The numeric measures in a fact table fall into three categories. The most flexible and useful facts are fully additive; additive measures can be summed across any of the dimensions associated with the fact table. Semi-additive measures can be summed across some dimensions, but not all; balance amounts are common semi-additive facts because they are additive across all dimensions except time. Finally, some measures are completely non-additive, such as ratios. A good approach for non-additive facts is, where possible, to store the fully additive components  of the non-additive measure and sum these components  into the final answer set before calculating the final non-additive fact. This final calculation is often done in the BI layer or OLAP cube.