Accuracy and precision

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Accuracy and precision are two terms that are often equivalently used, even though they do not have the same meaning and should be used accordingly. The accuracy of estimates is the degree of "closeness" to the actual (true) value, while the precision of estimates is the degree to which repeated estimates under unchanged conditions show the same results.A simple definition of “accuracy” is therefore the freedom from mistake or error: correctness. While “precision” is defined as the quality or state of being precise: exactness.

Example:

Accuracy:

Imagine you have the task to assess the mean height of students in a class. Unfortunately all you have for measuring the height is an old ruler, that has gotten a little longer with time. Thus, a bias is introduced by systematically overestimating the real height of each student. As a result the mean height for the whole class will also be too high. Here we say: the accuracy of mean height is low.

Example:

Precision:

Using a ruler (which varies its length due to bad manufacturing), you want to measure your own height. While measuring your height several times in a row, you recognize that you measure a different height with every measurement. The variation of each measurement around the mean height (your real height) gives you an idea about the precision of the measurement.