In science and engineering, the accuracy of a measurement is the degree of closeness or trueness of that measurement to the true value of the quantity being measured.
For example, a ruler that measures a known length of 1 meter as 1 meter would have high accuracy; a ruler that measures the same distance as 0.8 meter would have low accuracy. Another example would be if you are trying to measure pi, the accuracy would be determined by how close your measurements are to the known value of pi: 3.141592...
### Precision is about variability in repeat measurements
The precision of a measurement is the repeatibility of that measurement and expresses the variability within it. In other words, precision is being able to getting close to or the same measurements every time.
For example, if the ruler mentioned above that measured 1 meter as 0.8 meter, measured the distance as 0.80 m, 0.79 m, 0.80 m, 0.81 m and 0.80 m over successive measurements the ruler would be quite precise (the measurements have low variability), while being inaccurate. Additionally, using the pi example as above, if your measurments constantly get 3.12, then they are precise, but not accurate.
### Precision and accuracy are independent from each other
The precision of a measurement is independent of the accuracy of the measurement. Likewise the accuracy of a measurement is independent of its precision. This means that measurements can be precise and of low accuracy, accurate and of low precision, precise and accurate, and imprecise and inaccurate.
### Basketball analogy
A good analogy for understanding accuracy and precision is to imagine a basketball player shooting baskets. If the player shoots with accuracy, his aim will always take the ball close to or into the basket. If the player shoots with precision, his aim will always take the ball to the same location which may or may not be close to the basket. A good player will be both accurate and precise by shooting the ball the same way each time and each time making it in the basket.
