Generation of Referring Expressions (GRE) Reading: Dale & Reiter (1995) (key paper in this area)

The task: GRE NLG can have different kinds of inputs: –Flat data (collections of atoms, e.g., in the tables of a database) –Logically complex data In both cases, unfamiliar constants may be used, and this is sometimes unavoidable

No familiar constant available: 1.The referent has a familiar name, but its not unique, e.g., John Smith 2.The referent has no familiar name: trains, furniture, trees, atomic particles, … ( In such cases, databases use database keys, e.g., Smith$73527$, TRAIN-3821 ) 3. Similar: sets of objects (lecture 4).

Natural Languages are too economic to have a proper name for everything Names may not even be most appropriate So, speakers/NLG systems have to invent ways of referring to things. E.g., the 7:38 Trenton express Note: the problem arises whether the referent is a token or a type

GRE tries to find the best description GRE is microcosm of NLG: e.g., determines –which properties to express (Content Determination) –which syntactic configuration to use (Syntactic Realization) –which words to choose (Lexical Choice)

This lecture: Simplification 1: Content Determination only (until lecture 5). Simplification 2: Definite descriptions only (Pronouns, demonstratives, etc., are disregarded; until tomorrow)

Dale & Reiter (1995): best description fulfills the Gricean maxims. E.g., (Quality:) list properties truthfully (Quantity:) list sufficient properties to allow hearer to identify referent – but not more (Relevance:) use properties that are of interest in themselves * (Manner:) be brief * Slightly different from D&R 1995

D&Rs expectation: Violation of a maxim leads to implicatures. For example, – [Quantity] the pitbull (when there is only one dog). –[Manner] Get the cordless drill thats in the toolbox (Appelt). Theres just one problem: …

…people dont speak this way For example, –[Manner] the red chair (when there is only one red object in the domain). –[Manner/Quantity] I broke my arm (when I have two). General: empirical work shows much redundancy Similar for other maxims, e.g., –[Quality] the man with the martini (Donellan)

Example Situation a, £100 b, £150 c, £100 d, £150 e, £? SwedishItalian

Formalized in a KB Type: furniture (abcde), desk (ab), chair (cde) Origin: Sweden (ac), Italy (bde) Colours: dark (ade), light (bc), grey (a) Price: 100 (ac), 150 (bd), 250 ({}) Contains: wood ({}), metal ({abcde}), cotton(d) Assumption: all this is shared knowledge.

Violations of … Manner: *The £100 grey Swedish desk which is made of metal (Description of a) Relevance: The cotton chair is a fire hazard? ?Then why not buy the Swedish chair? (Descriptions of d and c respectively)

In fact, there is a second problem with Manner. Consider the following formalization: Full Brevity: Never use more than the minimal number of properties required for identification (Dale 1989) An algorithm:

Dale 1989: 1.Check whether 1 property is enough 2.Check whether 2 properties is enough …. Etc., until success {minimal description is generated} or failure {no description is possible}

Problem: exponential complexity Worst-case, this algorithm would have to inspect all combinations of properties. n properties combinations. Recall: one grain of rice on square one; twice as many on any subsequent square. Some algorithms may be faster, but … Theoretical result: algorithm must be exponential in the number of properties.

D&R conclude that Full Brevity cannot be achieved in practice. They designed an algorithm that only approximates Full Brevity: the Incremental Algorithm.

Incremental Algorithm (informal): Properties are considered in a fixed order: P = A property is included if it is useful: true of target; false of some distractors Stop when done; so earlier properties have a greater chance of being included. (E.g., a perceptually salient property) Therefore called preference order.

r = individual to be described P = list of properties, in preference order P is a property L= properties in generated description (Recall: were not worried about realization today)

P = < furniture (abcde), desk (ab), chair (cde), Swedish (ac), Italian (bde), dark (ade), light (bc), grey (a), 100£ ({ac}), 150£(bd), 250£ ({}), wooden ({}), metal (abcde), cotton ({d}) > Domain = {a,b,c,d,e}. Now describe: a = d = e =

P = < furniture (abcde), desk (ab), chair (cde), Swedish (ac), Italian (bde), dark (ade), light (bc), grey (a), 100£ (ac),200£ (bd),250£ ({}), wooden ({}), metal (abcde), cotton (d) > Domain = {a,b,c,d,e}. Now describe: a = d = (Nonminimal) e = (Impossible)

[ An aside: shared info will be assumed to be complete and uncontroversial. Consider Speaker: [[Student]] = {a,b,…} Hearer: [[Student]] = {a,c,…} Does this make a referable? ]

Incremental Algorithm Its a hillclimbing algorithm: ever better approximations of a successful description. Incremental means no backtracking. Not always the minimal number of properties.

