Æliens

...

---

---

query

• document database + string matching

problems

• synonymy -- topic T does not occur literally in document D
• polysemy -- some words may have many meanings

...

---

---

effective search

• precision -- how many answers are correct
• recall -- how many of the right documents are returned

precision and recall

  precision = ( returned and relevant ) / returned  
  recall = ( returned and relevant ) / relevant 
  

anomalies

• return all documents: perfect recall, low precision
• return 'nothing': 'perfect' precision, low recall

example

term/document	d0	d1	d2
snacks	1	0	0
drinks	1	0	3
rock-roll	0	1	1

complextity

compare term frequencies per document -- O(M*N)

reduction

• stop list -- irrelevant words
• word stems -- reduce different words to relevant part

...

---

---

...

---

---

user-oriented measures

• coverage ratio -- fraction of known documents
• novelty ratio -- fraction of new (relevant) documents
• relative recall -- fraction of expected documents
• recall effort -- fraction of examined documents