Monday, August 29, 2011

Features Reduction part 2: C 5 vs PCA - Road Map

In the last post I showed how reduce the complexity (dimension) of the problem through PCA algorithm.
I mentioned that PCA works well under the condition that the data set has homogenous density.
Are we in this scenario?
I analyzed our data set to verify how the points are distributed; to do that I used the first three components suggested by PCA for the classes having TOPICS = "GOLD" and "GRAIN".
To highlight better the density I soiled the 3D coordinates with a weak gaussian noise just to avoid the presence of coincident points. Here you are the results:
 In green "GRAIN" class, in violet "GOLD" class. Both soiled with weak gaussian noise.
As you can see both the data set are pretty much heterogenous! So the enthusiasm sprung from the higher reduction of PCA is maybe too much optimistic!
A countercheck of this quantitative analysis is provided by the second feature reduction algorithm we are testing: C 5.

A C 5 decision tree is constructed using GainRatio. GainRatio is a measure incorporating entropy. Entropy measures how unordered the data set is. Further details about C.5 are available at Quinlan's website: http://www.rulequest.com
An Extremely interesting book about Entropy application is  the famous "Cover - Thomas":

Elements of Information Theory, 2nd Edition

C 5  is really easy to manage. In the default version you don't need to tune parameters, even if a lot of configurations are available to improve the accuracy. An interesting feature (by my prospective) is the boosting mode. ... but we will discuss about that in the next time...

I wrote a wrapper in Mathematica to:
•  prepare the configurations files (.name file and .data) ;
• adapt the data set from LIBSVM format to C5 format;
• to display the tree output.
Contact me if you desire the notebook.
Let's show the output of C 5 using the default setting:

 The Decision Tree returned by C 5 The leaves (depicted with red triangles) represent the class assigned by the algo for this decision path. A = "GOLD", B= "GRAIN", C= "OIL". The left members of the node inequalities  represent the feature number.
The above Tree represents  the Decision paths for a data set composed by the Three classes having TOPICS = "GOLD","GRAIN","OIL".
To build the vector I used the TF DF feature set of the class "GOLD".
(in the next post I gonna explain why I'm building vectors of class X using feature set of the class Y :) ).
As you can see in the graph , only few features have been used:

Notice that C 5 returns different features respect PCA algo! Moreover it suggests us to use only 7 features: it is an amazing complexity reduction!!
I used C 5 just to analyze the important features, but C 5 is itself a classifier! In the below image the classification accuracy for the mentioned classes.

So C 5 gives us a set of rules to classify documents belonging to three different classes (the rules are depicted by the tree paths from the "start" node to each leaf) with an overall error rate less than 15%.
Is it a good result? Apparently yes, but ...if you have a look to the class "GRAIN" (B) the results are not so brilliant!!.
Don't worry we will see how to improve dramatically these accuracy.
At this point we have discussed about many algorithms, techniques and data set.
So where we are, and where we gonna go?