On Mon, 11 Mar 2013 11:46:03 -0400, Ed Pozharski <epozh...@umaryland.edu> wrote: ... >Notice that I >only prepared one sample, so if on that particular instance I picked up >4.8ul and not 5.0ul, this will translate into systematically >underestimating protein concentration, even though it could have equally >likely been 5.2ul.
Within the framework of such a short "Materials and Methods" description, the latter is _not_ a systematic error; rather, you are sampling (once!) a statistical error component. If possible, you should repeat the experiment and find out the magnitude of your pipetting error. If impossible, you can only estimate it (making reasonable assumptions based on past experiments). Of course, if - in repeated experiments - your pipetting more often gives (e.g.) lower volumes than 5.0 ul, then some kind of systematic error must be the reason. Systematic error in most cases has the property that it (on average) changes the result towards one side, whereas statistical error should not change the mean value. It is one of the goals of an experiment to identify all sources of systematic error, and to either model or eliminate them. If you are able to identify and model the systematic error, then you can "convert noise to signal". best, Kay
