Databases Reference
In-Depth Information
want to break it down based on the data you've collected. But how do
you construct decision trees from data and what mathematical prop‐
erties can you expect them to have?
Ultimately you want a tree that is something like
Figure 7-4
.
Figure 7-4. Decision tree for Chasing Dragons
But you want this tree to be based on data and not just what you feel
like. Choosing a feature to pick at each step is like playing the game 20
Questions really well. You take whatever
the most informative thing
is
first. Let's formalize that—we need a notion of “informative.”
For the sake of this discussion, assume we break compound questions
into multiple yes-or-no questions, and we denote the answers by “0”
or “1.” Given a random variable
X
,
we denote by
p X
= 1
and
p X
= 0
the probability that
X
is true or false, respectively.
