Page 158 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

4 ERRORS AND STATlSTlCS
Analyst ( 1 ) %A 49.01 ; 49.25; 49.08; 49.14

I I
Correct value 1 1 Average value
I I
1 I

The arithmetic mean is 49.12% and the results range from 49.01 % to 49.25%.

Analyst (2) %A 49.40; 49.44; 49.42; 49.42

1 1
Correct value 1 Average value I
I I
I
1
1
t
XXX

The arithmetic mean is 49.42% and the results range from 49.40% to 49.44%.

We can summarise the results of the analyses as follows.
(a) The values obtained by Analyst 1 are accurate (very close to the correct
result), but the precision is inferior to the results given by Analyst 2. The
values obtained by Analyst 2 are very precise but are not accurate.
(b) The results of Analyst 1 face on both sides of the mean value and could be
attributed to random errors. It is apparent that there is a constant
(systematic) error present in the results of Analyst 2.
Precision was previously described as the reproducibility of a measurement.
However, the modern analyst makes a distinction between the terms 'reproducible'
and 'repeatable'. On further consideration of the above example:
(c) If Analyst 2 had made the determinations on the same day in rapid
succession, then this would be defined as 'repeatable' analysis. However, if
the determinations had been made on separate days when laboratory
conditions may Vary, this set of results would be defined as 'reproducible'.
Thus, there is a distinction between a within-run precision (repeatability) and
a between-run precision (reproducibility).