Explaining absolute and relative effect sizes

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Explaining absolute and relative effect sizes – in a way you’ll never forget.  A blog by the Centre for Evidence-Based Medicine’s Jeremy Howick.

  • Format Websites
  • Language/s English
  • Target Audience Self-directed learning
  • Duration 5-15 mins
  • Difficulty Introductory

Key Concepts addressed


The blue whale is larger than any creature known to have roamed the earth. They are longer than two school buses, have tails as wide as a van, and hearts as big as a small car. If you get very close to a blue whale you might see tiny shelled creatures called barnacles attached to their skin. Most barnacles are about as big as the tip of your thumb. If I asked you who has the bigger sexual member: the blue whale or the barnacle, you might think I was joking. The blue whale’s is bigger than an adult human, while the barnacle’s is barely longer than your hand. Clearly the blue whale wins. But that’s only if we’re talking absolute sizes.

If we’re talking size relative to their body length, it’s a different story. The barnacle’s thingy is up to 30 times as long as its body. Barnacles don’t move, so their penises have to be longer than their bodies to impregnate a mate. The blue whale’s, on the other hand, is shorter than its body. So according to the relative measure, the barnacle’s is larger than the blue whale’s. When you read about medical treatment effects, they usually report relative not absolute effect sizes, which can be confusing.


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