Accuracy
Accuracy describes the agreement between the
determined value and the
true value.
Would you like to see this example? 
Click this link to go to the complete tutorial if you are an AUSeTUTE member. 
Not an AUSeTUTE Member?
 Find out how an AUSeTUTE Membership can help you here.
 Become an AUSeTUTE member here.

When talking about the accuracy of a measurement, Chemists like to know just how accurate the measurement is.
For this reason, we calculate the percentage relative error as shown below:
percentage relative error 
= 
true value  determined value true value 
x 100 
For an accurate measurement, the pecentage relative error will be very low, close to 0 %
For an inaccurate measurement, the percentage relative error will be high, for example, greater than 5 %.
Would you like to see this example? 
Click this link to go to the complete tutorial if you are an AUSeTUTE member. 
Not an AUSeTUTE Member?
 Find out how an AUSeTUTE Membership can help you here.
 Become an AUSeTUTE member here.

If we know the tolerance of a true measurement, we can decide that the determined value is accurate if it lies within the tolerance levels of the true measurement, and, inaccurate if it lies outside the tolerance levels of the true value.
Would you like to see this example? 
Click this link to go to the complete tutorial if you are an AUSeTUTE member. 
Not an AUSeTUTE Member?
 Find out how an AUSeTUTE Membership can help you here.
 Become an AUSeTUTE member here.

In order to determine how accurate a measurement is, we need to know the true value.
Often we don't know the true value, so we can't determine how accurate our results are.
Precision
Because it is very rare for Chemists to know how accurate a measurement is, they make a number of measurements under the same conditions until they arrive at a set of measurements that are in good agreement with each other.
The reproducibility of a measurement is known as precision.
When all the measurements are very similar, we say the determined value is known precisely.
If we cannot get measurements that are very similar, we cannot say the value is known precisely, instead we say the measurements are imprecise.
We can make generalisations about the precision of a set of measurements by calculating the range of the experimentally determined values:
range of values = largest value  smallest value
A set of measurements can be described as precise if the range of values is very small, that is, range is close to 0
A set of measurements will be described as imprecise if the range of values is large, that is, range is not close to 0.
Would you like to see this example? 
Click this link to go to the complete tutorial if you are an AUSeTUTE member. 
Not an AUSeTUTE Member?
 Find out how an AUSeTUTE Membership can help you here.
 Become an AUSeTUTE member here.

Examples of Accuracy and Precision
Accuracy describes the agreement between the determined value and the true value.
Precision describes the reproducibility of a measurement.
Therefore measurements can be described as one of the following:
 accurate and precise
 accurate but imprecise
 inaccurate but precise
 inaccurate and imprecise
Would you like to see this example? 
Click this link to go to the complete tutorial if you are an AUSeTUTE member. 
Not an AUSeTUTE Member?
 Find out how an AUSeTUTE Membership can help you here.
 Become an AUSeTUTE member here.
