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Lextek Language Identifier
P_ATLEAST
Name
P_ATLEAST
Synopsis
Returns records where any of the terms and subqueries that have at least a specified weight are present.
Arguments
P-Value
Minimum required weight
List of terms and subqueriesRanking Scheme
Ranks using the p-norm method. Conceptually you can consider each term a unique direction in n-space (where n is the number of terms). Each term's weight represents the distance in that direction. The p-value (p in the above formula) represents how much the final calculation is like the maximum weight or like the total distance of all vectors in n-space. For p = 2 the p-norm is identical to a vector space model. As p approaches infinity the returned weight becomes equal to the largest weight among the terms.
Picking an appropriate p-value can be tricky and often is the result of experimentation. A simplified way of thinking about the calculation is that it varies between a kind of average weight to the maximum weight. Generally a p-value of 2.5 or 3 is effective.
The p-norm weighting method is one of the most efficient methods available, especially if you've been careful in choosing how you weight your terms.
The only terms included in the p-norm calculation for P_ATLEAST are those found in each record with the required weight. Thus if you've indexed several documents as unique records you can have different weights for each one.
Comments
P_ATLEAST requires that any returned term or subquery have a weight in the record of at least the specified weight.
Example
P_ATLEAST( 2, 0.6, 'blue', 'green', 'red' );
See Also
atleast, atmost, r_atleast, r_atmost, p_atleast, p_atmost, or, value, r_or, p_or, m_or
