Lecture 8 (Measurement validity, reliability, screening) Flashcards
Measures of disease (3)
- Diagnostic test (symptoms)
- Screening test (absence of symptoms)
- Measurement in epi study
Validity of measure of disease/exposure
- Degree to which it distinguishes btwn those who are affected/those who aren’t
- Exposure: distinguishing among diff levels
Sensitivity
% of ppl w/disease who test +
% of exposed who test says exposed
Specificity
% of ppl w/o disease who test -
% non-exposed who test says non-exposed
Positive predictive value
% of ppl w/a + test who have disease
% who test + for exposure who actually are exposed
Negative predictive value
% of ppl w/a - test who don’t have disease
% who test - for exposure who are actually non-exposed
Screening tests (why, which diseases)
- Absence of symptoms
- Secondary prevention
- Not diagnostic
When:
- Diseases w/sig mortality
- Treatment has advantage
- Test = simple, fast, cheap, safe
Low value cut point (screening test) –>
high sensitivity, low specificity
High value cut point (screening test) –>
high specificity, low sensitivity
Receiver operating characteristic curve
- plot of sensitivity vs 1 - specificity
- Shows optimum cut point (shoulder of curve), mid point
Cost of low sensitivity
-Missed at early, treatable stage
Cost of low specificity
$
Possible hazards
Burden of tests, psych effects
Determinants of positive predictive value (2)
- Specificity of test
- Prev of disease
Higher positive predictive value when…
-Higher specificity and prev of disease
Bowlin et al: Behavioural Risk Factor Survey
- Poor validity/greatly underestimated prev: hypertension, hypercholesterolemia
- Fair validity, moderately underestimated prev: smoking, obesity
- No conclusions possible: diabetes
