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Paper Read: Case-Based Reasoning- Foundational Issues, Methodological Variations, and System Approaches

Posted by MHesham on November 28, 2010

Agnar Aamodt, Enric Plaza. 1994. Case-Based Reasoning: Foundational Issues, Methodological Variations, and System Approaches. In AI COMMUNICATIONS.

The paper presents

  • Overview of the field in terms of its underlying foundations, its current state-of the art, future trends.
  • The description of CBR principles, methods and systems in analytical scheme.


  • Case Based Reasoning (CBR):
    • CBR is a problem solving paradigm that instead of relying on general knowledge about the problem domain, CBR is able to utilize cases.
    • A new problem is solved by finding a similar past case, and reusing it in the new problem situation.
    • CBR is an approach to incremental learning, since new experience is retained each time a problem has been solved, making it available for future problems.
  • Case:
    • Case is a specific knowledge of previously experienced concrete problem situation.
    • In CBR terms, it denotes a problem situation.
    • A previously experienced situation, which has been captured and learned in a way that it can be reused in solving future problems.
    • Previous experience is known as retained case, solved case, past case.
    • New problem to be solved is called new case, or unsolved case.

Case-based problem solving

  • Reasoning by re-using past cases is powerful and frequently applied way to solve problems for humans.
  • Several studies have given evidence for the dominating role of specific, previously experienced situations in human problem solving.
  • It has been shown that human uses past-cases as models when learning to solve problems, in early learning.
  • Other results indicate that the use of past-cases isa predominant problem solving method among any domain experts as well.
  • Problem solving is not necessarily the finding of a concrete solution to an application problem.
  • The problem can be put by the user, such as criticize user solutions, generate set of solutions, interpret a problem situation, generate expectations in observable data.

Learning in Case-based reasoning

  • CBR is coupled to learning, it is regarded as a subfield of machine learning.
  • CBR denotes a machine learning paradigm that enables sustained and incremental learning by updating the case-base after a problem has been solved.
  • Learning in CBR occurs as a natural by product of problem solving.
  • When an attempt to solve a problem fails, the reason for the failure is identified and remembered in order to avoid the same mistake in the future.
  • CBR favor learning from experience, because it is easier to learn by retaining a concrete problem solving experience, than generalizing from it.
  • Effective learning in CBR requires a well worked methods in order to:
    • Extract relevant knowledge from the experience.
    • Integrate a case into an existing knowledge structure.
    • Index the case for later matching with a similar case.

Combining cases with other knowledge

  • Human learning and problem solving in general are processes that involve the representation and utilization of several types of knowledge, and the combination of several reasoning methods.

Fundamentals of case-based reasoning methods

  • Central tasks to all CBR methods:
    • Identify the current problem situation.
    • Find a past case similar to the new one.
    • Use the case to suggest a solution to the current problem.
    • Evaluate the proposed solution.
    • Update the system by learning from this experience.

Main types of CBR methods

  • The CBR paradigm covers a rage of different methods for organizing, retrieving, and indexing the knowledge retained in the past cases.
  • Exemplar-based reasoning:
    • The term is derived from a concept view (conceptual modeling) called “exemplar view”
    • Exemplar is a specific instance of a certain category, which is used to represent the category.
    • A concept is defined extensionally, as the set of its exemplars.
    • CBR methods that learn concepts are called exemplar-based.
    • Solving a problem is a classification problem by finding the right class for the unclassified exemplar.
    • The class of the most similar past case becomes the solution for the classification problem.
  • Instance-based reasoning:
    • A specialization of the exemplar-based reasoning into a highly syntactic CBR-approach.
    • To overcome the lack of guidance from general background knowledge, a large number of instances are used to go close to the concept definition.
  • Memory-based reasoning:
    • This approach emphasizes a collection of cases as a large memory, and reasoning as a process of accessing and searching this memory.
    • The utilization of parallel processing techniques is a characteristic of this method.
  • Case-based reasoning:
    • Case-based reasoning is sometimes used as a generic term to such systems. A typical case-based reasoning methods have some characteristics that distinguish them from other methods.
      • A typical case is usually assumed to have a certain degree of richness of information contained in it.
      • A certain complexity with respect to its internal organization.
      • Case-based methods are able to modify, or adapt a retrieved solution when applied in different problem solving context.
  • Analogy-based reasoning:
    • The analogy-based reasoning is sometimes used as a synonym to typical case-based reasoning.
    • The term can be used to characterize methods that solve new problems based on past cases from a different domain. While typical case-based methods focus on using past cases from the same domain.
    • Research on analogy reasoning is therefore a subfield concerned with mechanisms for identification and utilization of cross-domain analogies.
    • The major focus of study has been on the reuse of past, that is the mapping of a solution from an identified analogy (source) to the present problem (target).

