Stats equations to memorise

Question 1

How do you calculate NNT?

Answer 1

1/ARR

ARR = CER-EER

Question 2

How to calculate sensitivity?

Answer 2

Sensitivity = TP/(TP+FN)

Question 3

How to calculate specificity?

Answer 3

Specificity = TN/(TN+FP)

Question 4

How to calculate positive predictive value?

Answer 4

PPV = TP/(TP+FP)

Question 5

How to calculate negative predictive value?

Answer 5

NPP = TN/(TN+FN)

Question 6

How to calculate likelihood ratio for a positive result?

Answer 6

Likelihood +ive = sensitivity/(1-specificity)

Question 7

How to calculate likelihood ratio for a negative result?

Answer 7

Liklihood -ive = (1-sensitivty)/specificity)

Question 8

How to calculate odds ratio?

Answer 8

a) Odds of outcome in cases = number with/number without
b) Odds of outcome in controls = number with/number without
c) Odds ratio = a/b

Question 9

What study design usually gives its answer as odds ratio?

Answer 9

Case-control

Question 10

How do you calculate relative risk reduction?

Answer 10

Absolute risk change/control event rate

I.e., RRR = ARR/CER

= (ERR-CER)/CER

Question 11

How to calculate risk of an outcome in a group?

Answer 11

Number of people developing the condition as a proportion of the total.
Eg, 1000 people exposed to a risk factor. 200 of them die. Risk = 200/1000 = 0.2

Question 12

How do you calculate standard error?

Answer 12

SE = standard deviation/square root of n.

Question 13

False negative rate = ?

Answer 13

False negative rate = 1-sensitivity

Question 14

Calculate experimental event ratio or control event ratio? What is it expressed as?

Answer 14

Number who had the outcome/total number treated.

Expressed as a decimal or as a %

Question 15

How to calculate relative risk?

Answer 15

EER/CER

Question 16

How to calculate absolute risk?

Answer 16

EER-CER