What's the difference between relative and absolute risk reduction in cancer trial results, and why do press releases us

To understand these two terms, it helps to think of them as two different ways of looking at the same outcome.

Absolute Risk Reduction (ARR)

The ARR tells you the actual difference in risk between two groups.

Imagine a cancer trial with 1,000 people:

• Control group: 10 out of 500 people (2%) see their cancer progress.
• Treatment group: 5 out of 500 people (1%) see their cancer progress.

The ARR is 1% (2% minus 1%). It tells you that if 100 people take the treatment, 1 person will avoid the cancer progression who wouldn't have otherwise. It provides the "real-world" impact.

Relative Risk Reduction (RRR)

The RRR tells you how much the risk was lowered as a percentage of the original risk.

Using the same example:

• The risk dropped from 2% to 1%.
• That is a 50% reduction of the original risk.

The RRR is 50%. It tells you how effective the treatment is at modifying the risk, but it doesn't tell you how high the risk was to begin with.

Why do press releases favor the Relative Risk?

Press releases and marketing materials often highlight the RRR (the 50% figure) because it sounds much more impressive than the ARR (the 1% figure).

1. The "Big Number" Effect: Humans tend to perceive large percentages as more significant. Saying a drug "cuts the risk of progression by 50%" sounds like a medical breakthrough, whereas saying it "reduces the risk from 2% to 1%" sounds like a modest improvement.
2. Contextual Omission: By focusing solely on the relative number, the starting point (the baseline risk) is hidden. If the risk of a specific event is already very low, a high relative reduction can be mathematically large but clinically minor.
3. Persuasion: Pharmaceutical companies and research institutions have a vested interest in framing results in the most favorable light. RRR is a standard, statistically correct way to report data, but it is frequently used to make modest effects appear more substantial to investors, the media, and patients.

The takeaway: When reading health news, look for the absolute numbers or the "Number Needed to Treat" (NNT). If a headline claims a "50% reduction," always ask: "50% of what?" That extra step usually provides the true clinical picture.