` :There are various classifications that are called ``DRG``. The unit of classification is a DRG, which is pluralised to DRGs. This means that DRG is used both as a group noun (the classification) and a countable noun (the units of classification). So, ``DRG is a system ...`` refers to the classification, but ``DRGs are assigned ...`` refers to the units within the classification. Therefore, both are right and the article is correctly at the singular.   `