Data provide the foundation upon which understanding in science and engineering is built. A basic requirement is that data are expressed in units that are recognised and accepted internationally. This chapter considers the most commonly adopted system of units: The International System of Units. The chapter also considers how to express data, for example using scientific notation. The importance of presenting data clearly in tables is emphasised.

In some situations, it is possible to estimate the size of values likely to emerge from an experiment, prompting our attention to be alerted when the values obtained differ considerably from our estimation. Exploring the reasons for a discrepancy can lead to improved insight of the experiment or perhaps hint that a mistake has occurred, for example when converting units.