Why Percentages Can Be Misleading In Health Care

What you’re about to read and watch could potentially save your life, help you get the best treatment, or avoid an unnecessary treatment.

You often see health care advertisements that say things like “Drug A is 60% more effective than Drug B” or Lipitor reduced the risk of heart attack up to 45%.”  Have you ever really stopped and thought about what these numbers mean? Or how they are calculated?   Many of us see and use these numbers to make health care decisions without knowing that some numbers/percentages can be extremely misleading.

There are 2  types of health care percentages that come from clinical trials.   In order to truly make an informed decision, you need to know which one you’re getting because they mean very different things:

  1. Absolute Difference- The real difference between two treatments.  It’s the actual likelihood that something will happen or will not happen (i.e. If 10% of the people who took drug A had a stroke and 20% of the people who took no drug had a stroke, the Absolute Difference is 20%-10%=10%. In other words, Drug A reduced the risk of a stroke by 10%.)
  2. Relative Difference- Compares the Relative Difference between 2 outcomes.   This number tells you nothing about the actual risk.  (Using the same above example, the Relative Difference is calculated by 20%-10% divided by 20% which equals a Relative Difference of 50%.)

Based on these two definitions above, which number do you need to see to make an informed decision?

The best answer is: BOTH NUMBERS.  But if you only could see one number most experts now recommend you ask for the Absolute Difference.  Per above, you can see that a drug that decreases the risk of a stroke by an absolute difference of 10% can be spun as a “50% reduction” in stroke reduction.   Both numbers are mathematically correct, but the absolute number is closer to the truth.  The Absolute Difference is real.  The Relative Difference is statistics.

The kicker is, most of the time (maybe all of the time) drug advertisements will give you the Relative Difference number ONLY.  Why?  Because this is the  number they want you to see given the relative difference is always significantly larger then the Absolute Difference (50% vs 10%).   Would you be more likely to take Drug A if it “ reduces risk of stroke by 10%” OR if Drug A “reduces risk of stroke by 50%?”

We decided to put together this video tutorial to teach people how to use and understand the standard percentages and numbers that health care experts and doctors rely on every day to make their recommendations for patients. And most importantly, to show how Absolute and Relative Difference is calculated!  Anyone with basic math skills will understand this video.

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