Instant Notes: Analytical Chemistry

It should be noted that good precision does not necessarily produce good

accuracy (analyst A) and poor precision does not necessarily produce poor

accuracy (analyst B). However, confidence in the analytical procedure and the

results is greater when good precision can be demonstrated (analyst D).

Accuracyis generally the more important characteristic of quantitative data to

be assessed, although consistency, as measured by precision, is of particular

concern in some circumstances. Trueness is a term associated with accuracy,

which describes the closeness of agreement between the average of a large

number of results and a true or accepted reference value. The degree of accuracy

required depends on the context of the analytical problem; results must be shown

to be fit for the purpose for which they are intended. For example, one result may

be satisfactory if it is within 10% of a true or accepted value whilst it may be

necessary for another to be within 0.5%. By repeating an analysis a number of

times and computing an average value for the result, the level of accuracy will be

improved, provided that no systematic error (bias) has occurred. Accuracy

cannot be established with certainty where a true or accepted value is not known,

as is often the case. However, statistical tests indicating the accuracy of a result

with a given probabilityare widely used (vide infra).

Precision, which is a measure of the variabilityor dispersionwithin a set of

replicatedvalues or results obtained under the same prescribed conditions, can

be assessed in several ways. The spreador range (i.e. the difference between the

highest and lowest value) is sometimes used, but the most popular method is to

estimatethe standard deviationof the data (vide infra). The precision of results

obtained within one working session is known as repeatabilityor within-run

precision. The precision of results obtained over a series of working sessions is

known as reproducibilityor between-runs precision. It is sometimes necessary

to separate the contributions made to the overall precision by within-runand

B2 – Assessment of accuracy and precision 27

Correct

result

A

B

C

D

19.70 20.00

Titer (cm^3 )

20.30

Fig. 1. Plots of titration data to distinguish accuracy and precision.