This part of ISO 13909 sets out principles and procedures for testing the bias of test samples of hard coals or cokes, taken in accordance with other parts of ISO 13909. The use of univariate statistical methods only is addressed. The user is cautioned that the chance of falsely concluding that there is a bias, when no bias exists in any one of several variables measured on the same set of samples, is substantially greater than for a single variable. While several variables may be measured, the single variable on which the outcome of the test will be governed shall be designated in advance. NOTE In the text the term 'fuel' is used where both coal and coke would be applicable in the context and either 'coal' or 'coke' where only one is applicable.