Statistics

Question 0

Formula for positive Likelihood Ratio

Answer 0

LR = SENS / 1-SPEC

The higher the better, as you are more likely to have a positive test if you have the disease

Question 1

Formula for calculating NNT.

Answer 1

NNT = 1/ARR

ARR is the proportion of deaths in the exposed MINUS the proportion of deaths in the non-exposed group.

ARR = a/a+b MINUS c/c+d (THE ROWS)

Question 2

How to calculate Post-Test Probability

Answer 2

Need to calculate LR(+) first.
LR = sens / 1-spec

Pretest ODDS = Prevalence / 1-Prevalence

Post-test ODDS = Pretest Odds x LR

Then convert “Post-test ODDS” to “Post-test PROBABILITY”
Post-test PROB = Post-test ODDS / 1+ Post-test ODDS