What is risk? Absolute versus Relative

riskMy mom and I were talking this afternoon – we talk every day on my drive home from work (I celebrated the day I got Bluetooth in my car) – about Angelina Jolie.  It was difficult to miss the news this past week about her New York Times opinion piece describing why she decided to remove her ovaries and Fallopian tubes.  There have been a number of interesting articles both praising (here or here or here) or criticizing or clarifying her choice.  That’s not what I want to talk about and it’s not what my mom and I talked about.  What we talked about was risk.  Most stories talking about Angelina Jolie mention that because of the gene mutation she had, there was an 87% risk of her developing breast cancer.  Despite the fact that 87% is awfully specific (and based on limited data from a certain number of women with this mutation that were studied over time), what I want to focus on isn’t the number, but what the number refers to.  In particular, I want to point out that there are different ways of talking about risk – and this is important when reading about any scientific information in the news.

coin_flipLet’s start with a quick definition – risk is the chance that something will happen.  These are usually percentages.  There is a 50% chance when you flip a coin that it will land on heads.  The risk is 50%.  Of course, when applied to the chance of developing a disease, or having a particular treatment outcome, or surviving an accident, the numbers are a lot more difficult to calculate than a coin flip.  But they are also more confusing when describing the risk as well.

 

rosk_tableI’m sure you’ve read news stories that say something like “Drinking more than 3 caffeinated drinks a day increases your risk of a heart attack by 50%” (this is a completely fictional example!!!) Fifty percent. What a HUGE risk.  Except what they don’t tell you is that without drinking caffeinated drinks, your risk of having a heart attach is only 1%.  So a 50% increase means your risk only increases to 2%.  This is the difference between relative versus absolute risk.  50% is the risk relative to what the actual baseline risk, whereas the absolute risk tells you the actual chance of something happening.

Let’s look at another example.  “This new drug decreases the risk of blindness in diabetic patients by 50% over 5 years”.  This is promising news!  Except, again, the 50% is relative risk – what you want to know is what the chance of a diabetic patient going blind?  If the chance that a diabetic patient goes blind is 60%, then a decrease of 50% is huge. There is only a 30% chance of blindness now.  Ont he other hand, if like the previous example, the actual chance of going blind is 2%, the 50% decrease is less impressive.  This makes the decrease in risk no less important to the patients who take the drug and don’t go blind – but it does affect how you read a news story describing the effect of the drug and whether or not you may want to take an expensive drug.

Now let’s get back to Angelina Jolie. The actual risk for breast cancer in the general population over a lifetime is ~12%.  If you have the mutations in the genes (called BRCA1/BRCA2) that Angelina Jolie has, it increases the risk to 40-80%. This is the absolute increase in the chance of getting breast cancer.  And as you may notice – the risk has a range (based on a number of factors – family history, health history, etc that we’ll get into in another post).

So how can you be a more savvy reader? You can be tipped off to relative risk by phrases like “increased by”, “decreased by”, “more than” or “less than”.  This only tells us the difference compared to baseline, but gives NO indication of what that baseline risk is. Absolute risk, on the other hand, provides the best estimate of what the overall likelihood of something happening will be.

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2 thoughts on “What is risk? Absolute versus Relative

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