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In psychology, behavior is what an organism does, described through actions or responses in a specified setting. A keypress, a spoken answer and a shift of gaze can all be measured, but they provide different evidence about the processes producing them. An operational definition states what will count as the response in a particular investigation.

For an original example, a learning task might measure how often a participant chooses a rewarded symbol and how long each choice takes. Choice frequency and reaction time are distinct outcomes: two participants can make equally accurate choices while taking different amounts of time. Neither measure alone establishes whether a difference arises from attention, comprehension, motivation or motor demands. The interpretation requires a model and suitable comparison conditions.

This distinction is central to construct validity: recording a response reliably does not by itself show that it measures the intended psychological construct. A report of an emotion is also a response to a question, rather than an unmediated measurement of every component of the emotion.

The mind is the collection of mental processes and capacities involved in perception, attention, human memory, thought, emotion and consciousness. In scientific psychology, these processes are investigated through specified tasks, reports, behavior and measurements of the brain. The mind is not one directly observed score: remembering a location and noticing a flash require different explanations and measurements.

A useful experiment separates available information from the ability to report it. In 1960, George Sperling used briefly displayed arrays of symbols and partial report paradigms. Asking for only part of a display helped test whether a limited whole report reflected unavailable visual information or a limit on retaining and reporting it. Tones could specify which portion to report after the display had ended.^[ref]

For an invented sampling example, suppose a display has three rows and a randomly selected row can be reported with an average of three correct symbols. Under equal sampling and accessibility assumptions, multiplying by three estimates nine available symbols across the display. That is an inference from a task, not a direct inspection of a mental store. Its assumptions must be checked, and a cue delivered much later can test how availability changes with time.

The example links perception, sensory memory and reporting while keeping them distinguishable. An operational definition identifies the actual response to be measured; construct validity asks whether the proposed interpretation of that response is justified. The scientific study of the mind becomes more precise when these connections are explicit.

An operational definition specifies how a variable is measured or manipulated in a particular investigation. Defining stopping performance by a specified stop-signal task, scoring rule and estimation procedure makes the observation reproducible enough to inspect. The broader psychological construct of response inhibition need not be identical to that one procedure.^[ref]

For example, “number of correctly named digits from 1 to 10 under these instructions” operationally defines a numeral identification score. It leaves open whether the same child can compare quantities or count objects. An operational definition makes the measurement concrete; evidence about what that measurement represents is a separate question of construct validity.

A psychological construct is a theoretically specified attribute or process used to explain behavior and experience. Working memory, for example, is not directly observed as a number on an instrument; researchers infer aspects of it from performance under stated tasks. A psychological construct supplies an interpretation of observations, rather than merely renaming a test score.^[ref]

Different operational definitions can target the same proposed attribute. Their agreement, differences and relationships to other measures inform construct validity. If a memory task also requires reading, a low score might reflect reading difficulty as well as the intended memory demand. Clear theory and several complementary measurements help distinguish these explanations.