2 —
The Assessment of Analytical Data
A critical attitude towards the results obtained in analysis is necessary in order to appreciate their
meaning and limitations. Precision is dependent on the practical method and beyond a certain degree
cannot be improved. Inevitably there must be a compromise between the reliability of the results
obtained and the use of the analyst's time. To reach this compromise requires an assessment of the
nature and origins of errors in measurements; relevant statistical tests may be applied in the appraisal of
the results. With the development of microcomputers and their ready availability, access to complex
statistical methods has been provided. These complex methods of data handling and analysis have
become known collectively as chemometrics.
2.1—
Definitions and Basic Concepts
True Result
The 'correct' value for a measurement which remains unknown except when a standard sample is being
analysed. It can be estimated from the results with varying degrees of precision depending on the
experimental method.
Accuracy
The nearness of a measurement or result to the true value. Expressed in terms of error.
Error
The difference between the true result and the measured value. It is conveniently expressed as an
absolute error, defined as the actual difference between the true result and the experimental value in the
same units. Alternatively, the relative error may be computed, i.e. the error expressed as a percentage
of the measured value or in 'parts per thousand'.
Mean
The arithmetic average of a replicate set of results.