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The operational definition of an empirical term (i.e., a descriptor of some element of physical experience) is the description of a specific process, or set of validation tests, accessible to more persons than the definer (i.e., said to be publicly accessible), whereby it is (or can be) repeatedly measured or tested. For example, the weight of an object may be operationally defined by describing the operations of using a balance and standard weights. (Note that it is not the specific weight of the specific object which is operationally defined, but the process for arriving at it. Therefore, weight of an object labels an operational definition which is a nonspecific process.) But an historical event (e.g., the process of taking my weight last Tuesday) cannot be an operational definition because it cannot be repeated.
Operational definitions are also used to define system states or (nonspecific) physical objects by the description of a specific process of preparation, or validation testing, publicly accessible, which can be repeated. (In this case, the name of the state or 'nonspecific' object may be seen as a label for the preparation or testing process.) For example, 100 degrees Celsius may be crudely defined by describing the process of heating water until it is observed to boil. And a 'nonspecific' physical brick (or even a 'nonspecific' physical photograph of a brick) may be operationally defined by the description of the process for making it. But the substance 'iron' may be operationally defined by a set of measurements or tests.
Generally speaking, the contents of my mind cannot be operationally defined, because they are not accessible to anyone else (except by my verbal report). Hence, my mental image of a brick cannot be operationally defined, since it is not publicly accessible, e.g., it is invisible, to anyone but myself. Similarly, a particular, or specific, physical brick cannot be operationally defined by the process of making it, because that process is historical which, by definition, is not repeatable. (But see the example of The Constellation Virgo below for a discussion of how to avoid this difficulty.)
Nevertheless, philosopher Daniel Dennett has argued that first-person operationalism should be considered, even by the most tenacious opponents of operationalism, to be a legitimite form (perhaps the only legitimate form) of operationalism. In his Multiple Drafts Model of consciousness, he insists that such models of consciousness must be operationally defined. It is precisely the procedure of "writing down" data in memory that is criterial or definitive of consciousness. [Of course a behaviorist psychologist would point out that it is only the (potentially public) physical act or procedure of "writing down" (purportedly mental) data and recording any accompanying verbal report that is or may be operationally defined. In principle, the act of "writing down" data in memory is no different than the act of "viewing" a mental brick.]
It has been said (slightly imprecisely, except as noted using the parenthetical additions), that the operational definition of a (nonspecific physical) cake (of a 'specific type') is the recipe for making the cake, which we may regard as a (nonspecific) physical object preparation process. The saying, if it walks like a duck and quacks like a duck, it's a (nonspecific) duck, may similarly be regarded as involving a sort of measurement process or set of tests.
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 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 criticised 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 comment only made 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. 
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 mass, 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 mass could be analyzed into a statement of the following form:
- (*) The mass 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 mass.
Operationalism, in philosophy, was eventually rejected even by the logical positivists because defining terms in this way necessarily implied the analytic necessity of the definiens. According to operationalism, the definition in terms of "rational reconstruction" must not involve an empirical discovery, since that would entail the existence of the unobservable entity which was supposed to be defined away (e.g. mass). Definition had to be a matter of linguistic stipulation and therefore it had to be analytic and unrevisable. The problem is that scientists are constantly developing new instruments to measure the same theoretical entities and improve the accuracy of their measurements. Since this was methodologically unacceptable from the point of view of operationalism, operationalism as a general position with regard to the question of realism versus anti-realism in philosophy of science had to be abandoned.
However, this rejection of operationalism as a general project destined to ultimately define all theoretical entities in philosophy did not mean that operationalism ceased to have any pratical use or that it could not be applied in particular cases.
Relevance to standardisation
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 standardisation demanded for trade and for testing the reproducibility of scientific results. Standardisation bodies, therefore, specify physical quantities in terms of operational definitions in order to facilitate agreement and reproducibility.
The importance of using common, or standardised, 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. 
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- 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.
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- Thomson — Waddsworth, eds., Learning Psychology: Operational Definitions Research Methods Workshops
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