Case Maintennce的案例维护

1、case base maintenance (cbm)fabiana prabhakarcse 435november 6, 2006introductionnthe growing use of cbr applications has brought with it increased awareness of the importance of case-base maintenance (cbm).nlarge scale cbr systems are becoming more prevalent, with case library sizes ranging from thou

2、sands to millions of cases.nlarge case-bases raises concern about the utility problem for case retrieval, underlining the potential need to control case-base growth through case deletion policies. definition cbm is the process of refining a cbr systems case base to improve the systems performance. s

3、tandard cbr learningnthe system always add each new case to the case base.ndomain expert adds a variable number of new cases. nindexing of the cases.knowledge-based systems utility problemnthe cost associated with searching for relevant knowledge outweighs the benefit of applying the knowledge.tradi

4、tional deletion policiesna simple deletion policy is random deletion. according to this policy a random item is removed from the knowledgebase once the knowledge-base size exceeds some predefined limit. nmintons utility metric minton, 1990. chooses a knowledge item for deletion based on an estimate

5、of its performance benefits. utility=(applicationfreq*averagesavings)-matchcostremembering to forget ncompetency preserving case deletion policy for cbr systems (smyth and keane, 1995)coverage and reachabilityncoverage of a case is the set of target problems that can be solved by such case.nreachabi

6、lity of a target problem is the set of cases that can be used to provide a solution for the target problem.case competence categoriesnpivotal cases: its deletion directly reduces the competence of the system. a case is pivotal if it is reachable by no other case but itself.nauxiliary cases: do not e

7、ffect competence at all. a case is auxiliary case if the coverage it provides is subsumed by the coverage of one of its reachable cases.case competence categories (cont.)nspanning cases: do not directly affect the competence. their coverage spaces link regions of the problem space that are independe

8、ntly covered by other cases. if cases from this linked regions are deleted, then the spanning case might be necessary. nsupport cases: a special class of spanning cases. they exist in groups. the deletion of the group is analogous to removing a pivotal case.case competence categories (cont.)case com

9、petence categories (cont.)nthe case categories provide a means of ordering cases for deletion in terms of their competence contributions. 1.auxiliary cases (they make no direct contribution to competence)2.support cases3.spanning cases4.pivotal cases. modeling case competencencompetence categories a

10、re computed at start-up.nduring future problem solving as cases are learned, the case categories must be updated:1.re-compute the coverage and reachability sets of the appropriate cases;2.adjust the categories accordingly.the footprint deletion policynideally a deletion policy should work to remove

11、irrelevant cases guiding the case-base toward an optimal configuration of cases.ncompetence footprint is this optimal case-base. it provides the same competence of the entire case-base but with fewer cases.the footprint deletion algorithmdeletecase(cases):if there are auxiliary cases thenselectauxil

12、iary(auxiliarycases)elseif there are support cases thenwith the largest support groupselectsupport(suportgroup)elseif there are spanning cases thenselectspanning(spanningcases)elseif there are pivotal cases thenselectpivot(pivotalcases)endifthe footprint utility deletion policyncombine footprint and

13、 utility deletion:nmintons utility metric an item is selected based on an estimate of its performance benefits.utility = (applicationfreq * averagesavings) matchcostnthe footprint method is used to select candidates for deletion. if there is only one such candidate then it is deleted. nif, however,

14、there a number of candidates, then rather than selecting the one with the least coverage or largest reachability set, the candidate with the lowest utility is chosen. nin other words the utility metric is used within the selectpivot, selectspanning, selectsupport, and selectauxiliary procedures.furt

15、her applicationsnthe competence modeling approach may be used during the initial case acquisition stage of system development. it is often undesirable to store every available case in the initial case-base.1.utility problem;2.irrelevant cases may introduce noise into the retrieval stage and lead to

16、the selection of suboptimal cases or difficulties in tuning the similarity metric.nthe competence modeling approach may be used during the authoring process.cbr systems authoring processncase base authoring can be a long, difficult, and tedious process, and the only advice given to the author is oft

17、en of the “choose representative cases” variety. nthis can ultimately lead to the development of poor case bases, which offer limited coverage of the target problem space, and which include significant redundancy.cascade (case authoring support & development environment)nkeeps the knowledge engi

18、neer informed about how case authoring is progressing, and in particular, how case base competence is evolving.nextends the case competency model proposed by smyth and keane.competence groupsna competence group is a collection of related cases.nthe key idea underlying the definition of a competence

19、group is that of shared coverage. two cases exhibit shared coverage if their coverage or reachability sets overlap. the evolution of competencenin general as cases are added to the case base one of four things can happen:1.new groups are created;2.existing competence groups grow in size and coverage

20、;3.a number of existing groups merge to form a new super group;4.existing groups can grow in size but without increasing coverage. nconversely, as cases are deleted, groups may disappear altogether, or they may split into smaller sub groups.the competence visualization toolcompetence regionsthe comp

21、etence visualization tool examplesthe competence visualization tool examples (cont.)conclusionnexperience with the growing number of large-scale cbr systems has led to increasing recognition of the importance of case-base maintenance.nmultiple researches have addressed pieces of the cbm problem, considering such issues as maintaining consistency and controlling case-base growth.nthe authoring process can be improv