Operational definition
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An operational definition is a description of something — such as a variable, term or object — in terms of the specific process or set of validation tests used to determine its presence and quantity. Properties described in this manner must be publicly accessible so that persons other than the definer can independently measure or test for them at will.
For example, the weight of an object may be operationally defined in terms of the specific steps of putting an object on a scale. The weight is whatever results from following the measurement procedure, which can in principle be repeated by anyone. It is intentionally not defined in terms of some intrinsic or private essence. The operational definition of weight is just the result of what happens when the defined procedure is followed. In other words, what's being defined is how to measure weight for any arbitrary object, and only incidentally the weight of a given object.
Operational definitions are also used to define system states in terms of a specific, publicly accessible process of preparation or validation testing, which is repeatable at will. For example, 100 degrees Celsius may be crudely defined by describing the process of heating water until it is observed to boil. An item like a brick, or even a photograph of a brick, may be defined in terms of how it can be made. Likewise, iron may be defined in terms of the results of testing or measuring it in particular ways.
One simple, every day illustration of an operational definition is defining a cake in terms of how it is prepared and baked (i.e., its recipe is an operational definition). Similarly, the saying, if it walks like a duck and quacks like a duck, it must be some kind of duck, may be regarded as involving a sort of measurement process or set of tests.
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Limitations
If a definition invokes an historical event, such as having weighed an object sometime in the past, it is no longer repeatable, so it fails to qualify as operational. Similarly, a specific brick cannot be operationally defined by the process of making it, because that process is historical. (But see the example of The Constellation Virgo below for a discussion of how to avoid this difficulty.)
Operational definitions are inherently difficult — arguably, even impossible — to apply to mental entities, because they are generally understood to be accessible only to the individual who experiences them and are therefore not independently verifiable. According to this line of thinking, a person's mental image of a brick cannot be operationally defined because it cannot be measured from outside that person's mind. Philosopher Daniel Dennett has argued that first-person operationalism is possible and desirable, using the anthropological version of the scientific method to bring the mind fully into the third-person realm required by science. As part of the Multiple Drafts Model of consciousness, Dennett defines a process he calls heterophenomenology, by which the mental is defined operationally in terms of the observed behavior of the subject.
Usefulness
Despite the controversial philosophical origins of the concept, particularly its close association with logical positivism, operational definitions have undisputed practical applications. This is especially so in the sciences, where most (if not all) formal definitions (as distinct from arbitrary naming) must be operational definitions or traceable to operational definitions. Operational definitions are particularly important in the physical sciences.
Relevance to philosophy
The idea originally arises in the operationalist philosophy of P. W. Bridgman and others. By 1914, Bridgman was dismayed by the abstraction and lack of clarity with which, he argued, many scientific concepts were expressed. Inspired by logical positivism and the phenomenalism of Ernst Mach, in 1914 he declared that the meaning of a theoretical term (or unobversable entity), such as electron density, lay in the operations, physical and mental, performed in its measurement. The goal was to eliminate all reference to theoretical entities by "rationally reconstructing" them in terms of the particular operations of laboratory procedures and experimentation.
Hence, the term electron density could be analyzed into a statement of the following form:
- (*) The electron density of an object, O, is given by the value, x, iff P applied to O yields the value x,
where P stands for an instrument that scientists take as a procedure for measuring electron density.
Operationalism, defined in this way, was rejected even by the logical positivists, due to inherent problems: defining terms operationally necessarily implied the analytic necessity of the definition. The analyticity of operational definitions like (*) is essential to the project of rational reconstruction. Operationalism is not, for example, the idea that electron density is defined as whatever magnitude instruments of the sort P reliably measure. On that conception (*) would represent an empirical discovery about how to measure electron density, but -- since electrons are unobservables -- that's a realist conception not an empiricist one. What the project of rational reconstruction requires is that (*) be true purely as a matter of linguistic stipulation about how the term "electron density" is to be used.
