The target thickness for silicon wafers used in a certain type of integrated circuit is 245 µm. A sample of 50 wafers is obtained and the thickness of each one is determined, resulting in a sample mean thickness of 246.18 µm and a sample standard deviation of 3.60 µm. Does this data suggest that true average wafer thickness is something other than the target value?

A First Course in Probability (10th Edition)
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Author:Sheldon Ross
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Chapter1: Combinatorial Analysis
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The target thickness for silicon wafers used in a certain type of integrated circuit is 245 µm. A sample of 50 wafers is obtained and the thickness of each one is determined, resulting in a sample mean thickness of 246.18 µm and a sample standard deviation of 3.60 µm. Does this data suggest that true average wafer thickness is something other than the target value?

1. Parameter of interest: ? = true average wafer thickness
2. Null hypothesis: H0: ?       245
3. Alternative hypothesis: Ha: ?       245
4. Formula for test statistic value: 
z = 
x − 245
s/
  n
5. Calculation of test statistic value: 
z = 
246.18 − 245
3.60/
  50
 = 
 (rounded to two decimal places)
6. Determination of P-value: Because the test is two-tailed,
 
P-value = 2(1 − Φ(2.32)) = 0.0204
7. Conclusion: Using a significance level of 0.01, H0 would not be rejected since 0.0204 > 0.01. At this significance level, there is     evidence to conclude that true average thickness differs from the target value.
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