Accuracy and precision are related concepts that are often confused.  Accuracy and precision are both ways to measure data. Accuracy measures how close results are to the true or known value, often described by the percent error. Precision measures how close results are to one another, often measured by the standard deviation. Precision includes both repeatability and reproducibility.  To make data repeatable, all variables, such as the instrument, operator, and environmental factors, are kept constant.  For data to be reproducible, the same results must be obtained by different operators, on different instruments, and at different times. The image below shows the relationship between accuracy and precision.

Accuracy and precision combine to produce the margin of error (+/-). The accuracy and precision of data are often stated implicitly using significant figures. When the data are graphed, accuracy is seen in the amount of horizontal shift in the data, and precision is seen in the dispersion or spread of the data.