Our guest blogger this month is Amy Dyer, a Research Analyst in our Evidence team. Recently, we've been talking a lot about how important it is for men to know their risk of prostate cancer. And we've been quoting lots of statistics like one in eight men will get prostate cancer. But what do these numbers mean? Amy shines a light on the world of risk and some of the figures we use.
A few weeks ago I was sent a link to a BBC article that highlighted just how confusing statistics can be, even for doctors. And I agree, they can be. For all of us! But the right statistics in the right context can also help us understand complicated issues and explain them more clearly.
In my role as Research Analyst, I’m constantly looking at numbers and trying to understand what they mean to different people. And I know that clear statistics are really important to help men and their families understand health risks, such as the risk of being diagnosed with and getting prostate cancer. (In this context, when I’m talking about risk I mean a probability – the likelihood that something may occur.)
We want men to know as much as possible about their risk of getting prostate cancer so they can start the vital conversations with GPs that could save lives. Unfortunately we currently don’t have all the information we need to provide complete statistics for all risk factors and groups of men. To address this we are working hard to develop a risk calculation tool for GPs and we’re continuing to fund research projects investigating risk and awareness of risk.
In general there are two kinds of risks that are used when talking about a person's risk of being diagnosed with a disease: relative risk and absolute risk.
Relative risk compares the difference in the risk (or likelihood) of a disease between two groups. So we can say group one is X times more or less likely to be diagnosed than group two. For example, we know that men with a father or brother diagnosed with prostate cancer are two and-a-half times more likely to be diagnosed with prostate cancer themselves, compared with men who have no family history of the disease.
Absolute risk is an individual’s chance of developing a certain disease over a certain period of time (eg lifetime risk). For example, in the UK we know that one in eight men will get prostate cancer in their lifetime.
There has been a lot of research into how people's perception of the magnitude of a risk (how big it is) can be manipulated by changing the way in which that risk is communicated.
Relative risk alone can appear misleading if it is not put into context, and the impact of that risk isn’t always apparent. Differences between groups may appear large, when in fact the difference in outcome may actually be very small.
If we think about this in terms of money, knowing that you can double your money is only useful if you know how much money you have to start with. So, if you are told you have £1,000,000, your money is ‘common’, doubling your money means you gain another £1,000,000. However, if you only have £1, your money is ‘rare/uncommon’, and by doubling it you only gain an extra £1. In both cases you are getting twice as much money – the same relative change. But it would be difficult to understand the impact of doubling it without knowing the absolute value of the money you have in the first place. The website Understanding uncertainty has a great animation and explanation of how relative risk and statistics are used in everyday life, and goes into this further.
For prostate cancer, relative risk has often been used to compare the risks that men of different ethnicities have of being diagnosed. Black men are three times more likely to get prostate cancer than White men of the same age. However, without knowing how common it is to begin with, this statistic doesn’t really help Black men to understand their personal risk of prostate cancer. We need to look at other calculations of risk statistics, such as lifetime risk.
So we did a lot of number crunching to generate two very simple but important numbers which estimate a Black man’s lifetime risk of getting prostate cancer: one in four. This helped support the existing knowledge that Black men are three times more likely to be diagnosed with prostate cancer than White men of the same age. And it means we can more comprehensively understand the risk of prostate cancer that Black men face.