Since (*) is supposed to be analytic, it's supposed to be unrevisable. There is supposed to be no such thing as discovering, about P, that some other instrument provides a more accurate value for electron density, or provides values for electron density under conditions where P doesn't function. Here again, thinking that there could be such an improvement in P with respect to electron density requires thinking of electron density as a real feature of the world which P (perhaps only approximately) measures. But that's the realist conception that operationalism is designed to rationally reconstruct away.
In actual, and apparently reliable, scientific practice, changes in the instrumentation associated with theoretical terms is routine, and apparently crucial to the progress of science. According to an operationalist conception, these sorts of modifications would not be methodologically acceptable. Most logical empiricists were not willing to accept this conclusion. So they felt comelled to reject operationalism.
However, this rejection of operationalism as a general project destined to ultimately define all theoretical entities in philosophy did not mean that operational definitions ceased to have any practical use or that they could not be applied in particular cases.
Relevance to standardization
Physical quantities, such as temperature and electric current are commonly defined in textbooks in terms of their abstract definitions (vide infra). This leads to some practical difficulties for the standardization demanded for trade and for testing the reproducibility of scientific results. Standardization bodies, therefore, specify physical quantities in terms of operational definitions in order to facilitate agreement and reproducibility.
The importance of using common, or standardized, operational definitions was illustrated in 1999 when NASA lost a $125 million Mars orbiter because one engineering team used metric units while another used English units for a key spacecraft operation. [1]
Relevance to scientific practice
Operational definitions are at their most controversial in the field of psychology, where intuitive concepts, such as intelligence need to be operationally defined before they become amenable to scientific investigation, for example, through processes such as IQ tests. Such definitions are used as a follow up to a conceptual definition, in which the specific concept is defined as a measurable occurrence. John Stuart Mill pointed out the dangers of believing that anything that could be given a name must refer to a thing and Stephen Jay Gould and others have criticized psychologists for doing just that. A committed operationalist would respond that speculation about the thing in itself, or noumenon, should be resisted as meaningless, and would comment only on phenomena using operationally defined terms and tables of operationally defined measurements.
A behaviorist psychologist might (operationally) define intelligence as that score obtained on a specific IQ test (e.g., the Wechsler Adult Intelligence Scale test) by a human subject. The theoretical underpinnings of the WAIS would be completely ignored. This WAIS measurement would only be useful to the extent it could be shown to be related to other operationally defined measurements, e.g., to the measured probability of graduation from university. [2]
Relevance to business
On October 15 1970, the West Gate Bridge in Melbourne, Australia collapsed, killing 35 construction workers. The subsequent enquiry found that the failure arose because engineers had specified the supply of a quantity of flat steel plate. The word flat in this context lacked an operational definition, so there was no test for accepting or rejecting a particular shipment or for controlling quality.
In his managerial and statistical writings, W. Edwards Deming placed great importance on the value of using operational definitions in all agreements in business. As he said:
"An operational definition is a procedure agreed upon for translation of a concept into measurement of some kind." - W. Edwards Deming
"There is no true value of any characteristic, state, or condition that is defined in terms of measurement or observation. Change of procedure for measurement (change of operational definition) or observation produces a new number." - W. Edwards Deming
Examples
Temperature
The thermodynamic definition of temperature, due to Nicolas Léonard Sadi Carnot, refers to heat "flowing" between "infinite reservoirs". This is all highly abstract and unsuited for the day-to-day world of science and trade. In order to make the idea concrete, temperature is defined in terms of operations with the gas thermometer. However, these are sophisticated and delicate instruments, only adapted to the national standardization laboratory.
For day-to-day use, the International Practical Temperature Scale (IPTS) is used, defining temperature in terms of the electrical resistance of a thermistor, with specified construction, calibrated against operationally defined fixed points.
Electric current
Electric current is defined in terms of the force between two infinite parallel conductors, separated by a specified distance. This definition is too abstract for practical measurement, so a device known as a current balance is used to define the ampere operationally.