A descriptive framework

Description of the CBR methods and system has two main parts:

  • A process model of the CBR cycle
    • A dynamic model that identifies the main sub-process of a CBR cycle, their interdependency and products
  • A task-method structure for CBR
    • A task-oriented view, where a task decomposition and related problem solving methods are described.

The CBR Cycle

  1. Retrieve the most similar case or cases.
  2. Reuse the information and knowledge in that case to solve the problem.
  3. Revise the proposed solution.
  4. Retain the parts of this experience likely to be useful for future problem solving.


  • An initial description of a problem defines a new case.
  • This new case is used to retrieve a case from the previous cases.
  • The new case and the retrieved case are combined in a solved case through reuse which forms an initial solution to the problem.
  • Through revise the solution tested for success by being tested in the real environment, and repaired if failed.
  • During retain the useful experience is retained for future use, and the case base is updated with the new learned case, or a modification of the existing cases.
  • General domain-dependant knowledge supports the CBR process, this support vary from very weak (may be none) to very strong depending on the CBR method used.

A hierarchy of CBR tasks

  • This is a task-oriented view to the CBR model which gives a description for the detailed mechanisms from the CBR perspective, while the process model gives a global, external view to what is happening.
  • A system is viewed as agent which has goals, and means to achieve this goals.
  • A description to the system can be made from 3 perspectives: tasks, methods, and domain knowledge model.
  • Tasks are set up by the system goals, and a task is performed by applying one or more methods.
  • For a method to accomplish a task, it needs a general knowledge about the application domain as well as information about the current problem and its context.
  • A method is an algorithm that identifies and controls the execution of subtasks, it accesses and utilize the information needed to accomplish this.

CBR Problem Areas

  • In AI, there are no universal CBR methods for all application domains.
  • The challenge in CBR is to come up with methods that are suited for problem solving and learning in a subject domain (e.g Medical), and for a particular application environments (e.g Heart failure diagnosis).
  • Core problems address by CBR research can be grouped into 5 areas, a solution to them constitute a CBR method:
    • Knowledge representation.
    • Retrieval methods.
    • Reuse methods.
    • Revise methods.
    • Retain methods.

Representation of Cases

  • A case-based reasoned is heavily dependent on the structure and content of its collection of cases, refereed as case memory.
  • Problem solving is achieved by recalling a previous experience suitable for solving the current new problem, so the case matching and retaining process should be effective and efficient enough.
  • Problems in case representation:
    • Deciding what to store in a case.
    • Finding an appropriate structure for describing the case content.
    • Deciding how the case memory (case-base) should be organized and indexed for fast retrieval and reuse.
    • How to integrate the case memory into a model of general domain knowledge.

The Dynamic Memory Model

  • In dynamic memory model, the case memory is organized in hierarchical structure of ‘episodic memory organization packets’ also known as generalized episodes.
  • The basic idea is to organize specific cases which share similar properties under a more general structure (generalized episodes).
  • A generalized episodes (GE) contains 3 different objects:
    • Norms: features common to all cases indexed under the GE.
    • Indices: features which discriminate between GE`s cases.
    • Cases
  • An index may point to a more specific GE or to a case.


