In the past there has been much debate about the validity of evaluations based on relevance judgments provided by erring human beings.
Cuadra and Kattersupposed that relevance was measurable on an ordinal scale (one which arises from the operation of rank-ordering) but showed that the position of a document on such a scale was affected by external variables not usually controlled in the laboratory.
Lesk and Salton subsequently showed that a dichotomous scale on which a document is either relevant or non-relevant, when subjected to a certain probability of error, did not invalidate the results obtained for evaluation in terms of precision (the proportion of retrieved documents which are relevant) and recall(the proportion of relevant documents retrieved).
Today effectiveness of retrieval is still mostly measured in terms of precision and recall or by measures based thereon.
There is still no adequate statistical treatment showing how appropriate significance tests may be used (I shall return to this point in the Chapter on Evaluation, page 178).
So, after a few decades of research in this area we basically have only precision and recall, and a working hypothesis which states, quoting Cleverdon: 'Within a single system, assuming that a sequence of sub-searches for a particular question is made in the logical order of expected decreasing precision, and the requirements are those stated in the question, there is an inverse relationship between recall and precision, if the results of a number of different searches are averaged.'
Effectiveness and efficiency
Much of the research and development in information retrieval is aimed at improving the effectiveness and efficiency of retrieval.
Efficiency is usually measured in terms of the computer resources used such as core, backing store, and C.P.U. time.
It is difficult to measure efficiency in a machine independent way.
In any case it should be measured in conjunction with effectiveness to obtain some idea of the benefit in terms of unit cost.
In the previous section I mentioned that effectiveness is commonly measured in terms of precision and recall.
I repeat here that precision is the ratio of the number of relevant documents retrieved to the total number of documents retrieved, and recall is the ratio of the number of relevant documents retrieved to the total number of relevant documents (both retrieved and not retrieved).
The reason for emphasising these two measures is that frequent reference is made to retrieval effectiveness but its detailed discussion is delayed until Chapter 7.
It will suffice until we reach that chapter to