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In a trial, 2 in every 100 people taking a placebo had a heart attack over five years, compared with 1 in every 100 people taking the new drug. The drug costs £400 per year and causes muscle aches in 5% of people. (1) Calculate the relative risk reduction, absolute risk reduction and number needed to treat. (2) How would you explain this to a patient deciding whether to take it? You have 6 minutes.
Say your answer out loud first, then jot down the key points you made. Writing it helps you spot gaps — and it's what the AI feedback reads.
0 words
Understanding risk statistics, recognising how framing influences decisions, and communicating risk honestly for shared decision-making.
A structure to build your own answer on — not a script.
Relative risk reduction
From 2% to 1% = 50% relative reduction ('halves').
Absolute risk reduction
2% − 1% = 1 percentage point.
NNT
1 ÷ 0.01 = 100 people treated for five years to prevent one heart attack.
Harms and costs
5 in 100 get muscle aches; cost per heart attack prevented (100 × £400 × 5 years = £200,000).
Communicate
Natural frequencies; balance benefit and harm; patient's own risk and values; shared decision.
A strong but not perfect answer. Read the notes to see why it works — then make your own version with your experiences.
The heart attack rate went from 2 in 100 to 1 in 100. In relative terms that's a 50% reduction — so 'halves the risk' is technically true1.
But the absolute reduction is only 1 percentage point: from 2% to 1%. The number needed to treat is 1 divided by 0.01, which is 1002. So 100 people would need to take the drug for five years for one of them to avoid a heart attack. The other 99 wouldn't have had one anyway, or would have one despite the drug.
Against that, 5 in every 100 people get muscle aches. And cost-wise, 100 people for five years at £400 a year is £200,000 per heart attack prevented — which matters for the NHS, though it would also depend on how serious and costly the heart attacks prevented are.
To a patient, I'd explain it with the same numbers: 'Out of 100 people like you3 who take this for five years, about one will avoid a heart attack because of it, and about five will get muscle aches.' I'd ask what matters most to them — how worried they are about a heart attack, how they'd feel about side effects, and taking a daily tablet. Someone at higher risk would benefit more4, so the numbers would look different for them.
A company might prefer 'halves the risk' because it sounds much more impressive — which is why doctors need to look at the absolute figures.
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