While on the subject of sensitivity, specificty and PPV:
I know that there is a always a trade-off between sensitivity and
specificty in screening tests or, for me as a librarian, literature
searches: If I increase sensitivity by combining terms using the OR
operator I will always reduce specificity.
This does not seem to be the case for sensitivity and PPV.
In the exmaple below: When I COMBINE terms to increase sensitivity above,
say 40%, PPV (librarians prefer to call this 'precision') 'stabilizes' at
about 12%.
Does this make sense or is my math/stats weak? If correct: does knowing
this phenomenon help when designing screening programmes (or search
strategies)?
Reinhard Wentz
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On a database with some 1750 records with 64 known true+
Term
Records found True+ Sensitivity Precision(PPV)
experimental 58 11 17.2 18.9
before 125 16 25.0 12.8
impact 159 17 26.6 10.7
before
or impact 270 29 45.3 10.7 precision (PPV) stays the same?
before
or impact
or experi-
mental 313 38 59.4 12.1 precision (PPV) goes up!
______________________________
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Reinhard Wentz, Dipl. Bibl. Tel. 020 7242 9789 Ext 2291
Cochrane Injuries Group Fax. 020 7242 2723 (Attn: R. Wentz)
Institute of Child Health e-mail [log in to unmask]
30, Guilford Street
London WC1N 1EH
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