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.