Measurements
Uncertainty in Measurements
All measurement instruments have a certain degree of uncertainty in them. When we take measurements, it is possible to get pretty close to the exact measurement. However, there is always a point when two people measuring the same thing will get two different measurements. Observe the measurement at the right: Person 1 may measure the volume as 31.7 ml, while person 2 may say the volume is 31.8 ml. The two are in agreement on the 31., but they disagree on the tenths place. One says it is 0.7 and the other 0.8. This last number is an estimate, and both number are correct. When you take measurements, you will need to estimate one digit beyond the lowest mark on the measuring device. All measurements have one estimated number. Sometimes, when the amount of error is known, numbers may be written like this: 31.7 ± 0.1 ml This means that the actual value of the number could be as much a 0.1 ml above or below the estimated value that is written. |
Precision & Accuracy
The terms “precision” and “accuracy” do not mean the same thing. Precision is getting the same result over and over again. On the picture, the darts are all hitting the dartboard at the same spot. With precision, the results may be all the same, but they could all be wrong. Accuracy is getting close to the accepted value. On a dartboard, the idea is to hit the bulls-eye. In the picture at right, the darts are surrounding the bulls-eye, hitting on all sides. If an average were taken, the average would be the bulls-eye. In this case, the darts thrown are accurate because they come close to the accepted value (the bulls-eye). However, you must keep in mind that each individual experiment is inaccurate. When a dart thrower is both precise and accurate, the result looks something like this: |