FRCS Statistics, Ethics & Law

Question 1

A new diagnostic test for bowel cancer has a sensitivity of 85% and a specificity of 90%. What does the specificity of 90% tell you?

Accepted Answer

90% of patients without bowel cancer will test negative

Answer

Specificity is the proportion of truly disease-free individuals who test negative (true negative rate). A specificity of 90% means 90% of patients without bowel cancer will correctly test negative. The false positive rate is 10% (1 − specificity). High specificity minimises false positives.

Question 2

What is the number needed to treat (NNT) if a treatment reduces the absolute risk of complication from 20% to 15%?

Accepted Answer

20

Answer

NNT = 1 / absolute risk reduction (ARR). ARR = 20% − 15% = 5% = 0.05. NNT = 1/0.05 = 20. This means 20 patients must be treated for one additional patient to avoid the complication. Lower NNT values indicate greater treatment efficacy.

Question 3

In a randomised controlled trial, what is the purpose of blinding participants and investigators?

Accepted Answer

To reduce performance bias and detection bias by preventing knowledge of treatment allocation

Answer

Blinding prevents performance bias (if participants or clinicians modify behaviour based on knowing treatment allocation) and detection bias (if outcome assessors rate outcomes differently based on allocation knowledge). Double-blinding (both participant and investigator blinded) is the gold standard for reducing these biases.

Question 4

What is the difference between relative risk reduction (RRR) and absolute risk reduction (ARR)?

Accepted Answer

ARR is the percentage reduction in event rate between treatment and control groups; RRR is ARR divided by control event rate

Answer

ARR = control event rate − treatment event rate (e.g., 20% − 15% = 5%). RRR = ARR / control event rate (5%/20% = 25%). RRR appears more impressive than ARR but is less clinically meaningful because it does not reflect baseline risk. Both are needed alongside NNT for full interpretation.

Question 5

What type of study design provides the highest level of evidence in a hierarchy of evidence?

Accepted Answer

Systematic review and meta-analysis of RCTs

Answer

The evidence hierarchy (from lowest to highest) is: expert opinion, case reports, case series, case-control studies, cohort studies, RCTs, and at the apex, systematic reviews and meta-analyses of multiple high-quality RCTs. Systematic reviews synthesise the best available evidence and control for individual trial limitations.

FRCS Part A Practice Test

FRCS Part A Practice Test

FRCS Statistics, Ethics & Law