Non Aggregatable Measures

In insurance, the probability that an event happens is a non aggregatable measure. There are also several measures in investment banking which are not aggregatable, particularly those related to risk. Not aggregatable means you can’t sum it up. For example:

• The probability of building A in City X getting an earthquake in 2011 is 3×10^-6.
• The probability of building B in City Y getting an earthquake in 2011 is 4×10^-6.
• The probability of both building A and B getting an earthquake in 2011 is 5.6×10^-6. You can’t add them up.

Another example:

• The probability of building A in City X getting an earthquake in 2011 is 3×10^-6.
• The probability of building A in City X getting an windstorm in 2011 is 2×10^-6.
• The probability of building A in City X getting an earthquake or a windstorm in 2011 is 4.7×10^-6. You can’t add them up.

We would probably design the fact table as normal:

BuildingKey, CityKey, YearKey, PerilKey, Probability
1, 1, 1, 1, 3×10^-6
2, 2, 1, 1, 4×10^-6

But we need to remember that in the BI (reports, cubes) we can’t sum the measure up.

One thing that we need to ask ourselves is: On which dimension is it not aggregatable? For example, in the above example, is it aggregatable on year?

If a measure is not aggregatable on one dimension, it does not mean that it’s not aggregatable in all dimensions.