Analytical Chemistry

(c) Equation (2.9) is then applied to compare the means

From t-criteria the confidence level for this difference to be significant is less than 80%.

Conclusion: The new instrumental method is giving the same results as the accepted method.

Example 2.4

The technique of a trainee operator is being assessed by comparing his results with those obtained by an
experienced operator. Do the results obtained indicate a significant difference between the skill of the
two operators?

The trainee operator carried out six determinations yielding a mean of 35.25% with a standard deviation
of 0.34%. The experienced operator obtained a mean of 35.35% and a standard deviation of 0.25% from
five determinations.

(a) The F-test is used to compare the standard deviations (equation (2.8))

Fcrit from Table 2.4 is 6.26 and there is no significant difference in the standard deviations (at the 95%

level).

(b) Equation (2.10) now enables the two means to be compared. If s is first computed from the pooled
data (equation (2.3)) and found to be 0.29% then

Conclusion: The probability of the difference in means being significant is very low, and there is no
difference between the skill of the two operators.

Example 2.5

In an investigation of a determinate error a series of replicate measurements were made using a range of
sample weights. The results obtained are tabulated below.

Sample weight/g Analyte/%

0.113 9.67

0.351 9.96

0.483 10.04

0.501 10.03

0.711 10.09

0.867 10.12