Accurate and Precise

Think about two watches, one that has stopped and one that is always 5 minutes fast.  Which one is more precise?  Which one is more accurate?  Is there a difference?

In normal, everyday conversations we use the words accuracy and precision interchangeably and to mean the same thing.  This is all well and good in these situations, but when it comes to improvement projects, they are very different.

Did you know that a process can be accurate but not precise?  Precise but not accurate?  Both precise and accurate?  Neither precise or accurate?

To explain this further (to avoid boring you to tears, this is a simplified explanation), I will use an example of an archer.  Robin Hood is a bit too obvious, so we will call our fictional archer Jeffrey.

In front of Jeffrey is a target board.

If he has 5 shots and he hits the outer ring 5 times in almost the same spot is he accurate?  precise? both? neither?

Here, Jeffrey is not accurate but he is relatively precise.  He has repeatedly produced the same results.  Precision is about the repeatability of a process.

Now Jeffrey hits the bullseye twice, the inner ring once and the next ring out, twice.  Is Jeffrey accurate or precise here?

Here he is not precise, he is getting different results, but he is more accurate than last time because he is hitting his target.

If Jeffrey was to hit the bullseye 5 times then he would be both accurate and precise – he is repeatedly getting the target.

Now, think about two watches, one that has stopped and one that is always 5 minutes fast.  Which one is more precise?  You might say that it is the one that is always fast because it is always (repeatability) fast.  But, does that mean that it is more accurate than the stopped watch?  It can be argued that the stopped watch is more accurate because twice a day it is exactly right, where as the fast watch is never right.

So why are we interested in the accuracy and precision?  There is a belief that defects are caused by variations in processes.  Removing waste and defects can be said to improve efficiency and maximize the use of resources.  Six Sigma is one methodology that aims to do this by making processes as constant as possible.