S05 Screening & Diagnostic Tests (FILLED IN)

5 calculaung kappa c calculate expected agreement

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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...
View Full Document

This document was uploaded on 01/14/2014.

Ask a homework question - tutors are online