Some important statements:
In most cases, coal is very heterogeneous in terms of grains size distribution but also in chemical analysis.
Fines contains proportionally more inorganic material (ash). Therefore it is crucial to have the realistic proportion of fines also in the sample.
Please have a look at the below showing that a few grams in a laboratory plastic bag may only have to represent a full cargo or a 10.000 t sub-lot.
If sampling and/or preparation is incorrect so the subsequent analyses are incorrect even if they are absolutely correct performed.