Lecture 5 Frames

Associative networks, rules or logic do not provide the ability to group facts into associated clusters or to associate relevant procedural knowledge with some fact or group of facts. A frame is a structure for organizing knowledge with an emphasis on default knowledge (Minsky, 1975). A frame consists of a series of slots each of which represents a standard property or attribute of the element represented by the frame.

Frames New situations are interpreted on the basis of previous experience. This ability allows our knowledge to grow with each experience rather than to start from the initial condition in every case. Using a collection of knowledge consisting of a frame name and a set of attribute-value pairs, a frame represents a stereotypical situation from the world.

Frames The attributes of the attribute-value pairs are often called slots while the values are called fillers. The fillers can additionally be divided into facets: –Range –Default –If-Needed –If-Added –If-Changed

Example - 1

Frames These procedures also called demons represent a powerful concept in frames – the ability to combine procedural knowledge within the declarative knowledge structure of the frame.

Frames Procedural Information If needed procedures describe the process required to establish a value for the associated slot when a value is required. If added procedures are event driven procedures that are triggered by the fact that the associated slot has been assigned a value.

Frames Frames are used for organizing our basic understanding of the things that are typically true of some general class of elements.

Frames This frame describes a generalized subclass of automobiles, it does not need to represent all features shared by automobiles and coupes. It simply identifies those characteristics that distinguish it from a generic automobile.

Frames This frame is a specialization of COUPE which in turn a specialization of AUTOMOBILE, which in turn a specialization of VEHICLE. Most of the properties are inherited from the AUOMOBILE frame with COUPE adding the doors

Frames An important concept of frames is that properties associated with higher level, or merge general frames are considered to be fixed, while the lower level frames may vary but follow the general framework described by the higher frames. When a conflict in values occurs (penguins don’t fly but birds do), the more specialized value takes precedence over the general value.

Reasoning with Frames Reasoning with frames –Matching: retrieving the best applicable frame –Inheritance: applying general information to specific instances

Reasoning with Frames Matching Matching is a complex problem and requires complex algorithms. Description of a specific situation is matched to general description (structure match). There is the problem of establishing the default values for a frame accurately. For example for a tree most will say have leaves, but a person from north may say it has needles.

Reasoning with Frames Inheritance and defaults Organize frames in an is-a hierarchy (is generalization of, is specialization of) Compute properties and values by inheritance of predecessors in the hierarchy.

Reasoning with Frames Multiple Inheritance (advantages) Consider object from different views. Class car Knowledge about Transport objects Knowledge about law of objects Knowledge about mechanical objects

Reasoning with Frames Multiple Inheritance (problems) What in case of conflicting inheritance Republican Quaker person QuakerRepublican

Reasoning with Frames Multiple Inheritance (problems) What in case of conflicting inheritance “Nixon Diamond Problem” instance Nixon class person class quaker slot pacifist:yes class republican slot pacifist:no Is Nixon pacifist?

Multiple Inheritance (problems ) Give an ordering to the parents –Inherit only from the first parent that has a value –Fixed order (ie. From left to right) –Give weights to the links: “Nixon is mainly a republican, and only a quaker if he needed it” Make a separate node “republican-quaker”, with a IF-NEEDED demon for pacifist: Reasoning with Frames Class republican-quaker slot pacifist: IF-NEEDED: if election-year then pacifist = yes else pacifist = no end class

Reasoning with Frames Frames based systems are preferred for model based and case based reasoning. Frames can be used to implement rule based knowledge. Each rule is represented as a frame with its elements ( premise, action, certainty factor) as slots.

Advantages of Frames The advantages of frame based systems include 1.Facilitation of expectation driven processing – through the use of demons is able to specify actions that should take place when certain conditions arise during the processing of the information. 2.Organization of knowledge – information is more structured and organized. 3.Self driven – frames may be organized to determine non- applicability. Should a frame not applicable it can then suggest a frame which is more appropriate. 4.Storage of dynamic values. (especially useful for diagnostics and planning type problems)

Disadvantages of Frames The disadvantages of frame based systems include 1.Departures from prototype 2.Accommodation of new situations – frame based systems do not have prototypes built to guide the representation of new instances. 3.Detailing heuristic knowledge – difficult to describe heuristic knowledge with frames.