Formulae for MRCPsych

A/(A+C)x100

D/(D+B)x100

LR shows how much more likely someone is to get a positive test if he/she has the disease, compared with a person without disease.

Positive LR is usually a number greater than one and the negative LR ratio usually is smaller than one.

Sensitivity/(1-Specificity)

(1-Sensitvity)/Specificity

The proportion of patients with negative test who are actually free of the disease.

This refers to the proportion of those scoring negative that don't have a condition (i.e. The chance that a negative result will be correct).

Inversely proportional to prevalence.

D/(D+C)x100

A/(A+B)x100

The proportion of patients with positive test who actually have the disease.

Affected by prevalence - directly proportional.

(a+c)/(Total)

Pre-test probability/(1-pre-test probability)

Pre-test odds x likelihood ratio for a positive result

(A+D)/Total

CER - EER

EER/CER

CER - EER

1/ARR

Standard deviation/√sample size

= AR/EER

or (A/A+B) - (C/(C+D))

95.4%

(CER - EER)/CER

√variance