Asprin given to people with a headache does not always give relief. Need statistics to determine the likelihood of an event having arisen by chance. In any comparison in a medical context, differences are almost bound to occur.
All groups are discrete.
There is both no overlap between classifications and no intermediary values . Characteristics are the same as for nominal data except that there is some order to the classification. In this case improved is greater than same. For example the number of anaesthetics administered in the Royal Brisbane Hospital in 1994. Values have both order and meaningful value.