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基于改进信息熵离散化算法的研究
陈臣 1,周炎涛 1,2
1. 湖南大学计算机与通信学院,长沙(410082)
2. 海军工程大学电气与信息工程学院,武汉(430033)
摘 要:本文研究连续属性的离散化问题。首先,详细介绍了基于熵的离散化算法(EBD),
并对其存在的问题进行了分析。随后,给出了用于度量区间密度的定义;接着,在自适应思
想的启发下,对 EBD算法进行了改进,提出了基于熵的变阀值离散化算法, 区间密度的引
入使得该算法能够随样本集在区间上密度的变化适当调整熵的阀值。实验结果表明,与 EBD
算法相比,改进算法不仅保持简单性、一致性和精确性,而且容易操作。
关键词:信息熵;自适应;离散化
离散化技术用来减少连续属性值的个数,这对于使用基于决策树的分类挖掘方法非常有
益。例如ID3决策树算法,Konenko等人指出,在归纳建树阶段为一个非叶节点选择分裂属
性时,使用嫡函数将偏向于取值较多的属性,所以连续型属性由于拥有较多的取值而更容易
被选为分裂属性[1,2],从而由当前节点将导出很多的分支,使下层节点中的样本数据较快地
进入所谓的“纯”状态,即节点中的样本属于同一类别,但样本个数较少,甚至只有一个样本。
所以,最终生成的决策树所表示的规则缺乏适应性,这意味着规则由于支持度较低而不具有
实际意义,而且不容易被用户理解。也就是说,对于决策树对应的每一条规则,能够满足该
规则的数据很少,最后的结果是分类效果很差。由此可见,在建立决策树之前,对连续型属
性进行离散化是十分必要的。
1 离散化算法的目标
从直观的角度看,可以从以下几点[3]衡量一个离散化算法是否是成功的:
(1)完全离散化
就是指算法要能够完成数据集的多个连续属性的离散化处理。因为我们不大可能只需
要对数据集的某一个连续属性进行离散化处理,除非数据集只包含一个连续属性。
(2)具有最简单的离散化结果
一般来说,如果离散化处理完成后,属性空间的规模越小,由这些离散化处理所产生
出来的数据所生成的规则就越简单。因此,由这样的属性所获得的知识就更通用。
(3)一致性
具有连续属性的数据集通常都是一致的,如果应用于某个属性的离散化处理是不成功
的,一个不一致的数据集可能就会产生了。当这种情况发生时,我们就丢失了有价值的信息。
因此,离散化处理应该尽可能保证经过离散化处理后所得到的数据集的一致性水平与原始数
据集的一致性水平接近.
(4)预测精度
离散化在多大程度上能提高精度,精度可以通过有效模式的分类结果得到。对一个给
定的数据集,存在一种或多种最优的离散化结果,但遗憾的是,人们已经证明连续属性的最
优离散化问题是一个NP难题[4].因此,对具有丰富样本的信息系统而言,人们只能试图获得
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一个次最优的离散化算法,在保证离散化结果性能要求的前提下,用尽可能少的断点将属性
空间划分成尽可能少的子空间,达到前面的四条标准,就成了离散化算法的追求目标。
2 常用离散化算法
目前存在的离散化算法很多[5~8],文献[1]对离散化算法作了详细的分类和介绍。根据
离散化过程中是否考虑到类别属性,可以将离散化算法分为有监督和无监督两类。由于有监
督算法充分利用了类别属性的信息,所以在分类中能获得较高的正确率.EBD算法属于有监
督算法,它引入了信息论中熵的概念,充分利用了类别属性的信息,使得它更有可能将区间
的边界定位在准确位置,因而得到了广泛的应用。
下面简单介绍EBD算法[1]
给定一个样本集合 S,类别数为m;设 s为 S中的一个样本, As 为 s在属性 A上的取
值;属性 A为连续型属性, A的取值集合为 1 2{ | } { , ,..., }A A nS s s S a a a= ∈ = ,对 A进行基于
熵的离散化过程如下:
1) 对属性 A的所有取值从小到大进行排序,不妨设得到的序列为: 1 2, ,..., na a a ;
2) 认为每个 1 ( 1, 2,..., 1)
2
i i
i
a aT i n++= = − 为一个潜在的区间边界,称 iT 为候选分割
点。即 iT 将样本集合 S划分为两个子集 1 { | }i A iS s S s T= ∈ ≤ 和 2 { | }i A iS s S s T= ∈ > 。选择
iT ,使得将其作为分割点划分 S后的熵最小,熵的计算方法如下式:
1 2
1 2
| | | |( , ) ( ) ( )
| | | |
i i
i i i
S SE S T E S E S
S S
= + 其中, 1( )iE S 和 2( )iE S 按下式计算:
2
1
( ) log
m
ki kl kl
l
E S p p
=
= −∑ 式中, klp 为类别 l在子集 kS 中的概率。记 iT 为T ,划分后的两
个子集为 1S 和 2S 。
3) 当划分后得到的熵 ( , )E S T 大于阀值δ 时,递归地对 1S 和 2S 进行如上划分。
EBD算法有很多变种,其中较著名的有Fayyad和Irani提出的使用最小描述长度(MDLP)
原则作为划分终止准则的算法,以及Catlet提出的D-2方法。
