Cardenas’ formula [Cardenas, 1975] is a simple equation that is applicable to estimating the number of rows in a view:
Let n be the number of rows in the fact table.
Let v be the number of possible keys in the data space of the view.
Expected distinct values = v(1 – (1 – 1/v)^n)
Cardenas’ formula assumes a uniform data distribution. However, many data distributions exist. The data distribution in the fact table affects the number of rows in a view. Cardenas’ formula is very quick, but the assumption of a uniform data distribution leads to gross overestimates of the view size when the data is actually clustered. Other methods have been developed to model the effect of data distribution on the number of rows in a view.
Fonte:
http://media.techtarget.com/searchOracle/downloads/Teorey08.pdf