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An Overview of Morphology: Inflectional and Derivational Processes in Language

This lecture provides a comprehensive overview of morphological processes in linguistics, with a focus on inflectional and derivational morphology. Key topics discussed include the categorization of phi-features (such as person, number, and gender), comparative and superlative forms, as well as examples of irregular morphology. Special attention is given to stemming methods, particularly the widely used Porter algorithm, which addresses inflectional and derivational transformations in lexical items. The implications for information retrieval and semantic parsing are also explored.

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An Overview of Morphology: Inflectional and Derivational Processes in Language

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  1. LING 388Language and Computers Lecture 21 11/13/03 Sandiway FONG

  2. Administrivia • Room change next Tuesday • One time deal :-) • PAS 224 • Physics and Atmospheric Sciences Building

  3. Morphology • Inflectional Morphology: • Phi-features (person, number, gender) • Examples: movies, blonde, actress • Irregular examples: appendices, geese • Case • Examples: he/him, who/whom • Comparatives and superlatives • Examples: happier/happiest • Tense • Examples: drive/drives/drove (-ed)/driven

  4. Morphology • Derivational Morphology • Nominalization • Examples: formalization, informant, informer, refusal, lossage • Deadjectivals • Examples: weaken, happiness, simplify,formalize, slowly, calm • Deverbals • Examples: see nominalizations, readable, employee • Denominals • Examples: formal, bridge, ski, cowardly, useful

  5. Morphology and Semantics • Suffixation • Examples: • x employ y • employee: picks out y • employer: picks out x • x read y • readable: picks out y • Prefixation • Examples: • undo, redo, un-redo, encode, defrost, asymmetric, malformed, ill-formed, pro-Chomsky

  6. Stemming • Normalization procedure: • Inflectional morphology: • cities -> city, improves/improved -> improve • Derivational morphology: • transformation/transformational -> transform • Criterion: preserve meaning • Primary application: information retrieval (IR) • Efficacy questioned: Harman (1991)

  7. Stemming • IR-centric view: • Applies to open-class lexical items only: • Stop-words: the, below, being, does • Not full morphology: • prefixes generally excluded • (not meaning preserving) • Examples: asymmetric, undo,. encoding

  8. Stemming: Methods • Use a dictionary (look-up) • OK for English, not for languages with more productive morphology, e.g. Japanese • Write rules, e.g. Porter Algorithm (Porter, 1980) • Example: • Ends in doubled consonant (not “l”, “s” or “z”), remove last character • hopping -> hop, hissing -> hiss

  9. Stemming: Methods • Dictionary approach not enough • Example: (Porter, 1991) • routed -> route/rout • At Waterloo, Napoleon’s forces were routed • The cars were routed off the highway • Here, the (inflected) verb form is polysemous

  10. Stemming: Errors • Understemming: failure to merge • Adhere/adhesion • Overstemming: incorrect merge • Probe/probable • Claim: -able irregular suffix, root: probare (Lat.) • Mis-stemming: removing a non-suffix (Porter, 1991) • reply -> rep

  11. Stemming: Interaction • Interacts with noun compounding: • Example: • operating systems • negative polarity items • For IR, compounds need to be identified first…

  12. Stemming: Porter Algorithm • The Porter Stemmer (Porter, 1980) http://www.tartarus.org/~martin/PorterStemmer/ • For English • Most widely used system • Manually written rules • 5 stage approach to extracting roots • Considers suffixes only • May produce non-word roots

  13. Stemming: Porter Algorithm • Rule format: • (condition on stem) suffix1 -> suffix2 • In case of conflict, prefer longest suffix match • “Measure” of a word is m in: • (C) (VC)m (V) • C = sequence of one or more consonants • V = sequence of one or more vowels • Examples: • tree C(VC)0V • troubles C(VC)2

  14. Stemming: Porter Algorithm • Step 1a: remove plural suffixation • SSES -> SS (caresses) • IES -> I (ponies) • SS -> SS (caress) • S -> (cats) • Step 1b: remove verbal inflection • (m>0) EED -> EE (agreed, feed) • (*v*) ED -> (plastered, bled) • (*v*) ING -> (motoring, sing)

  15. Stemming: Porter Algorithm • Step 1b: (contd. for -ed and -ing rules) • AT -> ATE (conflated) • BL -> BLE (troubled) • IZ -> IZE (sized) • (*doubled c & ¬(*L v *S v *Z)) -> single c (hopping, hissing, falling, fizzing) • (m=1 & *cvc) -> E (filing, failing, slowing) • Step 1c: Y and I • (*v*) Y -> I (happy, sky)

  16. Stemming: Porter Algorithm • Step 2: Peel one suffix off for multiple suffixes • (m>0) ATIONAL -> ATE (relational) • (m>0) TIONAL -> TION (conditional, rational) • (m>0) ENCI -> ENCE (valenci) • (m>0) ANCI -> ANCE (hesitanci) • (m>0) IZER -> IZE (digitizer) • (m>0) ABLI -> ABLE (conformabli) - able (step 4) • … • (m>0) IZATION -> IZE (vietnamization) • (m>0) ATION -> ATE (predication) • … • (m>0) IVITI -> IVE (sensitiviti)

  17. Stemming: Porter Algorithm • Step 3 • (m>0) ICATE -> IC (triplicate) • (m>0) ATIVE -> (formative) • (m>0) ALIZE -> AL (formalize) • (m>0) ICITI -> IC (electriciti) • (m>0) ICAL -> IC (electrical, chemical) • (m>0) FUL -> (hopeful) • (m>0) NESS -> (goodness)

  18. Stemming: Porter Algorithm • Step 4: Delete last suffix • (m>1) AL -> (revival) - revive, see step 5 • (m>1) ANCE -> (allowance, dance) • (m>1) ENCE -> (inference, fence) • (m>1) ER -> (airliner, employer) • (m>1) IC -> (gyroscopic, electric) • (m>1) ABLE -> (adjustable, mov(e)able) • (m>1) IBLE -> (defensible,bible) • (m>1) ANT -> (irritant,ant) • (m>1) EMENT -> (replacement) • (m>1) MENT -> (adjustment) • …

  19. Stemming: Porter Algorithm • Step 5a: remove e • (m>1) E -> (probate, rate) • (m>1 & ¬*cvc) E -> (cease) • Step 5b: ll reduction • (m>1 & *LL) -> L (controller, roll)

  20. Stemming: Porter Algorithm • Misses (understemming) • Unaffected: • agreement (VC)1VCC - step 4 (m>1) • adhesion • Irregular morphology: • drove, geese • Overstemming • relativity - step 2 • Mis-stemming • wander C(VC)1VC

  21. Stemming: Porter Algorithm • Possible Term Project • The Porter Stemmer is a rule-based system • We know how to write rules • Implement the Porter Stemmer in SWI-Prolog

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