IRS13 Test Environment chapter E. M. Keen Harvard University Gerard Salton Use, reproduction, or publication, in whole or in part, is permitted for any purpose of the United States Government. these additional concept numbers sometimes have the effect of a recall device1 and sometimes that of a precision device. Fig. 7 gives an example of the use of concon, where the fir[OCRerr]t order matches of type A, and all second order matches, are recall devices because four additional matches between request and document are made possible by the use of concon. The one first order match of type B acts as a precision device because "cylinder" was already matched in request and document, but the concon pair "Unit + cylinder" gives added weight to "cylinder" in the document, and this in turn gives greater prominence to the match on "cylinder" in the final request/document matching correlation. The case of the phrase dictionary is also complex. Phrases normally act as a precision device of coordination, when the phrases recognized are used to completely replace all occurrences of the component concepts which do not meet the phrase criteria individually. When phrase identifiers are merely added to existing identifiers, phrases may act as a precision device by virtue of the weighting effect already discussed. In a few circumstances, phrases also act as a recall device. The weighting effect acts as a precision device only when the concepts being added to the document increase the weight of some, and not all, of the identifiers already in the document; this is, in general, the case with SMART. This disc'[OCRerr]ssion of devices shows that the clear-cut distinction between recall and precision devices is not easy to preserve, and its usefulness is probably now somewhat limited. This is particularly true because the effect of the various dictionaries is only detected in the performance measures if a certain type of cut-off is applied, namely, a cut-off that directly uses the