Database Reference
In-Depth Information
12)
Bummer. No rules found. Did we do all that work for nothing? It seemed like we had
some hope for some associations back in step 9, what happened? Remember from
Chapter 1 that the CRISP-DM process is cyclical in nature, and sometimes, you have to go
back and forth between steps before you will create a model that yields results. Such is the
case here. We have nothing to consider here, so perhaps we need to tweak some of our
model's parameters. This may be a process of trial and error, which will take us back and
forth between our current CRISP-DM step of Modeling and…
EVALUATION
13)
So we've evaluated our model's first run. No rules found. Not much to evaluate there,
right? So let's switch back to design perspective, and take a look at those parameters we
highlighted briefly in the previous steps. There are two main factors that dictate whether
or not frequency patterns get translated into association rules:
Confidence percent
and
Support percent
. Confidence percent is a measure of how confident we are that when
one attribute is flagged as true, the associated attribute will also be flagged as true. In the
classic shopping basket analysis example, we could look at two items often associated with
one another: cookies and milk. If we examined ten shopping baskets and found that
cookies were purchased in four of them, and milk was purchased in seven, and that further,
in three of the four instances where cookies were purchased, milk was also in those
baskets, we would have a 75% confidence in the association rule: cookies → milk. This is
calculated by dividing the three instances where cookies and milk coincided by the four
instances where they
could have
coincided (3/4 = .75, or 75%). The rule cookies → milk
had a chance to occur four times, but it only occurred three, so our confidence in this rule
is not absolute.
Now consider the reciprocal of the rule: milk → cookies. Milk was found in seven of our
ten hypothetical baskets, while cookies were found in four. We know that the coincidence,
or frequency of connection between these two products is three. So our confidence in
milk → cookies falls to only 43% (3/7 = .429, or 43%). Milk had a chance to be found
with cookies seven times, but it was only found with them three times, so our confidence
in milk → cookies is a good bit lower than our confidence in cookies → milk. If a person
