Accuracy and Precision
(Will be Complete on 8/23/05
Precision and accuracy are often used synonymously but they are actually different things. It is possible to have very precise measurements that are not at all accurate.
The accuracy is how close to "the truth" your measurements are. The accuracy of your measuring tool depends on how well it is kept to a known standard(calibrated). A household bathroom scale has a low degree of accuracy, whereas a triple beam balance has a high degree of accuracy.
Accuracy is a measure of how close you are to the true value (uncertainty) in a measurement.
Let’s say you have a graduated cylinder with 33.5 ml of water.
The accuracy of the measurement is 0.1 ml which means that the actual amount of water in the container is somewhere between 33.6 ml and 33.4 ml. It could be 33.49 ml. It could be 33.68 ml. No one knows. No one can ever know. All we know is that the measurement is some where between these values, this is called the range.
The precision refers to how repeatable your set of measurements
is. It depends on the accuracy of your instrument, it depends on the
care that
you are measuring with and it depends on the number of times you make
the measurement. "In physical science it means "repeatable,
reliable, getting the same measurement each time."
We can never make a perfect measurement. The best we can do is to come
as close as possible within the limitations of the measuring instruments.
Below is a model to illustrate the difference.
Suppose you are aiming at a target, trying to hit the bull's eye (the
center of the target) with each of five darts. Here are some representative
pattern of darts in the target.
