Precision and accuracy

Precision and accuracy in measurement are different, and often confused.

From here
Let's assume that we are trying to measure a property that has a value that is stable and that can be measured.

To measure a property we need both an instrument and a procedure. Using the same procedure with a different instrument or the same instrument with a different procedure will usually give a different result.

We'll call the combination of instrument and procedure a technique.

A technique for measuring a property yields accurate results if repeating the technique gives a series of measurements that, on average, converge on the "true value" of the property. This assumes that there is a knowable "true value."

A technique for measuring a property yields precise results if it repeating the technique gives a series of measurements that are close to one another and if the technique can differentiate properties that are close to one another, and not identical.

So a measuring technique can be accurate, precise, both accurate and precise, or neither.