3.3 Case Based Reasoning

*3.3. Case-Based Reasoning (CBR)Knowledge Component 3: Acquiring Data and KnowledgeIan F. C. SmithEPFL, Switzerland

*Module InformationIntended audienceNovice

Key wordsCase-based reasoning, informationretrieval, adaptation, case storageAuthorIan F.C. Smith, EPFL, Switzerland

Reviewers

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*Review Board:Renate Fruchter, ExCom Past Chair, Stanford University, USACarlos Caldas, TCCIT DIM Committee Chair, University of Texas Austin, USABill OBrian, TCCIT IC Committee Chair, University of Texas Austin, USAGuillermo Salazar, TCCIT Edu Committee Chair, Worcester Polytechnic Institute, USAWilliam Rasdorf, TCCIT JCCE editor, North Carolina State University, USAChimay Anumba, Loughborough University, UK.

The ASCE GCEC Officers:Tomasz Arciszewski, ExCom Chair, George Mason University, USAIan Smith, ExCom Vice-Chair, EPFL, SwitzerlandHani Melhem, ExCom Vice-Chair, Kansas State University, USA

The ASCE Technical Council on Computing and IT Officers:Renate Fruchter, ExCom Past Chair, Stanford University, USAKim Roddis, ExCom Chair, George Washington University, USARaymond Issa, ExCom Vice Chair, University of Florida Gainesville, USAHani Melhin, ExCom Secretary, Kansas State University, USAIan Flood, ExCom Member at Large, University of Florida Gainesville, USAIan Smith, ExCom Member, EU Liaison, EPFL, Switzerland

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*OutlineIntroduction

Steps in Case-Based Reasoning

CBR Cycle

CBR in Everyday Life

*Case-based reasoning (CBR) involves finding solutions to new tasks by reusing good solutions to old tasks.

CBR systems have many advantages over other types of computer systems for decision support.

Some of them are given in the following slides.Principle

*The process of storing cases often involves transformations that are computationally less expensive than those necessary with other representations.

A good case is often an easy shortcut in the search for good solutions.

The closed-world assumption associated with abductive tasks (Module 1.3) is explicitly related to the number of available cases.Advantages of CBR

*When only small changes are made to the old case, intrinsic advantages of the old case are transferred to the new task.

Cases are often the best way to represent knowledge, especially when the available models are unreliable.

The capability of the system can be enhanced by simply adding a case.Advantages (cont'd.)

*Large case-based reasoning systems are currently used for diagnosis, maintenance, planning and customer support.

There is much potential in civil engineering.Advantages (cont'd.)



CBR Cycle


*There are five main steps in the development and use of a CBR system.

RepresentationRetrievalAdaptationStorageMaintenance

Five Steps

*This step involves decisions related to how the case is stored in the computer.

For example, if cases are to be stored in a relational database, appropriate attributes have to be selected.

This is an important step since it can greatly influence the quality of support provided.

One has to know clearly for what purpose the cases are intended before finalizing the representation.Step 1: Representation

*Characteristics of both task and solution must be represented.

Task characteristics are used for case retrieval (Step 2) and solution characteristics are used for case adaptation (Step 3).

Step 1: Representation (cont'd.)

*This step involves the selection of a case (or cases) that have task characteristics which are similar to the new task.

Here, the challenge involves defining what the word similar means.

Hundreds of similarity metrics exist. The most common employ nearest neighbor methods. Step 2: Retrieval

*Below is a simple example of a nearest neighbor method for numerical attributes. For a simple nearest neighbor calculation:

For each task attribute, define a maximum value for the difference, Max.diff. This value sets the limit for similarity. Beyond this value, two attribute values are not similar. Steps 2: Retrieval (cont'd.)

*Define weight factors wf using a scale that describes the relative importance of each task attribute. An iterative loop begins here: For the given case, for each task attribute, calculate the difference (absolute value) between the value for the attribute in the case and in the new task. Divide this difference by Max.diff. Values greater than 1 are set to 1. This result is called Distance.

Steps 2: Retrieval (cont'd.)

*Evaluate the similarity variable for each task attributeSimilarity = 1-Distance

Calculate a weighted overall similarity measure between two cases [01]

and return to step 3.Steps 2: Retrieval (cont'd.)

*Report all cases having values for overall similarity that are above a pre-set level.

If possible, it is always preferable to report more than one case. Engineers want computers to give them choices, not answers.

At this point, users may choose to ignore certain cases according to considerations that are not represented in the task attributes.Steps 2: Retrieval (cont'd.)

*The adaptation stage is the most difficult to implement on a computer.

Here, the solution attributes of the retrieved case(s) are modified to suit the new task. This is seldom automated.

User interaction is often essential in order to bring in the domain knowledge necessary for creating a useful solution.Steps 3: Adaptation

*While this stage is poorly supported in many commercial systems, it is the crucial stage of case-based reasoning.

Without this stage, case-based reasoning would be no more than an exercise in information retrieval.Steps 3: Adaptation (cont'd.)

*The storage stage involves storing the newly found solution in the case base for future use.

If the solution is within the scope of solutions that were envisaged when case attributes were defined, this stage is straightforward. Steps 4: Storage

*The maintenance stage involves elimination of cases that are no longer relevant to the tasks that will come in the future.

This stage may also include the task of ensuring that the cases uniformly cover the entire range of possible attribute values.Steps 5: Maintenance



CBR Cycle


*The CBR CycleStorageAdaptationRetrievalData BaseNew TaskMaintenanceInitial Cases



CBR Cycle


*The concept of CBR is used in other fields where decisions are required in open worlds.

MedicineDoctors remember previous medical cases, especially for rare symptoms LawCase histories are always consulted Judgments often depend on precedents

ManagementLearning through study of previous casesCBR in Everyday Life

* Well suited tasksClassification DiagnosisPredictionControl

Harder for CBRSynthesis of systems Engineering designPlanning of complex projects

Suitability of CBR

*List the process steps for all CBR methods.

What are characteristics that indicate that a CBR approach may be suitable?

What is the end criterion for retrieval?

Review Quiz

*List the process steps for all CBR methods.

Retrieve the most similar case(s) comparing the present case to the library of past casesSelect a retrieved case to solve the current taskAdapt the old case to make a new solution Retain the new solution as a new caseMaintain the case baseAnswers to Review Quiz

*What are characteristics that indicate that a CBR approach may be suitable?

Records of previously solved problems existRemembering previous experience is usefulCases can be represented and stored without excessive use of resourcesAdaptation of cases is feasibleMaintenance that ensures complete and uniform coverage is possible

Answers to Review Quiz

*What is the end criterion for retrieval?Retrieval is complete when the best matching case is found.

Answers to Review QuizSubtasks involve:identifying a set of relevant problem descriptorsmatching the case and returning a set of sufficiently similar cases (given a similarity criterion)selecting the best case from the set of cases returned

*CBR involves finding solutions to new tasks by reusing good solutions to old tasks.

The capability of a system can be increased by simply adding a case.

There are 5 stages in the CBR methodology: Representation Retrieval Adaptation Storage MaintenanceSummary

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J. Kolodner. Case-Based Reasoning. Morgan Kaufmann Publishers, San Mateo, CA, 1993

Raphael, B. and Smith, I.F.C. Fundamentals of Computer-Aided Engineering, Wiley, 2003

Case-Based Reasoning on the Web, University of Kaiserslautern, http://www.cbr-web.org

Further Reading

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Documents

3.3 Case Based Reasoning