Common Registration Assessment, Part 1
Absolute risk reduction and number needed to treat
From event rates to ARR to NNT, with the rounding rule that catches people out. Three original questions.
Published 6 October 2026. Independent revision material, not GPhC questions, not for patient care.
Absolute risk reduction (ARR) is the actual difference between the event rates, and the number needed to treat (NNT) is one divided by that difference. The Pharmaceutical Journal's article on evaluating studies sets out both and warns that the ARR must be a decimal before you divide.
If the NNT is not a whole number, the article says to round up to the next whole number. The smaller the ARR, the bigger the NNT, and that is the point to keep in mind when you read the answer.
The method
- Subtract the treatment event rate from the control event rate to get ARR.
- Convert ARR from a percentage to a decimal, for example 4% becomes 0.04.
- Divide 1 by the decimal ARR to get the NNT.
- Round up to a whole number and say over what period and for which outcome.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
Exacerbations occur in 18% of patients on control and 14% of patients on a new inhaler over one year. What is the NNT? These figures are made up for the exercise.
Show the answer
Answer: 25
Working: ARR = 18% − 14% = 4% = 0.04. NNT = 1 ÷ 0.04 = 25. So 25 patients need to be treated for a year to prevent one additional exacerbation.
Question 2
Over two years, a fracture occurs in 7.5% of patients on placebo and 4.2% of patients on a treatment. What is the NNT, as a whole number? These figures are made up for the exercise.
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Answer: 31
Working: ARR = 7.5% − 4.2% = 3.3% = 0.033. NNT = 1 ÷ 0.033 = 30.3. This is not a whole number, so round up to 31, not down to 30.
Question 3
An event occurs in 2.4% of control patients and 1.9% of treated patients. What is the NNT, and what does the size of it tell you? These figures are made up for the exercise.
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Answer: 200
Working: ARR = 2.4% − 1.9% = 0.5% = 0.005. NNT = 1 ÷ 0.005 = 200. A small ARR gives a large NNT, so 200 patients must be treated to prevent one event. Whether that is worthwhile depends on how serious the event is and what the treatment risks.
The journal's NNT rules
The article gives the formula ARR = control rate minus treatment rate and NNT = 1 ÷ ARR, tells readers to use the decimal not the percentage, and says to round any decimal NNT up. It shows that the same ARR change can mean very different NNTs. The questions here use new numbers.
Where marks are lost
- Dividing 1 by the percentage, for example 1 ÷ 4, which gives 0.25.
- Rounding the NNT down.
- Subtracting the wrong way round and getting a negative ARR.
Frequently asked questions
Why round the NNT up?
A fraction of a patient cannot be treated, and rounding up keeps the figure on the cautious side, as the article advises.
Can an NNT be compared between studies?
Only with care. The article notes that duration, patient characteristics and outcome measures can differ.
Sources
- 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
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- 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.