On 11 March 2013 15:46, Ed Pozharski <[email protected]> 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 >
Ed, surely the point is that you don't know that you only picked up 4.8ul - as you say what you actually picked up for all you know could equally well have been 5.2ul (I'm assuming that you don't conduct a separate more accurate experiment to measure what was actually picked up by each pipetting). Statistics is about expectation as distinct from actuality, and the expected error is 0.2ul (or whatever: you would have to repeat the pipetting several times to estimate the standard deviation), regardless of what the actual error is. This expected error then feeds into the expected error of the measured concentration which results from performing the experiment in its entirety, using the usual rules of error propagation. Again the actual error in the concentration from a single experiment is unrelated to its expected error, except insofar that you would normally expect it to fall within (say) a +- 3 sigma envelope. Personally I tend to avoid the systematic vs random error distinction and think instead in terms of controllable and uncontrollable errors: systematic errors are potentially under your control (given a particular experimental setup), whereas random errors aren't. Cheers -- Ian
