The selected query word (850) is applied to the semantic network (142d) to provide a query expansion word in response to the selected query word.

If the keyword is matched to at least one inquiry word, at least one category corresponding to the matched inquiry word is obtained based on the correlation information.

Part of speech (710-720) of the selected query word is determined.

The speech input includes a sequence of spelled letters enunciated by the user that spell a query word.

The challenge-response protocol presents the user with an unexpected challenge word, which the user is then requested to respond to.

The query expansion word is applied to the database (142a) in accordance with the part of speech of the selected query word and the part of speech of the query expansion word.

The protocol involves the transmission of a plurality of polling words by a transmitter 3 to a receiver 12, and determination by the receiver 12 that a polling word has been transmitted to it during a sensing period.

The one or more sequences of the letters are post-processed so as to produce a plurality of recognized words approximating the query word.

A semantic network (142d) is provided for determining expansion words to expand the search of the database in response to said query word.

The partial Fréchet distance is adapted from the traditional Fréchet distance concept to match a subcurve or prefix of a query word.

The receiver 12 is thus arranged so that it is able to detect a polling word transmitted over a period of time for part of which the receiver 12 is idle.

When a query engine is invoked to search for an input query word, the variations are searched only if the user has set an option to consider such variations.