Mechanical hardness
Unlike temperature and electric current, there is no abstract physical concept of the hardness of a material. It is a slightly vague, subjective idea, somewhat like the idea of intelligence. In fact, it leads to three more specific ideas:
- Scratch hardness measured on Mohs' scale;
- Indentation hardness; and
- Rebound, or dynamic, hardness measured with a Shore scleroscope.
Of these, indentation hardness itself leads to many operational definitions, the most important of which are:
- Brinell hardness test—using a 10mm steel ball;
- Vickers hardness test—using a pyramidal diamond indenter; and
- Rockwell hardness test—using a diamond cone indenter.
In all these, a process is defined for loading the indenter, measuring the resulting indentation and calculating a hardness number. Each of these three sequences of measurement operations produces numbers that are consistent with our subjective idea of hardness. The harder the material to our informal perception, the greater the number it will achieve on our respective hardness scales. Furthermore, experimental results obtained using these measurement methods has shown that the hardness number can be used to predict the stress required to permanently deform steel, a characteristic that fits in well with our idea of resistance to permanent deformation. However, there is not always a simple relationship between the various hardness scales. Vickers and Rockwell hardness numbers exhibit qualitatively different behaviour when used to describe some materials and phenomena.
The Constellation Virgo
The constellation Virgo is a specific constellation of stars in the sky, hence the process of forming Virgo cannot be an operational definition, since it is historical and not repeatable. Nevertheless, the process whereby we locate Virgo in the sky is repeatable, so in this way, Virgo is operatonally defined. In fact, Virgo can have any number of definitions (although we can never prove that we are talking about the same Virgo), and any number may be operational.
References
- Ballantyne, Paul F. History and Theory of Psychology Course, in Langfeld, H.S. (1945) Introduction [to the Symposium on Operationism]. Psyc. Rev. 32, 241-243.[3]
- Bohm, D. (1996). On dialog. N.Y.: Routledge.
- Boyd, Richard. On the Current Status of the Issue of Scientific Realism in Erkenntnis. 19: 45-90.
- Bridgman, P. W. The way things are. Cambridge: Harvard University Press. (1959)
- Carnap, R. The Elimination of Metaphysics Through Logical Analysis of Language in Ayer, A.J. 1959.
- Churchland, Patricia, Neurophilosophy— Toward a unified science of the mind/brain, MIT Press (1986).
- Churchland, Paul., A Neurocomputational Perspective— The Nature of Mind and the Structure of Science, MIT Press (1989).
- Dennett, Daniel C. Consciousness Explained, Little, Brown & Co.. 1992.
- Depraz, N. (1999). "The phenomenological reduction as praxis." Journal of Consciousness Studies, 6(2-3), 95-110.
- Hardcastle, G. L. (1995). "S.S. Stevens and the origins of operationism." Philosophy of Science, 62, 404-424.
- Hermans, H. J. M. (1996). "Voicing the self: from information processing to dialogical interchange." Psychological Bulletin, 119(1), 31-50.
- Hyman, Bronwen, U of Toronto, and Shephard, Alfred H., U of Manitoba, "Zeitgeist: The Development of an Operational Definition", The Journal of Mind and Behavior, 1(2), pps. 227-246 (1980)
- Leahy, Thomas H., Virginia Commonwealth U, The Myth of Operationism, ibid, pps. 127-144 (1980)
- Ribes-Inesta, Emilio "What Is Defined In Operational Definitions? The Case Of Operant Psychology," Behavior and Philosophy, 2003.[4]
- Roepstorff, A. & Jack, A. (2003). "Editorial introduction, Special Issue: Trusting the Subject? (Part 1)." Journal of Consciousness Studies, 10(9-10), v-xx.
- Roepstorff, A. & Jack, A. (2004). "Trust or Interaction? Editorial introduction, Special Issue: Trusting the Subject? (Part 2)." Journal of Consciousness Studies, 11(7-8), v-xxii.
- Stevens, S. S. Operationism and logical positivism, in M. H. Marx (Ed.), Theories in contemporary psychology (pp. 47-76). New York: MacMillan. (1963)
- Thomson — Waddsworth, eds., Learning Psychology: Operational Definitions Research Methods Workshops[5]