In any discussion of the place of data and evidence in science or engineering, we must avoid the trap of failing to define terms and, as a consequence, rendering the argument unintelligible. We shall therefore begin by defining what we mean by data and evidence.
We take datum to mean the measurement of a parameter e.g. the volume of gas or the type of rubber. This does not necessarily mean a single measurement: it may be the result of averaging several repeated measurements and these could be quantitative or qualitative.
Data we take to be no more or less than the plural of datum, to state the obvious.
Evidence, on the other hand, we take as data which have been subjected to some form of validation so that it is possible, for instance, to assign a 'weight' to the data when coming to an overall judgement. This process of weighting will need to look wider than the data itself. It will need to consider, for example, the quality of the experiment and the conditions under which it was undertaken, together with its reproducibility by other workers in other circumstances and perhaps the practicality of implementing the outcomes of the evidence.

We begin our definition in the centre of the figure above with the ideas that underpin the making of a single measurement and work outwards. This seems a logical way to proceed but, please note, that we are not suggesting that this equates with the order of understanding necessary for carrying out an experiment or the order in which these ideas are best taught.

Making a single measurement

To make a single measurement, the choice of an instrument must be suited to the value to be measured. Making an appropriate choice is informed by an understanding of the basic principles underlying measuring instruments.

1  Underlying relationships

All instruments rely on an underlying relationship which converts the variable being measured into another that is easily read. For instance, the following (volume, temperature and force) are measured by instruments which convert each variable into length:
  • a measuring cylinder converts volume to a length of the column of liquid
  • a thermometer converts temperature to a change in volume and then to a change in length of the mercury thread
  • a force meter converts a force into the changing length of a spring
Other instruments convert the variable to an angle on a curved scale, such as a car speedometer. Electronic instruments convert the variable to a voltage.
Some instruments are not so obviously 'instruments' and may not be recognised as such. One example is the use of lichen as an indicator of pollution and another is pH paper where chemical change is used as the basis of the 'instrument' and the measurement is a colour. Other instruments rely on more complex and less direct relationships.