  • Case retrieval
    • When a new case description is given, the input case structure is pushed down the network structure starting from the root.
    • When one or more features of a case match the norms of a GE, the case is further discriminated based on the remaining features.
    • The case with the most common features with the input case is used.
    • As overall:
      • Case retrieval is done by finding the GE with the norms in common with problem description.
      • Indices under the GE are then traversed in order to find the case which contains most of the additional problem features not found in the GE norms.
  • Case retaining
    • It is the same as retrieval in the search method.
    • During storing a new case, when a feature of the new case matches the feature of an existing case, a GE is created and the 2 cases are discriminated under the created GE with different indices.
    • So if during case storage two cases or GEs end under the same index, a new GE is created for them.
    • As overall:
      • Storing a new case is done as the case retrieval, with the additional process of dynamically creating GEs.
  • So the memory structure is dynamic in the sense that similar features of two cases are generalized in a GE, and the cases are indexed under the GE by their different features.
  • The index structure may lead to an explosive growth of indices with increased number of cases.
  • The primary role of a generalized episode is an indexing structure for matching and retrieval of cases.
  • This knowledge organization structure is suitable for learning general and specific knowledge, and it is a simplified model of human reasoning and learning.

The Category and Exemplar Model

  • Cases are referred to as exemplars.
  • The view of this method is that the ‘real world’ natural concept should be defined extensionally.
  • Different features are assigned different importance in describing a case membership to a category.
  • The case-memory is structured as a network of categories, cases, and index pointers.
  • Each case is associated with a category, and each case is stored according to its prototypicality strength to its category.
  • Features are key-value pair.
  • Indices are of three kinds:
    • Feature links: pointing from a feature to cases or categories.
    • Difference links: pointing from a case to its neighbor that differs in only a few numbers of features.
    • Case links: pointing from a category to its associated case (called exemplar links).


  • In this organization, the categories are linked within a semantic network, which also contains features and intermediate states.
  • Case retrieval
    • Finding a case in the case base that matches an input description is done by combining the input features of the problem case into a pointer to a case or category that shares most of the features.
    • If a pointer points to a category, the links to its strongest prototypical exemplar are traversed, and these cases are returned.
    • General domain knowledge is used to match cases semantically.
  • Case retaining
    • A new case is stored by searching for a matching case.
    • If a case is found with only minor differences to the input case, the new case may not be retained, or the two cases may be merged.

Case Retrieval

  • The retrieval task starts with a problem description, and ends when a best matching previous case has been found.
  • Subtasks of case retrieval:
    • Identify Features: it is about coming up with a set of relevant problem descriptors.
    • Initially match: return a set of cases that are sufficiently similar to the new cases, given a threshold for case similarity.
    • Search
    • Select: works on the initial set of cases to find out the best match.
  • Some case-based reasoners retrieve a previous case based on superficial(apparent), syntactical similarities among problem descriptors(features). Others retrieve a previous cases based on features that have deeper, semantical similarities.
  • Semantical similarities:
    • In order to retrieve a case based on semantic similarities and relative importance of features, an extensive body of general domain knowledge is required to say how 2 cases match and how strong is the match.
    • It is referred as “knowledge extensive” approach, and it is able to use the contextual meaning for the problem description in matching.
    • For domains where general domain knowledge is available.
  • Syntactical similarities:
    • It is referred as “knowledge poor” approach.
    • For domains where general domain knowledge is very difficult or impossible.
  • It is important to know the purpose of the retrieval task, If the purpose is to be adapted for reuse then this can be accounted for in the retrieval method.
  • Identify Feature
    • Can be done by simply noticing the input features, but a more elaborate way is to understand the problem within its context.
    • Unknown features may be disregarded or requested for explanation by the user.
    • Understanding a problem involves:
      • Filtering noisy problem features (ignore ineffective features).
      • Infer relevant problem features (values of effective features).
      • Check whether the feature values make sense within the problem context.
      • Expect feature values not provided as input.
    • Inferring the values of not provided features can be done by using general knowledge model, or by retrieving a similar case from the case-base and use its features as the expected features.
      • Example on such feature expectation: previous case-1 has the value of feature A, B and C. I have inferred values A and B for the new case, and if the new case and case-1 match in A, and B then I can infer that the value of C in the new case is as of case-1.
  • Initially Match
    • The task of finding a good match is about finding a set of good candidate cases, then from this set of cases, find the best matching one (Select task).
    • The idea of matching is to use the input features as indices to the case-memory in a direct or indirect way. (there should be a mapping function).
    • The 3 principles of indexing:
      • Follow a direct index pointer from the input features to the case-base. (direct mapping).
      • Searching an index structure.
      • Search in a model of general domain knowledge. (search is guided by the underlying model of the domain).
    • Some systems combine between the 3 principles of indexing to achieve a certain goal.
    • Cases may be retrieved directly from input features, or from inferred features from the input.
    • Cases with the most matching features are a good candidates, but sometimes cases matching a fraction of features may be retrieved depending on the retrieval strategy.
    • If case is retrieved on the basis of a subset of features, a relevance test should be done. This test is about finding whether the retrieved solution is of the type as the expected solution for the new problem.
    • Similarity assessment can be more knowledge-intensive by understand the problem in its context more deeply and use the goals, constraints, etc to guide the matching.
    • Another option is to weight the features according to their importance in categorizing the problem.
  • Select
    • The best matching is done by evaluating the degree of initial match more closely.
    • This is done by attempt to generate explanations to justify non-identical features based on the knowledge in the semantic network.
    • If a match is not strong enough, the difference link between cases is followed to reach the best matching case.
    • The selection typical process is to generate consequences and expectations from each retrieved case, and attempts to evaluate consequences and justify expectations.
    • This generation, evaluation and justification of consequences and expectation are based on model of general domain knowledge, or by asking the user.
    • Rankings are made for the cases using some metric or ranking criteria.
    • Knowledge-intensive selection methods generate explanations that support this ranking process.
    • The case with the strongest explanation for being similar to the new case is selected as a solution to the new problem.
    • Other properties in the case that are considered in some CBR systems:
      • Prototypicality of case with its assigned class.
      • Difference links to related cases.
      • Relative importance and discriminately strengths of features.

