Number Needed to Treat (NNT): The Most Useful Statistic
Number Needed to Treat (NNT)
A drug company says: "Our drug reduces the risk of heart attack by 50%!" That sounds amazing. I'll take two!
But wait. That is a Relative Risk. It is marketing math. To understand the real benefit, you need the Number Needed to Treat (NNT).
The Symptom: The "50% Reduction" Lie
Imagine a disease that is very rare. Only 2 out of 100 people get it (2% risk). You take a drug that cuts the risk to 1 out of 100 (1% risk).
- Relative Risk Reduction: (2% - 1%) / 2% = 50%.
- Headline: "Drug cuts heart attack risk by HALF!"
- Absolute Risk Reduction: 2% - 1% = 1%.
- Reality: The drug only helps 1% of people.
The Mechanism: The Lottery Ticket
The NNT asks a simple question: "How many people must take this drug for ONE person to avoid the bad outcome?"
Formula: NNT = 1 / Absolute Risk Reduction
In our example:
- Absolute Reduction = 0.01 (1%)
- NNT = 1 / 0.01 = 100.
Translation: We have to treat 100 people to save 1 heart attack.
- 1 person is saved by the drug.
- 99 people take the drug, pay for the drug, and risk side effects, but get zero benefit. They wouldn't have had a heart attack anyway.
Real World NNTs (You might want to sit down)
| Treatment | Outcome Prevented | NNT (Approx) | | :--------------------------------------------- | :---------------- | :------------------------------------------- | | Antibiotics for Sepsis | Death | 1.1 (Amazing. Almost everyone benefits). | | Defibrillator for Cardiac Arrest | Death | 2 (Life-saving). | | Statins (for people with NO heart disease) | Heart Attack | 100+ (Most people get no benefit). | | Aspirin (for prevention) | Stroke | 2,000+ (Tiny benefit). |
The Prescription: Ask for the NNT
Before you start a long-term medication, ask your doctor: "What is the NNT?"
- Low NNT (1-5): This is a miracle drug. Take it.
- Medium NNT (20-50): It might be worth it if the side effects are low.
- High NNT (100+): Be very careful. You are likely taking a pill for no reason, but you still get the side effects (Number Needed to Harm).
At PaperScores, we calculate the NNT whenever possible. It cuts through the hype and tells you the "bang for your buck."