Length bias Length L gened ease Slowly progressive disease bias Rapidly progressive Screening identifies 2 cases of rapidly progressive disease and 5 cases of slowly progressive disease The incidence of rapidly progressive disease is equal to that of slowly progressive disease 8 March 2018 11/80
8 March 2018 11/80 Length bias
i Other bias ◆ Selection bias Health conscious High risk individuals ◆Over- diagnosis Aging population 8 March 2018 12/80
8 March 2018 12/80 Other bias Selection bias – Health conscious – High risk individuals Over-diagnosis – Aging population
Test e There should be a suitable test or examination o The test should be acceptable to the population 8 March 2018 13/80
8 March 2018 13/80 There should be a suitable test or examination. The test should be acceptable to the population
y Screening test o Reliability - get same result each time Validity-get the correct result Sensitive -correctly classify cases Specificity correctly classify non-cases screening and diagnosis are not identical 8 March 2018 1480
8 March 2018 14/80 Screening test Reliability – get same result each time Validity – get the correct result – Sensitive – correctly classify cases – Specificity – correctly classify non-cases [screening and diagnosis are not identical]
i Reliability o Repeatability-get same result Each time From each instrument From each rater o If don't know correct result, then can examine reliability onl 8 March 2018 15/80
8 March 2018 15/80 Reliability Repeatability – get same result – Each time – From each instrument – From each rater If don’t know correct result, then can examine reliability only