S05 Screening & Diagnostic Tests (FILLED IN)

# 5 calculaung kappa c calculate expected agreement

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Unformatted text preview: alence of injury dropped to 2%? –  Sn & Sp don’t change as they are not aﬀected by prevalence (since they relate to the test) –  What is the PPV and NPV? How did they change? –  PPV = (9/44) = 20.5% NPV = (553/556) = 99.5% LOWER HIGHER PPV & Prevalence Prev é༎, PPVé༎ •  Especially in low prevalence Prev é༎, NPVê༎ •  Mainly only at high prevalence •  ImplicaUons of this relaUonship: –  All interpreta2ons of a test result should occur within the context of that disease prevalence –  Screening programs should be targeted to groups with higher prevalence (or else many +ve tests will actually be disease - ve) Reliability (Repeatability) •  Recall class ac.vity on Friday •  Can the results be repeated? •  Three types of varia2on can occur: 1.  Intrasubject Varia2on •  E.g. Change in an individual over 2me 2.  Intraobserver Varia2on •  E.g. Diﬀerent interpreta2ons by same person; context 3.  Interobserver Varia2on •  E.g. Two people disagree on interpreta2on of a test Interobserver VariaUon •  How can we measure this phenomenon? 1.  Overall Percent Agreement –  % of ra2ngs where observers agree –  Generally, most evalua2ons are disease nega2ve, so ignore these observa2ons and evaluate percent agreement where one of the observers indicated a disease +ve result. a + b + c + d % Agreement = ____a____ x 100 a + b + c +ve Observer #2 % Agreement = ___a + d___ x 100 Observer #1 - ve +ve a b - ve c d Interobserver VariaUon 2. Kappa StaUsUc A staUsUc that measures agreement beyond what would be due to chance alone –  Deﬁni2on: _______________________________ •  Can be used to measure agreement between mul2ple observers •  Can also be used to measure agreement between two tests (recall use of mul2ple tests) –  Steps t...
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## This document was uploaded on 01/14/2014.

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