Case Reuse

  • The retrieval of a previous case solution to be used in the new case context focus on 2 aspects:
    • What parts of the retrieved case should be transferred to the new case.
    • The difference among the past and the current case.
  • Copy
    • In a simple classification task, the differences are abstracted away (ignored), and the similarities are considered relevant.
    • The solution class of the retrieved case is transferred to the new case as its solution.
    • Other systems adapt the retrieved case to the new case context by taking into account the differences, and adaptation of the solution to account for those differences.
  • Adapt
    • There are 2 ways to reuse past cases:
      • Transformational reuse
        • Reuse the past case solution.
        • It doesn’t look how a problem was solved, it focus on the equivalence of solutions.
        • The past case solution is not directly a solution, there exist some knowledge in the form of transformational operator {T} that when applied to the past case solution transform it into a solution for the new case.
        • This requires a strong domain-dependant model in the form of the {T} operator.
      • Derivational reuse
        • Reuse the past methods that constructed the solution.
        • It looks how the problem was solved in the retrieved case.
        • The retrieved method holds information about the methods used for solving the retrieved problem including operators used, sub goals considered, alternatives generated, failed search path, etc…
        • Derivational reuse re-instantiate the retrieved methods to the new case and replays the old plan in the new case context.
        • During replay, the successful alternatives, paths and operators will be explored first, while unsuccessful ones will be avoided.

Case Revision

  • When a case solution generated by the reuse phase is not correct, an opportunity for learning from failure arise.
  • The revision task is divided in 2 subtasks:
    • Evaluate the case solution generated by reuse.
    • If success, then learn and retain it (a separate phase called Retain).
    • If failure, then repair the case solution using domain-specific knowledge.
  • Evaluate Solution
    • Takes the result from applying the reused case in the real environment.
    • Applying a case is a task outside the CBR, this can be done by asking a teacher or by performing it directly in the real world.
    • Results of applying the suggested solution may take time depending on the type of application:
      • In Medical CBR, in diagnosis, a case result may appear after hours, or may be months, so the case must be marked as unevaluated, so that it can be evaluated later when its results are out.
      • A solution may be applied to a model of the domain, using simulation program.
  • Repair Fault
    • Involves detecting the errors of the current solution and retrieving or generating explanation for them.
    • Failure detection in CHEF system:
      • Casual knowledge is used to generate explanations of why goals of the solution plan where not achieved.
      • Learns the general situations that will cause the failures using explanation-based system.
      • These learnt general situations are used during the reuse phase, to predict the shortcomings of using a certain solution plan.
      • This allows for early detection of errors so that they can be predicted, avoided and handled.
    • Solution repair task:
      • uses the explanations to modify the solution in such a way that failures don’t occur.
    • Solution repair in CHEF system:
      • Failed plan is corrected by a repair module, that adds steps to the plan that will assure that the causes of the errors will not occur.
      • The repair module possesses domain-knowledge about how to compensate or disable the cause of errors in the domain.
    • The revised plan can be retained directly if approved to be successful by the revise module or it can be repaired and revised again.

