Berry

Preparing Data for Mining 579
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y = 0.1007x + 49.625
R 2 = 0.0135
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Figure 17.15 The slope of the line of best fit provides a good measure of
changes over time.
This example shows a very typical use for calculating the slope—finding the
slope over the previous year’s usage or billing patterns. The tabular format
shows the calculation in a way most suitable for a spreadsheet. However,
many data mining tools provide a function to calculate beta values directly
from a set of variables in a single row. When such a function is not available, it
is possible to express it using more basic arithmetic functions.
Although monthly data is often the most convenient for such calculations,
remember that different months have different numbers of days. This issue is
particularly significant for businesses that have strong weekly cycles. Some
months have five full weekends, for instance, while others only have four. Different
months have between 20 and 23 working days (not including holidays).
These differences can account for up to 25 percent of the difference between
months. When working with data that has such cycles, it is a good idea to calculate
the “average per weekend” or “average per working day” to see how
the chosen measure is changing over time.
TIP When working with data that has weekly cycles but must be reported by
month, consider variables such as “average per weekend day” or “average per
work day” so that comparisons between months are more meaningful.