Class Discovery (MAHOUT) edited by abdelhakim deneche
Page: http://cwiki.apache.org/confluence/display/MAHOUT/Class+Discovery
Changes:
http://cwiki.apache.org/confluence/pages/diffpagesbyversion.action?pageId=100778&originalVersion=6&revisedVersion=7
Content:
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h1. Intro
See http://www.cs.bham.ac.uk/~wbl/biblio/gecco1999/GP-417.pdf
CDGA uses a Genetic Algorithm to discover a classification rule for a given
dataset.
A dataset can be seen as a table:
|| ||attribute 1||attribute 2||...||attribute N||
|row 1|value1|value2|...|valueN|
|row 2|value1|value2|...|valueN|
|...|...|...|...|...|
|row M|value1|value2|...|valueN|
An attribute can be numerical, for example a "temperature" attribute, or
categorical, for example a "color" attribute. For classification purposes, one
of the categorical attributes is designated as a *label*, which means that its
value defines the *class* of the rows.
A classification rule can be represented as follows:
|| ||attribute 1||attribute 2||...||attribute N||
|weight|w1|w2|...|wN|
|operator|op1|op2|...|opN|
|value|value1|value2|...|valueN|
For a given *target* class and a weight *threshold*, the classification rule
can be read :
{code}
for each row of the dataset
if (rule.w1 < threshold || (rule.w1 >= threshold && row.value1 rule.op1
rule.value1)) &&
(rule.w2 < threshold || (rule.w2 >= threshold && row.value2 rule.op2
rule.value2)) &&
...
(rule.wN < threshold || (rule.wN >= threshold && row.valueN rule.opN
rule.valueN)) then
row is part of the target class
{code}
*Important:* The label attribute is not evaluated by the rule.
The threshold parameter allows some conditions of the rule to be skipped if
their weight is too small. The operators available depend on the attribute
types:
* for a numerical attributes, the available operators are '<' and '>='
* for categorical attributes, the available operators are '!=' and '=='
The "threshold" and "target" are user defined parameters, and because the label
is always a categorical attribute, the target is the (zero based) index of the
class label value in all the possible values of the label. For example, if the
label attribute can have the following values (blue, brown, green), then a
target of 1 means the "blue" class.
For example, we have the following dataset (the label attribute is "Eyes
Color"):
|| ||Age||Eyes Color||Hair Color||
|row 1|16|brown|dark|
|row 2|25|green|light|
|row 3|12|blue|light|
and a classification rule:
|weight|0|1|
|operator|<|!=|
|value|20|light|
and the following parameters: threshold = 1 and target = 0 (brown).
This rule can be read as follows:
{code}
for each row of the dataset
if (0 < 1 || (0 >= 1 && row.value1 < 20)) &&
(1 < 1 || (1 >= 1 && row.value2 != light)) then
row is part of the "brown Eye Color" class
{code}
Please note how the rule skipped the label attribute (Eye Color), and how the
first condition is ignored because its weight is < threshold.
h1. Running the example:
NOTE: Substitute in the appropriate version for the Mahout JOB jar
# cd <MAHOUT_HOME>/examples
# ant job
# {code}<HADOOP_HOME>/bin/hadoop dfs -put
<MAHOUT_HOME>/examples/src/test/resources/wdbc wdbc{code}
# {code}<HADOOP_HOME>/bin/hadoop dfs -put
<MAHOUT_HOME>/examples/src/test/resources/wdbc.infos wdbc.infos{code}
# {code}<HADOOP_HOME>/bin/hadoop jar
<MAHOUT_HOME>/examples/build/apache-mahout-examples-0.1-dev.job
org.apache.mahout.ga.watchmaker.cd.CDGA
<MAHOUT_HOME>/examples/src/test/resources/wdbc 1 0.9 1 0.033 0.1 0 100 10
{code} *TODO*: Fill in what these parameters mean. See the CDGA class
javadocs. Also fill in where to find the output and what it means.
h1. The info file:
To run properly, CDGA needs some informations about the dataset. Each dataset
should be accompanied by an .infos file that contains the needed informations.
for each attribute a corresponding line in the info file describes it, it can
be one of the following:
* IGNORED
if the attribute is ignored
* LABEL, val1, val2,...
if the attribute is the label (class), and its possible values
* CATEGORICAL, val1, val2,...
if the attribute is categorial (nominal), and its possible values
* NUMERICAL, min, max
if the attribute is numerical, and its min and max values
This file can be generated automaticaly using a special tool available with
CDGA.
{code}$ <HADOOP_HOME>/bin/hadoop jar
<MAHOUT_HOME>/examples/build/apache-mahout-examples-0.1-dev.job
org.apache.mahout.ga.watchmaker.cd.tool.CDInfosTool dataset_path{code}
* the tool searches for an existing infos file (*must be filled by the user*),
in the same directory of the dataset with the same name and with the ".infos"
extension, that contain the type of the attributes:
** 'N' numerical attribute
** 'C' categorical attribute
** 'L' label (this also a categorical attribute)
** 'I' to ignore the attribute
each attribute is in a separate
* A Hadoop job is used to parse the dataset and collect the informations. This
means that *the dataset can be distributed over HDFS*.
* the results are written back in the same .info file, with the correct format
needed by CDGA.
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