Facts can be divided into three basic types, as follows :
Additive : Facts that are fully additive are the most flexible and useful. We can sum up additive facts across any dimension associated with the fact table.
Semi-additive : We can sum up semi-additive facts across some dimensions associated with the fact table, but not all.
Non-Additive : The Fact table contains non-additive facts, which cannot be summed up for any dimension. The ratio is an example of a non-additive fact.
