How big are my uncertainties?

'How big are my uncertainties?' is the question all students should ask themselves every time they carry out practical work. At the start of the course this may be answered simply by considering the resolution of the measuring instrument but immediately we are faced with two difficulties. Firstly, some Awarding Organisations (AOs) call this the precision of the instrument rather than the resolution and secondly, do we give the measurement uncertainty as +/- the resolution or +/- half the resolution?  My answer would be +/- half the resolution for analogue scales and +/- the resolution for digital scales however AOs may disagree and some university departments would too. In fact practicing scientists and engineers won't necessarily even agree on what resolution, precision and even accuracy mean! So the advice is this - for the purposes of passing exams, make sure the students are familiar with the AO's practical guide. For the purposes of good science, explain to them that the whole business of uncertainties can be subjective so scientists and engineers must be clear about how they have arrived at their final answers and what the degree of uncertainty is. Apart from the AO's guides, an excellent source of clarification is the ASE / Nuffield Foundation 'Language of Measurement' booklet from The ASE Bookshop. Also, for those who wish to take the subject further, a recommended title is 'Measurements and their Uncertainties' by I G Hughes and T P A Hase, OUP 2010.