statistics

Question 1

Decreasing the pre-test probability has what effect on positive predictive value?

Answer 1

Decreases PPV.

Question 2

Decreasing the pre-test probability has what effect on negative predictive value?

Answer 2

Increased NPV (inversely proportional)

Question 3

Equation for PPV

Answer 3

True positives/True positives + False positives

TP/TP + FP

Question 4

Equation for NPV

Answer 4

True negatives/True negatives + False negatives

TN/TN + FN

Question 5

Equation for sensitivity

Answer 5

True positives/True Positives + False negatives

TP/TP + FN

Question 6

Equation for specificity

Answer 6

True negatives/true negatives + false positives

TN / TN+FP

Question 7

What is the usual outcome measure in a cohort study?

Answer 7

Relative risk

Question 8

What is the usual outcome measure in a case-control study?

Answer 8

odds ratio

Question 9

Describe a type 1 error

What is it increased by?

Answer 9

rejecting the null hypothesis when it is true i.e. false positive

“alpha” error

increased if the number of end-points are increased. (i.e. likely that by chance one of these end points will be reached) and increading significance level

Question 10

Describe a type 2 error

What is it decreased by:

Answer 10

accepting the null hypothesis when it is false i.e. false negative

“beta” error

Decreased risk by increasing power and sample size (increased sample size = increased power)

Question 11

Define the power of a study

What is the equation?

Answer 11

Probability of correctly rejecting the null hypothesis when it is false, i.e. the probability of detecting a statistically significant difference

Power = 1 – the probability of a type II error

Question 12

What is the equation for NNT?

Answer 12

NNT = 1 / Absolute Risk Reduction

Question 13

What is the equation for absolute risk reduction?

Answer 13

If the outcome of the study is undesirable

• Control event rate - experimental event rate
  i. e. (b/b+d) - (a/a+c)

If the outcome of the study is desirable

• EER - CER
  i. e. (a/a+c) - (b/b+d)

Question 14

What is the equation for relative risk?

Answer 14

Relative risk = EER / CER

Question 15

When are odds ratios most commonly used?

Answer 15

Case control studies

Question 16

What is the equation for relative risk reduction?

Answer 16

Experimental event rate - control event rate/control event rate

(EER – CER) / CER

Question 17

What is the equation for likelihood ratio of a positive result?

Answer 17

sensitivity/(1 - specificity)

Question 18

What is the equation for likelihood ratio of a negative result?

Answer 18

(1 - sensitivity)/specificity

Question 19

Which remains constant across different populations with different risk of underling disease - absolute or relative risk reduction?

Answer 19

Relative risk reduction remains constant (ratio)

Absolute risk reduction varies with underlying risk of disease in population

Question 20

What level of evidence do case studies provide?

Answer 20

LEVEL III

Ia- evidence from meta-analysis of randomised controlled trials

Ib- evidence from at least one randomised controlled trial

IIa- evidence from at least one well designed controlled trial which is not randomised.

IIb- evidence from at least one well designed experimental trial

III- evidence from case, correlation and comparative studies.

IV- evidence from a panel of experts.

Question 21

What are grade B recommendations based on?

Answer 21

Evidence from non-RCT (IIa,IIb,III)

Grade A- based on evidence from at least once RCT (Ia,Ib)

Grade C- based on evidence from a panel of experts (IV).