Suppose the Blast search returned 100 hits. Of these, 17

were false positives and we knew that there were 165

sequences in

the database which should have returned a hit with our

sequence.

To calculate the sensitivity and selectivity, we must

determine the number of true positives (ntp), the number of

false

positives (nfp) and the number of false negatives (nfn). We

are told that the number of false positives was 17, hence

the

number true positives must have been 100-17 = 83, as there

were 100 hits. Therefore we know that the search algorithm

found

83 of the 165 sequences it should have found, hence the

number of false negatives was 165-83 = 82. So, we know that

ntp = 83,

nfp = 17 and nfn=82. Using the equations in the notes, we

can calculate:

Sensitivity = ntp/(ntp+nfn) = 83/(83+82) = 83/165 = 0.50 (2

d.p)

Selectivity = ntp/(ntp+nfp) = 83/(83+17) = 83/100 = 0.83