Relative versus absolute risk

Introduction

A headline saying that alcohol increases the risk of breast cancer by 30% sounds pretty scary. But what does it actually mean? Relative risks are often reported in newspaper headlines but without the context of absolute (or baseline) risk, this information is meaningless. Absolute risk numbers are needed to understand the implications of relative risks and how specific factors or behaviours affect your likelihood of developing a disease or health condition.

Absolute risk

is the risk of an individual developing a disease such as CVD, cancer or being involved in an accident over a certain period of time.
It is your personal risk of developing a disease over a time period, i.e. the absolute risk for breast cancer is a look at an individual’s risk independent of other groups.
It is a measure of the cumulative risk that a certain event/disease will happen during a given period of time. One measure of absolute risk is the lifetime risk, which is the absolute risk of a certain disease/cancer over the period of someone’s life.

In cancer research, it is the likelihood that a person who is free of a certain type of cancer will develop that cancer by a specific age. For example, a woman with no known risk factors for breast cancer has a cumulative risk of getting breast cancer over a lifetime of 90 years of about 12-13%. This means one out of every eight women will get breast cancer by age 90 years.
The higher the absolute risk is, the more likely is that something will happen, even though it is not guaranteed to take place.
https://www.cancer.gov/publications/dictionaries/cancer-terms/def/cumulative-risk

This will change depending on different factors. The most important is a person’s age. Since most cancers are more common in older age groups, your absolute risk of cancer will be higher as you get older. Thus, a woman’s absolute risk for breast cancer is not always 12%. Other factors such as age, family history, lifestyle factors, etc. influence the absolute risk.
Women who have genetic changes in the BRCA1 or BRCA2 genes have around a 60% absolute lifetime risk of developing breast cancer. This absolute risk is high. But this risk is over a whole lifetime, so a woman with altered BRCA genes who develops breast cancer, may not do so until she is in her 70s or 80s.

Calculating absolute risk

Absolute risk is always written as a percentage. It is the ratio of people who have a medical event compared to all the people who could have an event.

For example, if 26 out of 100 people will get dementia in their lifetime, the absolute risk is 26/100 or 26%. The higher the denominator (the bottom number), the lower the absolute risk.
https://www.statisticshowto.com/calculate-relative-risk#:~:text=Absolute%20risk%20is%20different%20from,relative%20risk%20increase%20of%2025%25.

Relative risk

In epidemiological research, risk refers to the likelihood that an outcome will occur within a certain time interval. To examine associations between exposure (i.e. drinking alcoholic beverages) and outcomes (i.e. total mortality, cancer, cardiovascular disease, etc.), the relative risk is assessed, which is the risk for individuals exposed to a factor compared with the risk among those not exposed. The relative risk expresses how much more or less likely the outcome is to occur in exposed individuals.

Cancer: relative risk

In scientific publications, it is important to understand that the figures describing the risk increases of cancer present the relative risk. These figures tell us how much more or less likely the cancer occurs in a population consuming alcoholic beverages compared to abstainers.
This is different from the overall likelihood of getting cancer during a lifetime, i.e., the absolute lifetime risk. Even non-drinkers have a certain risk to get cancer.

Understanding relative risk – The importance of absolute risk versus relative risk

The impact of a relative risk depends on the underlying absolute risk of a disease.

• When a disease is rare (such as breast cancer among young women), a high relative risk leads to only a few extra cases among those with the risk factor.
• When a disease is more common (such as breast cancer among older women), even a small relative risk can lead to many more cases among those with the risk factor.

We can think about relative risk in terms of money:
If you only have one dollar, this makes dollars “rare.” If you double your money, you only gain one extra dollar.
But, if you have one million dollars, this makes dollars “common” and doubling your money means you gain one million extra dollars.
In both cases, you double your money, but the increase in dollars is quite different.
The same is true with disease risk. The higher the absolute risk of getting a disease, the greater the number of extra cases that will occur for a given relative risk.

Example of the impact of a relative risk

For example, inactive women have a 25% greater risk of breast cancer than active women (a relative risk of 1.25).
Since older women are more likely to get breast cancer, a lack of exercise has a greater impact on breast cancer risk in older women than in younger women.
Let us look at the women in a study ages 70-74 years.
The study finds 500 women per 100,000 who are inactive develop breast cancer for one year. This is the absolute risk for women with the risk factor, lack of exercise.

The study also shows 400 women per 100,000 who are active develop breast cancer. This is the absolute risk for women without the risk factor.

500/100,000 (inactive) = 1.25 Relative risk
400/100,000 (active)

So, the relative risk is 1.25 for women who are inactive compared to those who are active. Among women ages 70-74, being inactive led to 100 more cases of breast cancer per 100,000 women in one year (500 cases – 400 cases = 100 cases).

Now let us look at the women in the study ages 20-29.

The study finds 5 women per 100,000 who were inactive developed breast cancer for one year. And 4 women per 100,000 who were active got breast cancer. Here again, the relative risk is 1.25.

However, in women ages 20-29, being inactive led to only 1 extra case of breast cancer per 100,000 women (5 cases – 4 cases = 1 case).
So, the same relative risk of 1.25 led to many more extra cases of breast cancer in the older women (100 extra cases) than in the younger women (1 extra case).
The impact of the same relative risk (1.25) was different depending on the underlying absolute risk.
https://ww5.komen.org/Breastcancer/Understandingrisk.html

Why the numbers matter

Relative risk
“New wonder drug
Reduces heart attack risk 50%”

Absolute risk
“New wonder drug
Reduced heart attacks from 2 per 100 to 1 per 100”

The absolute risk is more useful at conveying the true impact of an intervention yet is often under-reported in the research and in the news.
https://www.healthnewsreview.org/toolkit/tips-for-understanding-studies/absolute-vs-relative-risk/