So, what does a measurement being accurate or precise mean? Accuracy is the true value of a measurement or calculation. Precision is the consistency of a set of measurements. For example, lets say that you and I are playing darts. All four of my darts land very close together, but far away from the bullseye. All four of your darts land further away from each other, but they are all surrounding the bullseye. So, all of my darts are precise, and all of your darts are accurate. In order to excel at darts, we would have had to have both accuracy and precision.
Precision is also based on significant digits. But what are significant digits? It is the number of reliable digits in a measurement or calculation. It is defined by 1/10th the finest unit (so the last number is always an estimate). This relates to precision because your calculation will be more precise, if you have more significant digits. This means that 4.0 is more precise than 4, because 4.0 has 2 significant digits and 4 only has 1. Significant digits can have a huge impact on elements. As instruments improve, scientists are able to produce measurements with more precision. In the article “Atomic weight changed for 19 elements”, by Megan Gannon, it states that the weight of gold “is being updated from 196.966 569(4) amu to 196.966 569(5) amu, where the numbers in parentheses represent the uncertainty in the last digit of the atomic weight”. So, the change in the last significant digit changed the atomic weight of gold. More advanced tools for measurement has allowed for scientists to be able to read to the next estimated significant digit, and therefore get a more precise atomic weight for certain elements.