95% Confidence Level and Systematic Error

We are using the book Analytical Chemistry 7th edition by Skoog, West, Holler, and Crouch

This question requires all equations to be written out. Thanks so much!


To test the quality of work of a commercial laboratory, 15 replicate analyses of a purified benzoic acid (68.8% C, 4.953%H) sample were requested.  The absolute standard deviation of the method is s=4ppt for carbon and s=6ppt for hydrogen.  The means of the reported results are 68.5% C and 4.882% H.  At the 95% confidence level, is there any indication of systematic error in either analysis?


for 95% confidence the degrees of freedom are as follows:

Number of observatoins           95% Confidence
3                                 0.970
4                                 0.829
5                                 0.710
6                                 0.625
7                                 0.568
8                                 0.526

I don't remember if you subtract the total results by 1 or not for this question.


Don't know if you need this or not

Degrees of Freedom   Factor for Confidence Interval95%
1                      12.7
2                      4.30
3                      3.18
4                      2.78
5                      2.57
6                      2.45
7                      2.36
8                      2.31
9                      2.26
10                     2.23
11                     2.20
12                     2.18
13                     2.16
14                     2.14