statistics

Definitions

	Actually true	Actually not true	Test total
Test positive	A 30	B 15	45
Test negative	C 10	D 45	55
Actual total	40	60	100

· true positive

· test positive and actually true

· A

· false positive

· test positive but actually not true

· B

· false negative

· test negative but actually true

· C

· true negative

· test negative and actually not true

· D

· sensitivity

· test positive and actually true

· A/A+C = 30/40

· specificity

· test negative and actually not true

· D/B+D = 45/60

· accuracy

· test positive and actually true and negative and actually not true

· A+D/A+B+C+D = 75/100

· positive predictive value

· likelihood that patient with positive test has disease

· A/A+B = 15/45

· negative predictive value

· likelihood that patient with negative test does not have disease

· D/C+D = 45/55

study

	study detected difference	study detected no difference
null hypothesis true	type 1 error	confidence
null hypathesis false	power	type II error

· power

· correctly rejects null hypothesis

· recognises that treatment makes difference

· power to change treatment

· confidence

· correctly accepts null hypothesis

· recognises that treatment makes no difference

· confidence to continue current treatment

· type I error

· incorrectly rejects a true null hypothesis

· does not recognise that treatment makes no difference

· type I person likely to think treatment works when it really doesn’t

· type II error

· incorrectly accepts a false null hypothesis

· does not recognise that treatment makes difference

· type II person likely to not recognise treatment works when it really does