 In this video we'll look at the concepts of Accuracy and Precision. So we've looked at sources of error in measurements, random error and systematic error. Now each of these has an effect on your final measurement, on its precision and its accuracy. So systematic error affects accuracy. Accuracy is the difference between your measured value and the true value. Random error affects precision and it may also affect accuracy. Precision is how close together or reproducible your results are if you measure the same thing several times. The classic analogy here is of arrows in a target. So the first target here shows an archer with high accuracy and high precision, or in other words low systematic error and low random error. The shots are clustered together, this is like measurements that are very close to one another, and they cluster right on the bullseye, which you could think of as the true value of the measurement. The fact that they're clustered together tells you that they're very precise, they're very close to one another, and the fact that they cluster on the bullseye means that they're accurate, they're hitting the true value, or are very close to the true value. The second box shows an archer with high precision but low accuracy. So the archer's shots are very reproducible, they cluster, meaning she has low random error. She's probably experienced with skill at reducing the random fluctuations of her own body. However, something is causing a high systematic error, each of her shots is to the left and below the bullseye. Perhaps there's a steady wind blowing to the left. If she identifies this problem she can compensate for it. The third box shows an archer with low precision, shown as a wide scattering of shots. Imagine an inexperienced archer not skilled in reducing the random fluctuations of her body, or maybe an experienced archer but on a gusty day with the wind blowing in many directions. However, if these were measurements and we averaged them out, the average would fall fairly close to the bullseye, so the accuracy is quite good. As an experimenter this is a good example of why you should take many measurements. The more you take the more likely it is that they'll average out to something useful. The fourth box shows low precision and low accuracy. The shots are scattered so they're not very precise, and if you averaged them they would not be near the bullseye. This suggests perhaps an inexperienced archer shooting in a steady wind. The archer's lack of experience produces the random error, and the wind contributes an additional systematic error.