Cancer Study Headline, Translated: Absolute vs Relative Risk

Two numbers, one study

A headline says a treatment "cuts the risk of death by 30%." Another says a habit "doubles your risk."

Both are relative risks. Both can be true and still tell you almost nothing about what happens to a person, because a relative number is a comparison between two groups and carries no information about how common the outcome was to begin with.

Absolute risk is the missing half. It is the chance of the outcome itself: how many people out of a hundred, or a thousand.

A worked example from the federal data

NCI's colorectal cancer screening summary gives two absolute figures. About 4.0% of Americans are expected to develop colorectal cancer in their lifetime, and the lifetime risk of dying from it is 1.4%.

Now imagine a study reporting that some exposure raises colorectal cancer risk by 50%.

Applied to that baseline, a 50% relative increase moves lifetime risk from about 4% to about 6%. Two people in a hundred. That is a real effect and worth knowing, and it is a very different sentence from "raises your risk by half."

Run the same arithmetic on a rare cancer and the relative figure gets more misleading, not less. Doubling a risk of 1 in 100,000 produces a risk of 2 in 100,000.

The same trick works in reverse

Relative numbers also undersell things.

NCI reports that annual low-dose CT screening reduced lung cancer deaths by about 20% to 24% in randomized trials. That is a relative reduction, and it sounds modest against a headline like "cures 90% of patients."

But lung cancer kills about 125,000 people a year in the United States. A 20% reduction in a very common cause of death is a large absolute number of people, even though the percentage looks small.

So the rule is not "relative bad, absolute good." The rule is that you need both to know what you are looking at.

Number needed to treat, and its cousin

There is a third format that does the translation for you.

NCI's breast screening summary gives the number of women who need to be invited for screening to prevent one breast cancer death. For ages 39 to 49 it is about 1,904. For ages 50 to 59, about 1,339. For ages 60 to 69, about 377.

That single set of figures explains why screening guidance differs by age far better than any percentage does. The test is the same. The number of people who must undergo it for one death to be avoided is five times higher at 40 than at 65.

Watch for the widening gap

Absolute and relative differences can drift apart over time, which is where a lot of confusion lives.

In the ATAC breast cancer trial, anastrozole beat tamoxifen on time to recurrence with a hazard ratio of 0.79 in hormone-receptor-positive women — a relative figure that did not change much over the follow-up.

The absolute difference did change. The gap in recurrences was 2.7 percentage points at five years and 4.3 percentage points at ten years.

Same trial, same drugs. The relative measure says how much the risk was reduced. The absolute measure says how many women that reduction actually accounted for, and it depends on how long you wait.

Five questions that decode almost any headline

• What was the absolute risk in each group, in plain counts?
• What was the relative difference, and is that the number in the headline?
• How many people were studied, and for how long?
• Was there a comparison group, or is this a single-arm result?
• Does the population studied resemble me — age, stage, other conditions?

Our page on hazard versus risk covers a related trap: something can be capable of causing cancer without meaningfully changing anyone's risk at ordinary exposures.

When to get checked

A headline is not a symptom, and no news story should change a treatment plan on its own. But a few thresholds are worth acting on regardless of what you have been reading.

Book an appointment for blood in the stool or urine, even once. For a cough or hoarseness lasting more than three weeks. For a lump anywhere that is still there after two to three weeks. For unexplained weight loss of more than about 5% of body weight over six to twelve months. For bloating, pelvic pain, or feeling full quickly that happens most days for more than two or three weeks.

For screening, the anchors are ages rather than symptoms. Colorectal screening generally starts at 45 for people at average risk. Breast and cervical screening timing depends on age and history. Lung screening applies to people with a heavy smoking history, set by the US Preventive Services Task Force at 20 or more pack-years. Our page on colorectal cancer screening covers the options for one of them.

Family history changes all of these, and it is the single most useful thing to bring to the conversation.

What this story cannot tell

This page cannot tell you whether any specific headline applies to you. That depends on the study's population, and most articles do not report it clearly.

It cannot substitute for a care team's judgement about a treatment plan, and it is not a reason to start, stop, or change any treatment.

And it cannot make a study good. Converting a relative risk into an absolute one clarifies what a result means; it does not fix a small sample, a missing control group, or a short follow-up. Our page on cancer statistics covers how registry figures are built and what they do and do not describe.

Sources

• NCI PDQ: Colorectal Cancer Screening (Health Professional Version)
• NCI PDQ: Breast Cancer Screening (Health Professional Version)
• NCI PDQ: Lung Cancer Screening (Health Professional Version)
• NCI: Cancer Statistics
• Cuzick J et al., Lancet Oncol 2010, ATAC 10-year analysis (NCBI E-utilities)
• NCI SEER Cancer Stat Facts: Lung and Bronchus Cancer

How this article was prepared

An AI-assisted editorial system helped prepare this page. No named medical reviewer has reviewed it unless one is listed.

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