 Okay, so archery is all very well, but let's think about an actual experimental situation. Imagine an experimenter has to repeatedly measure out 200 mls of water during an experiment, and she needs to make sure that each sample is as close to 200 mls as she can get it. Pause the video and read the four scenarios here, and think about potential random and systematic errors for each one. Decide whether the accuracy will be high or low in each case, and whether the precision will be high or low. Okay, so let's go through them. In the first situation, she uses a 1 liter beaker to measure her 200 mls, and it's a cheap, poorly calibrated one at that. Measuring 200 mls in a 1 liter beaker is difficult because the gradations are far apart, and the beaker is so wide that small differences in the height of the water translate to a large difference in the volume. Sometimes the volumes will be a bit above the 200 mil mark, and sometimes they'll be below. So that means a large random error, and hence the precision is going to be low. In addition, however, we're told that the beaker is poorly calibrated. The manufacturer has perhaps not checked the gradations on the side. Perhaps where the 200 mil mark is, is actually 250 mls. So this would lead to a systematic error, and hence to low accuracy. This is the worst possible situation. In the second situation, things have improved a bit. She's using at least a good quality piece of glassware, so we hope that it's well calibrated, and it's closer to the volume that she wants to measure. This makes it more likely that the 200 mil mark actually represents 200 mils. However, it's still a beaker. As before, the large diameter of the container means that very small differences in the height of the water mean large changes in the volume, which means that the measurements will still have a large random error. If she measures out many, many samples, these variations should average out, but it's not an ideal situation. In the third situation, she's changed glassware. She's now using a measuring cylinder. This piece of equipment is designed for measuring volumes of liquid precisely. A measuring cylinder has much more finely spaced gradations than a beaker, and it's narrower, so you can more easily make small adjustments to the volume of the liquid. So in this situation, the random error is low. She will be precise. However, there's a problem. She's measuring hot water. Now, as you know, matter expands when it heats up and it contracts when it cools down, so it's volume changes with temperature, and water is no exception. Every time she measures out 200 mils of hot water and then lets it cool down, she'll actually have a smaller volume than she thought, and this will cause a systematic error. All of her volumes will be smaller than she had thought, so her accuracy will be low. It's worth noting here that volumetric glassware like measuring cylinders and burettes and volumetric flasks are calibrated to be used at room temperature, usually 20 or 25 degrees C, for precisely this reason. And in fact, most measuring instruments will have a temperature range in which they are most accurate because of the expansion and contraction of matter with temperature. Okay, in our final situation, she's finally got it right. She's using the right kind of glassware, so her random error is reduced, and she has eliminated the systematic error caused by using hot water. So her accuracy should be good. In this situation, her volumes will cluster closely around 200 mils.