following data :
Standard deviation = 13.981
Sample size = n = 7 ( since there are 7 observations against each sample )
UCL for the mean given the 97.55 confidence interval
= Xbar + Z x Standard deviation / Square root ( sample size )
= 409.5 + 2.2414 x 13.981 / Square root ( 7 )
= 409.5 + 2.2414 x 13.981 /2.645
= 409.5 + 11.84
= 421.34
| 
 UCL FOR MEAN = 421.3  |