Absolute Risk & Baseline Risk
The same relative benefit is large in high-risk patients and trivial in low-risk ones — baseline risk decides whether a treatment matters.
Absolute risk is the actual probability of an event in a group — e.g. 2 in 100 = 2%. Baseline (control) risk is the absolute risk without treatment, and it governs how much a relative effect is worth: the same relative risk reduction removes many events in a high-risk group but almost none in a low-risk group. Relative effects travel; absolute benefit depends entirely on where you start.
~4 events prevented / 100 at 20% baseline risk same ~1/5 RRR at 1% baseline risk → well under 1 prevented / 100 — baseline risk is everything
100 high-risk patients in whom ~20 would have a major vascular event untreated. Apply a relative reduction of roughly a fifth and you colour in ~4 figures — four real events prevented per hundred. Drop the baseline risk to ~1 in 100 and that identical relative reduction colours in only a sliver of a single figure: the same percentage, a vanishingly small absolute payoff.
Read the relative effect (the RRR) and the baseline risk together — never the relative number alone. Absolute benefit ≈ baseline risk × RRR, so as baseline risk falls, the figures you can colour in shrink toward zero and the NNT balloons. The treatment hasn’t changed; the population’s starting risk has.
Being dazzled by a relative risk reduction while ignoring baseline risk is the single commonest appraisal error. “Halves your risk” of a 1-in-10,000 event prevents 5 events in 100,000 — almost nothing — yet the same headline in a high-risk group is genuinely practice-changing. Baseline risk is what turns a true relative effect into a benefit worth the harms, cost and number needed to treat.
Absolute benefit ≈ baseline risk × RRR- Same RRR → big ARR in high-risk, tiny ARR in low-risk
NNT = 1 ÷ ARRrises sharply as baseline risk falls
Statins — CTT meta-analysis (Lancet 2012) — lowering LDL with a statin cut major vascular events by about 21% per 1 mmol/L (RR 0.79, 95% CI 0.77–0.81), a relative reduction that held across baseline-risk strata. But the absolute benefit tracked baseline risk: in low-risk people (5-year risk <10%) it prevented only ~11 events per 1000 treated over 5 years, whereas the same proportional reduction in high-risk secondary-prevention patients prevents far more — identical RRR, very different absolute value. A constant relative risk reduction does not mean a constant benefit: weigh it against each patient’s baseline risk before treating.
- Confusing relative with absolute risk — quoting “21% reduction” as if it were the absolute benefit.
- Forgetting benefit is baseline-risk-dependent — the same RRR is worth far less in low-risk groups.
- Falling for framing effects — relative figures sound impressive precisely because they hide the baseline.
Quick check
A drug halves the risk of an event — is that clinically important?
Answer: It depends on the baseline risk. Half of a large baseline risk prevents many events; half of a tiny risk is still tiny. You cannot judge importance from the relative reduction alone.
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