A useful theory need not be consistent with other theories, but it must be consistent with itself. An internally consistent theory is one whose components are logically compatible. Its limitations of scope are carefully defined and it does not offer explanations that lie beyond that scope. Also, an internally consistent theory uses language in a consistent manner; that is, it does not use same term to mean two different things, nor does it use two separate terms to refer to the same concept.
A good theory will use concepts and terms that have been clearly and operationally defined. An operational definition is one that defines units in terms of observable events or behaviors that can be measured. For example, an extravert can be operationally defined as any person who attains a predetermined score on a particular personality inventory.
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