尽管在一些应用问题中,EBD算法相当成功,但是由于这种方法需要人为取定阀值δ ,
因此在划分区间时会导致一些问题。例如,离散化后两个原始样本条件属性值相同,但是分
类属性值(决策属性)却不相同,这种现象在数据挖掘领域中称为“数据冲突”,从统计角度讲,
这种现象的发生严重改变了原始的数据分布。导致数据冲突的根本原因是阀值δ 取值过大,
离散化结果未能真实的反映出原始数据的差别。为了避免这一现象的发生,必须缩小阀值δ 。
但是,具体应当选择哪些属性的阀值进行缩小,缩小到何种程度,这是很难确定的,需要根
据数据分布情况进行交互式的选择。综上所述,EBD算法在易操作性方面还有所欠缺,所以
应当寻找一种新的方法,使得离散化过程在简单性、一致性、精确性和易操作性方面达到较
好的结合点。
3 改进的离散化算法
首先估计 ( , )E S T 的取值区间,当 1S 和 2S 中的样本都属于同一类时, ( , ) 0E S T = ;当
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1S 和 2S 中的样本各属于不同类时, 2( , ) logE S T m= ,所以可知 [ ]2( , ) 0, logE S T m∈ (其
中m为类别数)。
EBD算法中阀值δ 所起的作用相当于对于原始数据的“分辨率”。δ 越小,分辨率越大,
离散化后的数据就越接近于原始数据[7],虽然这时产生数据冲突的可能性较小,但离散化
后的区间个数偏多,最为极端的情况是取 0δ = ,此时离散化结果最接近原始数据甚至完全
相同;δ 越大,分辨率越小,由于区分数据之间差别的能力下降,产生数据冲突的可能性将
增大,最极端的情况是 2log mδ = ,此时原始数据的取值区间仅被划分一次,生成两个子
区间。
由于EBD算法采用恒定的阀值,在递归分割区间的过程中没有考虑区间内样本的密度成
都,因而导致区间划分不是过“细”就是过“粗”,而且选择适当的阀值变得相当困难。为了使
离散过程具有自适应能力(即根据区间内样本的密度程度适当的变化阀值),本文尝试给出
一种基于熵的变阀值离散化算法。为了衡量区间上的样本密集程度,作如下定义:
定义 设 I 为实数集上的一个区间,S为 I 内所有样本组成的集合, ( )f I 为区间 I
内的样本频度,即: ( ) | |f I S= , ( )l I 为区间 I 的长度,则称
( )( )
( )
f ID I
l I
= ()
为样本集 S在区间 I 上的密度。
1) 计算 ( )E S ,令 0 ( )E Sδ = ;
2) 对属性 A的所有取值从小到大进行排序,不妨设得到的序列为: 1 2, ,..., na a a ;
3) 认为每个 1 ( 1, 2,..., 1)
2
i i
i
a aT i n++= = − 为一个潜在的区间边界,称 iT 为候选分割
点。即 iT 将样本集合 S划分为两个子集 1 { | }i A iS s S s T= ∈ ≤ 和 2 { | }i A iS s S s T= ∈ > 。选择
iT ,使得将其作为分割点划分 S后的熵最小,熵的计算方法如下式:
1 2
1 2
| | | |( , ) ( ) ( )
| | | |
i i
i i i
S SE S T E S E S
S S
= + 其中, 1( )iE S 和 2( )iE S 按下式计算:
2
1
( ) log
m
ki kl kl
l
E S p p
=
= −∑ 式中, klp 为类别 l在子集 kS 中的概率。记 iT 为T ,划分后的两
个子集为 1S 和 2S 。
4)计算待划分区间 iS 的阀值δ , 0( )D lδ δ= ;(其中 ( )D l 按()式计算)
5) 当划分后得到的熵 ( , )E S T 大于阀值δ 时,递归地对 1S 和 2S 进行如上划分。
4 实验结果
我们以加利福尼亚Irvine分校提供的Iris数据集(共有4个连续型属性,150个样本)为样本
数据,利用ID3决策树对数据集进行分类,并以以下几个方面进行比较分析:
(1) 简单性分析:
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表 EBD算法与改进算法的简单性比较
EBD 改进EBD
区间总数 29 24
该算法对数据集的离散化后得到的区间总数要少,这样就降低了离散过程中的迭代次
数,也就降低了计算的复杂程度;
(2) 精确性比较:
表 EBD算法与改进算法的精确性比较
EBD 改进EBD
预测正确率
叶节点数目 18 23
该算法略微能提高分类的正确率,这是由于按此算法离散化后生成的决策树复杂程度
较高,由 23个叶节点组成;
(3)易操作性分析:
从算法实施过程中,无需反复调整各属性的阀值,它可以在离散化过程中自动确定。
因此,该算法在实际应用中是有效且便捷的。
5 结论
本文分析了EBD算法存在的缺陷,并通过实验的方式验证了以自适应的方式适当变换离
散化过程的阀值,可以有效的提高算法的准确性和可操作性。
需要指出的是,目前仍然没有一种离散化方法是“普适性”的,即对任意的数据集均能取
得良好的离散化结果。该算法并不能完全避免“数据冲突”现象的发生,只能说在一定程度上
降低了数据冲突发生的可能性。要完全的避免数据冲突,需要在离散化过程中全面考虑数据
