A random sample of n=15 indicate a sample mean treadwear index of xbar =141.8 and a sample standard deviation of s= 19.3.
90% confidence interval of population mean
Xbar +/- [ tcritical * ( s / sqrt ( n ) ) ]
141.8 +/- [ 1.7613 * ( 19.3 / sqrt ( 15 ) ) ]
133.02 < mu < 150.58
No, there is no grounds to allegate wrong-doing because the 90% confidence interval contains 150, which means that the sample mean is not statistically significantly different from the claimed tread wear index.
Comparing means by z and t distributions is an endeavor to compute probabilities; these methods cannot be legitimately used to determine whether a single observation is within or outside of expected limits.
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