Error in measurement is a limitation all practitioners must be aware of to ensure accuracy and repeatability in your test results.
The smallest worthwhile change that indicates an improvement or lack there-of may be bigger than what you are expecting.
The web abounds in information on this, but the following Powerpoint slideshows from Dr Will G Hopkins outline the nuts and bolts of this very well.
Hopkins reminds us of the importance of both test validity and reliability, and that reliability is paramount.
Dr Chris Gore