集的所有属性,这可以作为后续工作进一步研究。
参考文献
[1] Huan L.,Farhad Manoranjan : An Enabling Technique[J].Data Ming and Knowledge
Discovery, 2002,6: 393~423
[2] 史忠植.知识发现[M].北京:清华大学出版社,2002.
[3] Cheielewski M, Crzrmala-Busse J. Global Discretization of Continuous Attributes as Preprocessing for
Machine Learing[J].International Journal of Approximate Reasoning, 1996,15
[4] and mining: Concepts and Techniques[M].Morgan Kaufmann, San Francisco, CA,
2001.
[5] Tay . , Shen L. , A modified Chi2 algorithm for discretization. IEEE Transactions on Knowledge and Data
Engineering, (3):666~670
[6] Su Chao-Ton , Hsu Jyh-Hwa. An extended Chi2 algorithm for discretization of real value attributes. IEEE
Transactions on Knowledge and Data Engineering, (3);437~441
[7] Zhao Jun,Wang Guo-yin,Wu Zhong-fu. New algorithms for data discretization based rough set theory. Journal
of chongqing University Aatural Science Edtion, (3):18~21.
[8] 刘震宇 廓宝龙,杨林耀 一种新的用于连续值属性离散化的约简算法.控制与决策,2002,17(5):545-549
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An Algorithm of Discretization based on Entropy
Chen Chen1,Zhou Yantao1,2
of Computer and Communication,Hunan University, ChangSha(410082)
2. Information and Electrical Engineering College of Naval Engineering University,Wuhan
(430033)
Abstract
In this paper, discretization methods of continuous attributes are researched. Firstly , we introduce
Entropy-Based Discretization algorithm (EBD) and discuss some limits in it. Secondly, the concepts of
density are defined. Then, in the self-fit idea , we propose a new algorithm based on the EBD
algorithm ,that can adjust the threshold of entropy according to the variation of the density of sample
set . At last, we apply this algorithm to two datasets. Experimental results show that , by comparing
with EBD and this algorithm ,not only maintains simplicity, consistency and accuracy but also is easily
operated
Keywords: entropy,self-fit,discretization;
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/ITA <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>
/NOR <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>
/SVE <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>
>>
>> setdistillerparams
<<
/HWResolution [2400 2400]
/PageSize [ ]
>> setpagedevice