Repeat After Me…

So, in my last post (Agreement Is Difficult) we started to talk about agreement which measures “closeness” between things.  We saw that agreement is broadly defined by accuracy and precision. Today, I would like to talk a little more about the latter.

 The Food and Drug Administration (FDA) defines precision as “the degree of scatter between a series of measurements obtained from multiple sampling of the same homogeneous sample under the prescribed conditions”. This means precision is only comparable under the same conditions and generally comes in two flavors: 

1- Repeatability which measures the purest form of random error – not influenced by any other factors. The closeness of agreement between measures under the exact same conditions, where “same condition” means that nothing has changed other than the times of the measurements.

2- Reproducibility is similar to repeatability but represents the precision of a given method under all possible conditions on identical subjects over a short period of time. So, same test items but in different laboratories with different operators and using different equipment for example.

Now, when considering agreement if one of the readings you are collecting is an accepted reference then you are most probably interested in validity (we will talk about this a future post) which concerns the interpretation of your measurement. On the other hand if all of your readings are drawn from a common population then you are most likely interested in assessing the precision of the readings – including repeatability and reproducibility.

As we have just seen, not all repeats are the same! Think about what it is that you want to report before you set out to study agreement – or you could be destined to do it over again as does Tom Cruise in his latest movie Edge of Tomorrow where is lives, dies, and then repeats until he gets it right… 

See you in the blogosphere,

Pascal Tyrrell