Incremental Algorithm Logical completeness: A unique description is found in finite time if there exists one. (Given reasonable assumptions, see van Deemter 2002) Computational complexity: Assume that testing for usefulness takes constant time. Then worst-case time complexity is O(n p ) where n p is the number of properties in P.

Better approximation of Full Brevity (D&R 1995) Attribute + Value model: Properties grouped together as in original example: Origin: Sweden, Italy,... Colour: dark, grey,... Optimization within the set of properties based on the same Attribute

Incremental Algorithm, using Attributes and Values r = individual to be described A = list of Attributes, in preference order Def: = Value i of Attribute j L= properties in generated description

FindBestValue(r,A): - Find Values of A that are true of r, while removing some distractors (If these dont exist, go to next Attribute) - Within this set, select the Value that removes the largest number of distractors - If theres a tie, select the most general one - If theres still a tie, select an arbitrary one

Example: D = {a,b,c,d,f,g} Type: furniture (abcd), desk (ab), chair (cd) Origin: Europe (bdfg), USA (ac), Italy (bd) Describe a: {desk, American} (furniture removes fewer distractors than desk) Describe b: {desk, European} (European is more general than Italian) N.B. This disregards relevance, etc.

P.S. Note the similarity with Van Rooy & Dekkers semantic of answers: Let A and B be truthful answers to a question, then A is a better answer than B Utility(A) > Utility(B) or Utility(A) = Utility(B) & B A (More about this in the next lecture …)

Exercise on Logical Completeness: Construct an example where no description is found, although one exists. Hint: Let Attribute have Values whose extensions overlap.

Example: D = {a,b,c,d,f} Contains: wood (abe), plastic (acdf) Colour: grey (ab), yellow (cd) Describe a: {wood, grey,...} - Failure (wood removes more distractors than plastic) Compare: Describe a: {plastic, grey} - Success

Complexity of the algorithm n d = nr. of distractors n l = nr. of properties in the description n v = nr. of Values ( for all Attributes ) Alternative assessment: O(n v ) (Worst-case running time) According to D&R: O(n d n l ) (Typical running time)

Minor complication: Head nouns Another way in which human descriptions are nonminimal –A description needs a Noun, but not all properties are expressed as Nouns –Example: Suppose Colour was the most-preferred Attribute, and target = a

Colours: dark (ade), light (bc), grey (a) Type: furniture (abcde), desk (ab), chair (cde) Origin: Sweden (ac), Italy (bde) Price: 100 (ac), 150 (bd), 250 ({}) Contains: wood ({}), metal ({abcde}), cotton(d) target = a Describe a: {grey} The grey ? (Not in English)

D&Rs repair: Assume that Values of the Attribute Type can be expressed in a Noun. After the core algorithm: - check whether Type is represented. - if not, then add the best Value of the Type Attribute to the description

Versions of Dale and Reiters Incremental Algorithm have often been implemented Still the starting point for many new algorithms. (See later lectures.) Worth reading!

Limitations of the algorithm 1.Redundancy does not arise for principled reasons, e.g., for - marking topic changes, etc. (Corpus work by Pam Jordan et. al.) - making it easy to find the referent (Experimental work by Paraboni et al. - Next lecture)

Limitations of the algorithm 2.Targets are individual objects, never sets. What changes when target = {a,b,c} ? (Lecture 4) 3.Incremental algorithm uses only conjunctions of atomic properties. No negations, disjunctions, etc. (Lecture 4)

Limitations of the algorithm 4.No relations with other objects, e.g., the orange on the table. (Lecture 3) 5.Differences in salience are not taken into account. (Lecture 3) 6.Language realization is disregarded. (Lecture 5)

Discussion: How bad is it for a GRE algorithm to take exponential time? –More complex types of referring expressions problem becomes even harder –Restrict to combinations whose length is less than x problem not exponential. –Example: descriptions containing a most n properties (Full Brevity)

However: –Mathematicians view: structure of problem shows when no restrictions are put. –What if the input does not conform with these restrictions? (GRE does not control its own input!)

Compare with Description Logic: - Increasingly complex algorithms … - that tackle larger and larger fragments of logic … - and whose complexity is conservative Question: how do human speakers cope?