Biomedical Engineering Reference
In-Depth Information
Information Theory specifies the amount of meaningful information that can be communicated from
the Web server to the browser as a function of the signal-to-noise level and the bandwidth of the
medium. The greater the strength of the desired signal compared to that of the noise—that is, the
higher the signal-to-noise ratio—the greater the amount of relevant data that can be propagated
from the database through the Internet to the user (see Figure 4-8 ).
Searches generally fail in one of two ways: Either they retrieve too much noise with the desired data,
so that the time it takes to look through results isn't worth the trouble, or they retrieve the wrong
data, because the search criteria were incorrect. The best searches are sensitive enough to return all
or most of the desired data and selective enough to limit undesired data or noise to the least level
possible.
One way to limit the amount of noise returned by a search is to increase the selectivity of a search by
using a Boolean operator, such as AND, OR, or NOT. As illustrated in Figure 4-9 , the OR operator
provides the least amount of selectivity. Conversely, the AND operator provides the greatest
selectivity, returning data that contains all of the keywords submitted in a query. The NOT operator
generally provides an intermediate amount of selectivity. The relative selectivity of the Boolean
operators assumes that there is a significant signal-to-noise ratio—that there are a significant
number of Web sites or nucleotide sequences that fulfill the search requirements compared to the
other results that may be returned by the search.
Figure 4-9. Boolean Operators. Most search engines support the Boolean
AND, OR, and NOT operators, illustrated graphically here. Shaded areas in
each image represent the data returned by the search.
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