A man making a decision to have a shingles vaccine told me that his physician said the vaccine would reduce his chance of shingles by 50%. But, 50% of what? What does 50% really mean?
To become a medical decision maker, you must understand numbers. But, numbers can either confuse or enlighten depending how they are communicated. Relative numbers, rather than absolute numbers are often communicated in TV commercials or in your physician’s office, but relative numbers make things sound better than they really are, and, in essence are a sort of untruth.
A relative number refers to another number; it is a “relative to something else” number. A relative number can’t stand alone; you need a reference point. For example, say your favorite candy bar costs $2 and is on sale for 25% off? The 25% can’t refer to the absolute cost because the 25% number is bigger than the $2 number. You will need to multiply the 25% times the $2 to get your savings. What is the absolute savings if the relative drop is 25%; 50 cents.
But, what if the price of the item before taking off 25% is bigger in the first place? For example, if your favorite shirt costs $200 and it goes on sale for 25% off , what is the amount you save; $50. The relative value, 25%, is the same for both items but the absolute amount you save varies 100 fold. The “25%” value is termed relative as it is related (relative) to something else. The “something else” was the before the sale cost of the items.
While costs are an important aspect of making a medical decision, differences in clinical outcomes between compared options for care are most important and must be known before considering costs. What would you pay, for example, for a shingles vaccine? You would have to know how much better the vaccine was compared to no vaccine in terms of reducing the chance of getting shingles. So, you need to know the percent chances of shingles for both strategies, vaccine and no vaccine. In the shingles example in a previous blog post, about 3% of people got shingles if not vaccinated, but about for 1.5% if vaccinated. The absolute difference is 1.5% (3%-1.5%).
However, what is the relative difference? The relative difference is large, a 50% drop in the chance of getting shingles; this is calculated by taking the difference, 1.5%, and dividing it by 3%, the number without the vaccine. This may have been the reasoning behind this patient’s physician saying the vaccine reduced the chance of shingles by 50%. The absolute difference, alternatively, is small compared to the relative number. Relative numbers are always bigger numbers than absolute difference numbers. So, relative numbers may bias your thinking about the value of a treatment.
This blog post alerts you that a number is not just a number. Relative and absolute numbers are not the same. As a decision-maker you should not accept a relative number. Relative numbers are a miscommunication. Unfortunately, relative numbers are often used when medical treatments are advertised, and, unfortunately, they are used by physicians. The learning point is that if you are told a number, ask if it is a relative or absolute value.
