Accuracy vs. Precision

Let's take a closer look at the dartboards.

#1 -- Not Precise, Not Accurate
This dart thrower needs some lessons -- the darts are nowhere near the centre of the board, and they're all over the place.

#2 -- Precise, But Not Accurate
The darts here are precise -- that is, they're really close together. However, they're not accurate -- they're far from the centre of the board.

#3 -- Not Precise, But Accurate
The darts are not precise -- they're not close together, but they're fairly accurate by being close to the bull's-eye.

#4 -- Precise And Accurate
The darts are precise, since they're close together, and they're accurate, since they're right at the centre of the board.

Next we'll formalize these distinctions and see how they apply to error analysis in general.