It helps me to remember this if I associate it with the word correctness .
Precision is a measure of the degree to which a set of many values agree with each other.
I often think of this as scatter . The two are not the same. Just because a bunch of values are tightly clumped together does NOT mean that they must be clumped around the correct value. It's usually very hard to detect systematic bias in a data set, since one usually does not know the true value. Of course, in real life, you almost always don't know the true value.