Accuracy and Precision
Accuracy and precision are used in context of measurement, e.g. the size of a project.
Accuracy and precision are alike only in the fact that they both refer to the quality of a measurement, but they are very different indicators of a measurement.
Accuracy is the degree of closeness to true value. Precision is the degree to which an instrument or process will repeat the same value. In other words, accuracy is the degree of veracity while precision is the degree of reproducibility.
If a measurement is accurate it means that it agrees closely with the accepted standard for that measurement. For example, if we estimate a project's size to x and the actual size of the finished project is equal to or very close to x, then it is accurate, but it might not be precise. Thus, the closer a system's measurements to the accepted value, the more accurate the system is considered to be.
A measurement that is precise means that it agrees with other measurements of the same thing. Again in the sense of projects, if we estimate the size of many of them and they, in the end, are all close to or equal to what we estimated, then we can start to get a sense of the precision of our estimates. But first and foremost we want each of them to be as accurate as possible.
Accuracy can be determined by one measurement while many measurements are needed to determine precision. For instance, by looking at the image above, just by one bullet fired, one knows if it is accurate or not, but a number of bullets have to be fired to know if the result is precise or not. Bullets that hit closer to the bullseye are considered more accurate. If a large number of bullets are fired, precision would be the size of the bullet cluster and not how close they are to the bullseye.
Remember that our measurement system is intended to give us a high confidence in the data that we use to decide on estimation accuracy, so we can determine root causes and improve over time.
In short we can say that we want all our estimates to hit the target first (be accurate to within a certain limit), and then we can concentrate on the precision afterwards. In this sense it is not possible to reliably achieve accuracy in individual measurements without precision.
I hope this has clarified how to use the two terms properly and as always, feel free to contact us for more information.
Image credit: Quality In Practice