Advanced Dimension Design - Professional SQL Server Analysis Services 2005 with MDX - page 319

Custom Rollups
The name "custom rollup" is very much self describing. Custom refers to
the "user defined" nature of a rollup, such that a measure value for a
member is not a simple sum of values of its children as you move up a
hierarchy. Rollup describes how those calculations typically start at the
leaf or lower-level node and move (roll) up toward the root. There are
several ways in which you can apply a custom rollup to a hierarchy: by
using the attribute property CustomRollup column, using unary operators
(used for parent-child hierarchies), and by using MDX scripts to specify
custom rollup for members in a level. Note that unary operators can be
used on non-parent-child hierarchies too.
A business scenario will help you better understand the need for and
concept of the custom rollup. Perhaps you are familiar with the word de-
preciation . Technically, the definition of depreciation is "mapping an as-
set's expense over time to benefits gained through use of those assets."
It simply means that the value of an asset decreases over time. When
working on the financial side of business intelligence it is only a matter
of time before you encounter the concept of depreciation, so if you don't
already know it, study this carefully. As you may already know, the value
of a car decreases over the years. In fact, after the moment you drive off
the lot, the new car's value starts to decrease. This is a common example
of depreciation.
There are two types of depreciation you should be familiar with and un-
derstand. They are called straightline and accelerated. Typically, busi-
nesses keep two (sometimes more) sets of accounting topics, which, by
the way, is completely legal. One set of topics is for the IRS and one set
is for investors. The topics for the IRS often use accelerated depreciation
because this provides optimal tax benefits (less taxable income is initially
reported) and the topics for investors use straight-line depreciation be-
cause this yields higher net earnings per share for that quarter or year
and a more favorable ROE (Return on Equity), which is the net income
divided by the shareholder's equity.
Accelerated depreciation on a delivery van, illustrated in Figure 8-1 , can
be thought of as "front loaded" depreciation; the percentages associated
with each year indicate the percentage of total value depreciated or "writ-