If you ask a footballer what are the dimensions of the goal he will probably say something rude. The effect of his remark may well be that he is more concerned in getting the ball into it than wearing himself out remembering its measurements. He may add that as long as both goals are the same standard size that is all that matters; the knowledge of its numerical definition does not help you to kick a goal.

Similarly, in research, common dimensions are fair to both sides and cancel out; you do not need to know their absolute values.

I was once concerned in the design of a new type of centrifugal pump for use in paper mills. Paper is made of pulp and this is transported about the factory by mixing it with differing amounts of water as it goes through the various processes. In all paper mills, and especially in old ones, the number of centrifugal pumps to be found is astronomical and, even worse, hardly any two of them are of the same size. The design of each has not only to be matched to the widely varying ratio of pulp to water, which sometimes attains a porridge-like consistency and at others that of watered down milk, but also must be tuned to the local requirements of head and quantity per minute. The maintenance department's store is packed with a huge range of spare motors, pump casing, impellers etc., but hardly ever has the one you want if needed quickly.