A closer look · Treatment benefit
What does a
50% reduction mean?
Keep the same percentage reduction. Change the starting risk. See why the number of people affected can be very different.
Invented numbers for learning. Imagine an unwanted event, such as cancer returning, over five years. These figures do not describe any real cancer, study, treatment or individual. They cannot estimate your risk or guide a treatment choice.
Without the additional treatment
20 in 100
80 do not have the event
With the additional treatment
10 in 100
90 do not have the event
Each symbol represents one person in a fictional group of 100. The groups illustrate average expectations, not which particular people would benefit.
The same difference, described two ways.
10 fewer events per 100 people over five years. The absolute reduction is 10 percentage points; the relative reduction is 50%.
Read the explanation
Absolute change: how many fewer?
In the default fictional example, the event occurs in 20 out of 100 people without the additional treatment and 10 out of 100 with it. That is 10 fewer events per 100 people over five years: an absolute reduction of 10 percentage points.
“Percentage points” describes the difference between the two percentages: 20% minus 10%.
Relative change: compared with what?
Ten is half of twenty. That same difference can therefore be described as a 50% relative reduction. It does not mean that 50 out of every 100 people avoid the event because of treatment.
The starting risk, outcome and period of follow-up are needed to understand what the percentage means.
Two more invented examples
- 4 events become 2 per 100: a 50% relative reduction and a 2-percentage-point absolute reduction.
- 40 events become 20 per 100: the same 50% relative reduction, but a 20-percentage-point absolute reduction.
Real estimates also have uncertainty. This simple display does not show confidence intervals, competing risks, treatment side effects or the burden of treatment. Those belong in a clinical discussion.
Pause and consider
Two treatments are each described as reducing an event by 50%. Does that tell you they prevent the same number of events per 100 people?
No. A relative percentage alone does not tell you the absolute difference. You need the starting risk, the event being measured and the time period. Expected benefit also needs to be considered alongside harms and personal priorities.
Useful questions for a consultation
- What outcome are we discussing, and over what period?
- What are the expected numbers with and without this treatment?
- How uncertain are those estimates, and how do side effects compare?
Sources & clinical review
Clinically reviewed and approved by Dr AlHilali on 4 November 2026.
- NICE NG197: communicating risks, benefits and consequences — absolute risks, natural frequencies and consistent framing.
- NCI: understanding cancer prognosis — limits of applying group statistics to individuals.
- The existing clinically approved DrAlHilali.com risk article supplies the patient-facing context.