We're going to say here that even though we try to be as accurate as possible, every measurement or calculation we do in chemistry has some level of uncertainty. Now, this uncertainty we call experimental error. Basically, any type of calculation we do is never going to be perfect. Some of the circumstances leading to this imperfection are within our control while others are not. Now before we talk about the different types of errors, we first have to talk about how we can look at our data, our calculations and determine if they are good or not. We're going to say when we investigate the quality of an experimental decision or calculation, we have to take into consideration two major principles.

We're going to say the first major principle deals with the reproducibility of our calculations. This is called precision. Precision is just a way of looking at our data or calculations and seeing how close they are to one another. If I've run an experiment 10 times and gotten 10 results, how close are those ten results to one another? This can oftentimes lead us to determine if our calculations are correct or not. Now, in terms of simplicity, we can look at a dartboard. On this dartboard, we have 3 strikes. These 3 strikes are very close to one another. We'd say the reproducibility is very high. We'd say that our strikes here are precise because they're very close to one another.

The second principle deals with how close our measured calculation is to the actual value. This is accuracy. In this image, we have our dartboard still. All the strikes are dead center. If dead center represented our actual value or our true value, we'd say that these strikes are very accurate. We could also say that they're precise as well because they're all very close to one another. We could say here that this would be a very accurate and precise list of strikes. The one above would be precise but not necessarily accurate because although they land in the same general area, none of them hit the bull's eye. They're not accurate.

Later on, we'll learn that we can calculate how good our calculations are, how reproducible and precise they are by looking at the standard deviation of these values. But that's for later on. For now, just realize that when it comes to any calculation, it's never going to be perfect. That's because there are things that we may have done incorrectly and there are also circumstances within the experiment which make it impossible to be totally accurate and sometimes not very precise. Of the two, precision is the one that we can try our best to control. Accuracy sometimes may not be totally within our control. Knowing this, take a look at example 1. Look through the experiment that this individual has done to determine if it's precise or accurate. Once you've figured that out, come back and take a look at how I approach the same question.