Metric

• Recall

  Fraction of relevant documents that are retrieved; measures completeness of retrieval; high recall indicates few false negatives.

• Precision

  Fraction of retrieved documents that are relevant; measures quality of retrieved set; high precision indicates few false positives.

• Precision at k

  Precision measured over top k results; practical metric reflecting user experience when viewing limited results.

• Perplexity

  Exponentiated negative average log probability; measures how well a language model predicts a sample. Lower is better.

• Mean Average Precision

  Mean of average precision scores across queries; standard evaluation metric balancing precision and ranking quality. Abbreviated MAP.