Different aspects determine the effectiveness of Information Retrieval:

1. Precision: The ratio of relevant objects associated with the descriptor to the total number of relevant objects. (This indicates that the system may fail to find all relevant objects.)
2. Recall: The ratio of the number of the objects associated with the descriptor to the total number of relevant objects. (This indicates that the system may find objects that are not relevant after all.)
3. Exhaustivity: The degree to which the contents of the objects are reflected in the index expressions. (This indicates that meaningful words may not be searchable through the index.)
4. Power: The ratio of a descriptor's specificity to its length. (How long does a descriptor have to be in order to be precise?)
   Eliminability: The ability to determine irrelevance of a descriptor and stop the search. (How easy can one exclude part of the database from the search without losing relevant nodes.)
5. Clarity: The ability to grasp the intended meaning of the descriptor.
6. Predictability: The ability to predict where relevant descriptors can be found in the index. (This could also apply to predicting where relevant nodes can be found in the hyperdocument.)
7. Collocation: The extent to which the relevant index terms are near each other in the index. (Or to which the relevant nodes are near each other in the hyperdocument.)

So far no experiments have been conducted to determine the usefulness of these measures for hypertext-based Information Retrieval.

From http://wwwis.win.tue.nl/2L670/static/ir-measures.html