Case Retainment – Learning

  • It is the process of incorporating what is useful to retain from the new problem solving episode in the existing knowledge.
  • The learning from success or failure from the proposed solution is triggered by the result of the evaluation and possible repairs.
  • Involves selecting which information from the case to retain, in what form to retain it, how to index the case for later retrieval for similar problems, and how to integrate the new case in the memory structure.
  • Extract
    • In CBR the case base is updated no matter how the problem was solved.
    • If it was solved by the use of a previous case, then a new case may be built, or the used case for solution can be generalized to include the new case.
    • If the problem was solved without using a previous case, by asking the user may be, then an entirely new case should be created.
    • It is important to know what to use as the source of learning?
      • Relevant problem descriptors (features) and problem solutions are good candidates.
      • Sometimes it is required to include in the new case the explanation of why a solution is a solution for a problem.
      • In some systems:
        • Explanations are included in the retained case, and they are used in later modification of the solution.
        • The explanation structure is used to search up the model for other states that explains the input data of the new case, and look for the causes of this states as answer to the new problem.
        • This focuses the search and speeds up the explanation process, compared to searching in the entire domain model.
      • The last thing that can be extracted for learning is the problem solving method.
    • Failures recognized from the revise task, can be extracted and retained, either as a separate failure case, or within total problem cases.
    • When a failure is encountered, the system gets a reminding to a similar failure case, and uses it to improve its understanding of the failure causes.
  • Index
    • It is a central and much focused problem in case-based reasoning.
    • It is about deciding what type of indices to use for future retrieval, and how to structure the search space of indices.
    • Direct indices skips the structuring of the indices search space, however, the type of indices used should be identified.
    • A trivial solution is to use all the input features as indices.
    • In memory-based reasoning, all the cases are matched in parallel on the relevant features, the cases with the few features in common are filtered out.
  • Integrate
    • It is the final step of updating the knowledge base with new case knowledge.
    • Modifying the indexing of the current cases improves the CBR system, and allows it to become a better similarity assessor.
    • Index strengths or importance relevant to the case are adjusted due to the success or failure of using the case to solve the input problem.
    • Features that are judged as relevant to the case their association with the case is strengthened, while weaken of those that lead to unsuccessful cases.
    • In PATDEX system:
      • There is a relevance matrix that links features to the diagnosis (problem solutions) for which they are relevant, this matrix represents the weights of such links, these weights are strengthened or weakened based on the feedback of success or failure.
    • In knowledge intensive-CBR approaches:
      • Learning can take place within the general conceptual knowledge model.
      • The system incrementally refines and extends its general domain knowledge model.
    • The case just learnt can be tested by re-entering initial problem to the system, and finds whether the system will behave as expected.

Integrated approaches

  • Most CBR systems use general domain knowledge in addition to the knowledge represented by cases.
  • The overall architecture of CBR system has to decide interactions between the CBR method and the other components.
  • CASEY system:
    • It integrates model-based casual reasoning in the diagnosis, when the case-based method fail to find a correct solution, the mode-based method is executed to find a solution, the solution is then stored as a new case for future use. Because the model-based method is complex and slow, the case-based method represents a speed up in reasoning when it can.
  • BOLERO system:
    • It integrates a rule-based system (base-level) with a case-based planner(meta-level).
    • The base-level contains domain knowledge, while the meta-level contains strategic knowledge.
    • The case-based planner is used to control the space searched by the base-level achieving a speed up.
    • The control is passed to the meta-level whenever a new information is know by the base-level assuming that the system will able to deduce a better strategic plan based on the new information.

Example applications and tools

  • Application
    • The CBR used because the knowledge needed to perform the task resides in the head of few people.
    • There is no theory, and very few generally applicable schemes for doing this job.
    • Building up experience in the form of previously successful and unsuccessful situations is important.


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