Common Registration Assessment, Part 1
Relative versus absolute risk reduction
The same result can sound big or small depending on how it is framed. Three original questions.
Published 6 October 2026. Independent revision material, not GPhC questions, not for patient care.
A relative risk reduction of 50% sounds impressive, but it can describe a fall from 4% to 2% or from 40% to 20%. The Pharmaceutical Journal's article on communicating risk gives the example of a treatment that cuts an adverse outcome from 10% to 5%, a relative reduction of 50% but an absolute risk of 5%.
The same journal series on evaluating studies warns that a paper reporting only a 'risk reduction' may hide which kind it is. The skill is to work out the other measure yourself.
The method
- Find out whether the stated reduction is relative or absolute.
- If it is relative, multiply the control rate by it to find the fall, then subtract to get the treatment rate.
- ARR is control rate minus treatment rate, and NNT is 1 ÷ ARR as a decimal, rounded up.
- Say which measure you are quoting when you explain the result.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A treatment reduces an event rate from 4% to 2%. What are the relative risk reduction and the absolute risk reduction? These figures are made up for the exercise.
Show the answer
Answer: RRR 50%, ARR 2 percentage points
Working: RRR = (4 − 2) ÷ 4 = 0.5, which is 50%. ARR = 4% − 2% = 2 percentage points. The same RRR of 50% would apply to a fall from 40% to 20%, but the ARR would then be 20 percentage points.
Question 2
A leaflet says a drug reduces the risk of an event by 40%. The event rate in the control group is 5%. What is the treatment event rate, the ARR and the NNT? These figures are made up for the exercise.
Show the answer
Answer: 3%; 2%; 50
Working: If 40% is a relative reduction, the fall is 40% of 5% = 2 percentage points, so the treatment rate is 5% − 2% = 3%. ARR = 2% = 0.02. NNT = 1 ÷ 0.02 = 50.
Question 3
Hypoglycaemia occurs in 4.8% of control patients and 2.4% of patients on a new drug, and the manufacturer reports a 'risk reduction of 50%'. Is that a relative or absolute reduction, and what are the ARR and NNT? These figures are made up for the exercise.
Show the answer
Answer: Relative; ARR 2.4%; NNT 42
Working: RRR = (4.8 − 2.4) ÷ 4.8 = 0.5, so 50% is the relative reduction. ARR = 4.8% − 2.4% = 2.4% = 0.024. NNT = 1 ÷ 0.024 = 41.67, rounded up to 42. The absolute figure is a much smaller number than the headline.
What the sources say
The communicating risk article contrasts a relative risk reduction of 50% with an absolute risk of 5% and advises discussing risk in terms of absolute risk. The evaluating studies article explains why a bare 'risk reduction' needs checking. The numbers on this page are different.
Where marks are lost
- Treating a stated 'risk reduction' as absolute without checking.
- Subtracting 40 from 5 when the 40% is a relative figure.
- Forgetting to round the NNT up.
Frequently asked questions
Why do papers sometimes report only the relative figure?
It is larger and looks more impressive. The article recommends checking for conflicts of interest when a study emphasises it.
Which should I use with a patient?
The communicating risk article advises discussing risk in absolute terms.
Sources
- The Pharmaceutical Journal, communicating risk: how pharmacists should use data in conversations with patients (6 August 2024)
- The Pharmaceutical Journal, understanding risk and clinical utility, dropouts, data handling and the discussion section when evaluating studies (11 September 2026)
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional drugs. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
Try 15 free questions with worked answers. No sign-up required.
More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Electrolyte content over time
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Loading dose and volume of distribution
- Top-up doses from measured drug levels
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
See also the formula sheet, the eight sample questions and the approved calculator page.