6.1 Result

• Calibration

• Non-parametric statistical techniques

• Outlier

• Raw data

• Reporting limit

• Test result

The result of an analytical measurement is usually obtained by the application of appropriate statistical procedures to the raw data obtained in the laboratory, monitoring station etc. In many cases some of the data are derived from measurements on standards (materials of known composition), the remainder coming from test materials. In a calibration experiment several standard materials are examined using a carefully defined sequence of steps, involving appropriate sampling and preparation of the materials followed by an instrumental analysis protocol. One of the standards is a blank material, containing solvent, reagents etc. but no analyte. The results of these measurements are plotted as a calibration graph or analytical growth curve. Test materials ( unknowns ) are then analysed using exactly the same sequence, and the results are used with interpolation on the calibration graph to yield a test result for each test material.

A problem that arises in many cases is that small raw data samples may contain one or more results which appear to be divergent from the remainder; the problem may arise in a calibration experiment or in a set of replicate measurements on a test material. Assuming that the cause of the divergence is not immediately apparent ( e.g. as a result of a transcription error or instrument failure) it is then necessary to decide whether such an outlier can be rejected entirely from the raw data before data...