NLP Tasks and Why They're Hard

• Speech
  • Given a continuous acoustic/auditory input stream, how are segments, syllables, words extracted from it?
  • Why it's hard
    • The input contains far more than what is needed.
    • For a given phonetic/phonological unit, there is great variability: gender, dialect, other speaker differences, and rate all make a difference.
    • For a given segment, there may be strong co-articulation effects: segments may depend on their contexts.
    • A high level of noise in the input doesn't bother people.
• Analysis (parsing, understanding)
  • Given a stretch of text (a sentence, a paragraph, a conversation), produce a representation embodying the semantic content of the text (what is this anyway?) and possibly also the presumed intentions of the speaker/writer (and what are these?).
  • Given a sentence, produce a representation for its syntactic structure.
  • Why it's hard
    • Hearers can understand sentences they have never heard before.
    • The input may be ill-formed; within limits, this does not bother people.
    • Many sources of knowledge (morphology, syntax, linguistic semantics, linguistic pragmatics, world knowledge) need to brought to bear.
    • The input may contain ambiguity (structural, lexical, pragmatic).
    • The input often underdetermines the intended output.
    • Speakers often make use of metaphor and metonymy.
    • The hearer needs to have (in some sense) a model of the speaker.
• Generation (production)
  • Given some communicative intent (pragmatic and/or semantic), produce some text.
  • Why it's hard
    • Speakers can talk about thoughts they've never talked about before.
    • The speaker needs to have (in some sense) a model of the hearer.
    • Speakers plan what they say based on mutual knowledge.
• Machine translation
  • Given some text in one language, produce a corresponding text in another.
  • Why it's hard
    • One word or structure in the source language may correspond to two or more in the target language. This may require a deep understanding of the source text.
    • Languages have different requirements about what is obligatory in their grammars.
• Language acquisition
  • Given samples of a language, learn a grammar.
  • Given examples of words used in context, learn the meanings and structural properties of the words.
  • Trust me; it's hard.