This is a picture of 10g of coffee ground coarsely and then sifted into 3 sizes.
It shows the dramatic difference in particle size. Now it might illustrate the state of the burrs, and some grinders are better than others, but I wanted to show it because it illustrates the unevenness of grinding. And it’s quite shocking I think!!
It really brought home to me how an extraction percentage is not a true value but an average. That is, take an ext% reading of 18%, it’s not that we’ve extracted 18% from each particle (we may not have extracted 18% from any!). We’ve extracted different amounts from each. From some it may be 20%, from others 13%, and from others still upwards of 27%!! All these added together and averaged give us the extraction yield. In this case: 18%.