Common Registration Assessment, Part 1
Odds ratio and confidence intervals
Odds are not the same as risk. Three original questions on odds ratios and what a confidence interval says.
Published 6 October 2026. Independent revision material, not GPhC questions, not for patient care.
The Pharmaceutical Journal's series notes that risk and odds differ in the denominator. Risk uses everyone in the group, and odds use only those who did not have the event.
An odds ratio is the odds in the treatment group divided by the odds in the control group. The same rule for significance applies as for relative risk: if the 95% confidence interval includes 1, the result is not significant.
The method
- For each group, count the people with the event and the people without it.
- Odds = people with the event divided by people without the event.
- Odds ratio = treatment odds divided by control odds.
- For a ratio, check whether the confidence interval includes 1. For a difference, check whether it includes 0.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
In a study, 40 of 200 patients on a new drug and 60 of 200 on control had the event. What is the odds ratio, to two decimal places? These figures are made up for the exercise.
Show the answer
Answer: 0.58
Working: Treatment group: 40 with the event and 160 without, so odds = 40 ÷ 160 = 0.25. Control group: 60 with and 140 without, so odds = 60 ÷ 140 = 0.4286. OR = 0.25 ÷ 0.4286 = 0.58.
Question 2
15 of 100 patients on treatment and 25 of 100 on control had the event. What is the odds ratio, to two decimal places? These figures are made up for the exercise.
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Answer: 0.53
Working: Treatment odds = 15 ÷ 85 = 0.1765. Control odds = 25 ÷ 75 = 0.3333. OR = 0.1765 ÷ 0.3333 = 0.53. Using 15 ÷ 100 and 25 ÷ 100 would give the relative risk of 0.60, not the odds ratio.
Question 3
A trial reports a mean difference in systolic blood pressure of −3.2 mmHg between treatment and control (95% confidence interval −5.1 to −1.3). Is the difference statistically significant? These figures are made up for the exercise.
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Answer: Yes
Working: For a difference, the value that means no effect is 0. The interval −5.1 to −1.3 does not include 0, so the difference is statistically significant. The interval shows the true difference is likely to be a fall of between 1.3 and 5.1 mmHg.
The journal's worked examples
The article's second worked example asks for the odds ratio of severe hypoglycaemia with a new oral drug in 238 patients against 295 controls, and its first explains why an interval containing 1 is not significant. The numbers on this page are different.
Where marks are lost
- Using the group total as the denominator for odds.
- Checking for 1 in the interval when the result is a difference, where the no-effect value is 0.
- Treating an odds ratio and a relative risk as the same number.
Frequently asked questions
When are odds and risk similar?
When the event is rare, because the people without the event are close to the whole group.
Which no-effect value do I look for?
1 for ratios such as RR, OR and hazard ratio, and 0 for differences.
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
- Absolute risk reduction and number needed to treat
- 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.