Accuracy and Precision

As you look at numbers, and especially as you work with them during the course of learning chemistry, it is important that you understand the terms accuracy and precision..

Accuracy

Accuracy is simply correctness. If you weigh 114 pounds and your bathroom scale says 114 pounds when you stand on it, then it is accurate. If it says 127 pounds when you stand on it, it is not accurate.

Precision

Precision can be defined in several ways. In the context of measurement, precision is how detailed the measurement is. If a small pebble is weighed twice on two different scales and one scale says that the weight is 1 gram and another says that the weight is 0.9937 grams the second is more precise -- that is it gives us more detail. That does NOT mean that the first is not accurate. If that scale weighs only to the nearest gram, then 1 gram is the correct, accurate measurement (that is the mass to the nearest gram), it just isn’t as precise as the other.

The precision of measuring devices

An important thing to understand is that the last digit of any measurement is always an approximation or estimate. This is true even for an electrical device that has a digital display that gives you a reading. 

If you take your pencil and put it on an accurate electronic balance the mass might be 8.65 g. If you remove the pencil and then place it down again the value reported might be the same, but it also might be 8.64 or 8.66 g. 

This could be because you placed the pencil on a different part of the balance pan, because some dust fell, because there are oils on your hands that were transferred to the pencil, some eraser bits fell off, or for any number of other reasons. What that means to us is that the 8.6 portion of the measurement is quite dependable, but that the 5 is a little “iffy.”

To understand how scientists deal with this "iffy" part of measurements you need to understand significant figures.