2.3 Errors in Analytical Chemistry
49
- For a sample (n < 30), which is the most usual case,
- Student's t depends on (a) the number of degrees of freedom (n - 1); (b) the confidence level requ ired,expressed as a percentage or as a probability [P = 1 - (%/1 OO)J.
Confidence interval
t values
90%
95%
98%
99%
Likelihood
0.10
0.05
0.02
0.01
Degrees of freedom
V= n - l
1
6.31
12.71
31.82
63.66
2
2.92
4.30
3.96
9.92
3
2.35
3.18
4.54
5.84
4
2.13
2.78
3.75
4.60
5
2.02
2.57
3.36
4.03
6
1.94
2.45
3.14
3.70
7
1.89
2.36
3.00
3.50
8
1.86
2.31
2.90
3.36
9
1.83
2.26
2.82
3.25
10
1.81
2.23
2.76
3.17
12
1.78
2.1 8
2.68
3.05
14
1.76
2.14
2.62
2.98
16
1.75
2.12
2.58
2.92
18
1.73
2.10
2.55
2.88
20
1.72
2.09
2.53
2.85
30
1.70
2.04
2.46
2.75
50
1.68
2.01
2.40
2.68
00
1.64
1.96
2.33
2.58
• The confidence interval depends on:
(A) Sample size (n). As n increases, the interval decreases because .In is in the denominator [and t decreases as a result (there are fewer degrees of freedom) ].
(B) The confidence level (probability).