What is precision? Some exam boards adopt the view that the essence of precision is related to equipmet used for making measurements. The lack of clarity and consitency often leads to teachers misrepresenting presion. A typical definition is " How small a measurement is. Millimetres are more precise that centimetres, beacause the are smaller". Although this is useful when considering how to improve the precision of a set of measurement it is not in itself a definition of precision. Just as accuracy is the closeness of a measurement (or result) to the true value, precision is the closeness of a set of repeat measurements (or results). Where a result is a mathematical value derived using one of more experimental measurements. So what? The definitions of accuracy and precision are introduced in order to evaluate experimental measurements and results. The evaluation of accuracy is facilitated by a consistent approach across exam boards. The main hurdle is often the concept if true value, which is sometimes unknown, exisitng as a concept. The evaluation of precision does not require any reference to measurement equipment but only reference to other measurements in the same set. I am not sure why the defiinition precision is sometimes based on the nature of measurement equipment rather than the closeness of the measurements.but differences in the scientific and everyday use of the the terms accuracy and precision may be a contributing factor. Thus when evaluating accuracy, the comparision is between individual measurements or results (or the average of a set of measurements or results and the true value. Accuracy can be improved by using more accurate equipment. For the mannufacture of a 25cm3 pipette involves a more accurate comparision to the true value 25ml, than does the manufacture of a 25cm3 measuring cylinder. This the accuracy of an experiment involving the measurement of 25cm3 would be improved by using a pipette instead of a measuring cylinder. When evaluating precision, the comparison is between and individual result or measurement and the measurements or results in the same set. This is often expressed as the range of % range about the mean value. Precision is improved by using equipment which gives measurements and results closer to each other. Thus many suggestions for modification of experimental methods or equipment have an impact on both accuracy and precision. It is of course important that students understand thal although the ideal is a accurate and precise measurements and results, that both accurate and inprecise, inaccurate and inprecise and inaccurate but precise measurement/results do exist. A more accurate and precise use of the definition of precision in the glossary's of science text books and teacher generated resources would improve the ability of students to understand and evaluate experimental metods and equipment.