610 CHAPTER 20 Cognitive radio evolution retrieved from a corpus by a query. Precision is the fraction of documents retrieved that are relevant. Recall of 1.0 indicates that all relevant documents are retrieved, while precision of 1.0 indicates that no irrelevant documents have been retrieved. Adapting this well-known metric to QoI, one may apply precision and recall to items provided to a user by a wireless device, with or without the assistance of one or more networks and with or without prior caching. Users may provide feedback by obtaining more items, rejecting, or not using the items retrieved. 20.5.3 Quality: Accuracy Accuracy characterizes the quantitative aspects of the information provided. Accu- racy is reduced by errors such as lack of factual correctness (e.g., spell the President's name wrong). Numerical accuracy in QoI reflects any numerical errors, whether from the original source, via transmission, or via misformatting the results. Numerical precision may limit accuracy. If the accuracy required by the user is met, then accuracy = 1.0. The rate of degradation of the accuracy metric from 1.0 may depend on the situation and may take any form (linear, quadratic, exponential, fractal, defined by table lookup, etc.). 20.5.4 Timeliness Timeliness is defined in terms of the user's timeline along which the information is to be employed. If the information is needed immediately, then the quality may be characterized as inversely proportional to excessive time delay. To avoid division by 0, one may consider timeliness to be 1.0 if the information is available before a minimum delivery time: T min (time, place, social setting, topic). (20.2) Situations include time, place, and social setting, such as shopping or needing medical attention. Suppose the shortest time delay in such a setting is so the maximum contribution of timeliness to QoI would be 1/. If timeliness is normal- ized by , then maximum timeliness is 1.0. In medical situations, there typically is a window after which the information is of marginal value if any, so the timeliness parameter may fall off sharply after such a window. Similarly, in some situations, timeliness may be decremented from 1.0 if the delivery time is less than . There is value in meeting users' exact timelines in the same sense that a wakeup call should be delivered when requested, and a user is not happy if the wakeup call is 15 minutes early. 20.5.5 Quality: Validity Validity is 1.0 if the information provided is true and approaches 0 if false with fuzzy set membership, for validity values in [1.0, 0.0].