The term Operational Definition was coined by Percy Bridgman and appeared in his book, The Logic of Modern Physics published in 1927 by Macmillan. I take the following from that book:
“What do we mean by the length of an object? We evidently know what we mean by length if we can tell what the length of any and every object is, and for the physicist nothing more is required. To find the length of an object we have to perform certain physical operations. The concept of length is, therefore, fixed when the operations by which length is measured are fixed: that is, the concept of length involves as much as and nothing more than the set of operations by which length is determined. In general, we mean by an concept nothing more than a set of operations; the concept is synonymous with the corresponding set of operations.
If the concept is physical, as of length, the operations are actual physical operations, namely, those by which length is measured; or if the concept is mental, as of mathematical continuity, the operations are mental operations, namely those by which we determine whether a give aggregate of magnitudes is continuous.”
So the operational definition of a concept (e.g. Dry, safe, weight, length, hardness, poverty, intelligence, etc.) would indicated that the concept is synonymous with the operations used to measure it.
It is useful to observe that if a different set of operations are used, the definition changes and thus our idea of the concept changes as well.
“There is no true value of anything measured or observed.” was an oft-heard Deming quote.
This is not some obscure academic point. We have heard much lately about whether or not this or that food is safe. What means safe? What does dry mean or slippery? Lawyers need operational definitions badly yet are largely ignorant of the implications.
This problem is everywhere as Bridgman pointed out in 1929. The idea of operational thinking changes the way we think about any concept and is, perhaps, Einstein’s most profound idea, at least for humankind.
“Let anyone examine in operational terms any present-day discussion of religious or moral questions to realize the magnitude of the reformation awaiting.” - Bridgman
What is right? What is wrong? Is it safe?