We live with uncertainty every day. Will the weather be fine for a barbecue at the weekend? What is the risk to our health posed by a particular item of diet or environmental pollutant? Have we invested our money wisely?

It is understandable that we would like to be able to eliminate, or at least reduce, uncertainty. If we can reduce it significantly, we become more confident that a desirable event will happen, or that an undesirable event will not. To this end we seek out accredited professionals, such as weather forecasters, medical researchers and financial advisers.

However, in science and technology uncertainty has a narrower meaning, created by the need for accurate measurement. Accurate measurement, which implies the existence of standards of measurement, and the evaluation of uncertainties in a measurement process are essential to all areas of science and technology. The branch of science concerned with maintaining and increasing the accuracy of measurement, in any field, is known as metrology. It includes the identification, analysis and minimisation of errors, and the calculation and expression of the resulting uncertainties.

Whether or not a measurement is regarded as ‘accurate’ depends on the context. Supermarket scales used for weighing fruit or vegetables need not be better than 1% accurate. By contrast, a state-of-the-art laboratory balance is able to determine the value of an unknown mass of nominal value one kilogram to better than one part in ten million. These figures, 1% in one case and one part in ten million in the other, are numerical measures of the degree of accuracy: low in the first case and high in the second, but each of them fit for its particular purpose.