I’m sure you’ve been in a situation where someone has asked something like “Is 49 litres a good result?”

You don’t know, of course, so you ask “Compared to what?”

Maybe they respond “Compared to 43 litres!”

That sounds impressive, but you don’t want me to chastise you, so you say, “That still tells me nothing because I don’t know the variation inherent in the process. Give me another typical result!”

They might then say “Uhh, 47 litres.”

Now you let your guard down and think, “Oh, 49 is above both the typical results. Very good!”

And then i chastise you!

So you turn on your brain instead.

You have received two typical numbers: 43 and 47. These don’t tell you much about how the inherent variation, but they do tell you a little. The distance between them is four. If we multiply that by 1.3, we get our estimation of the standard deviation, which is something like 5 litres. That means 49 litres is less than one standard deviation away from the midpoint of 45 litres. That’s a normal result, not unusually good or bad.