 ### Clustering and classification

#### Self-assessment question 1

1

##### The effect of prevalence on accuracy measures

Which of the following accuracy measures are affected by the prevalence of positive cases?
Hint: Use the onscreen calculator and keep a and c constant but vary b and d to change the prevalence. It is easier to interpret if the proportions are held constant. For example, set a = 90 and c = 10 in all calculations and then set b and d to 10 and 90, followed by multiples of these numbers (20 and 180, 100 and 900, etc).

 a) CCR b) Sensitivity c) Specificity d) PPP e) NPP f) Kappa a) Correct, it is unaffecteda) NO, this is unaffected but as the number of negatives increases the positive results have a decreasing effectb) Correct, it is unaffectedb) Since it only considers a and c it unaffected by the number of negative cases.c) Correct, it is unaffectedc) Since it only considers b and d it unaffected by the number of positive cases.d) Correct, it is affectedd) Even if the proportion of correctly identified positive and negative cases remains unchanged this measure will decrease as prevalence declines.e) Correct, it is affectede) Even if the proportion of correctly identified positive and negative cases remains unchanged this measure will increase as prevalence declines.f) Correct, it is affectedf) Even if the proportion of correctly identified positive and negative cases remains unchanged this measure will decrease as prevalence declines. 