Why Stimulus Classes Matter
Organisms do not respond to every stimulus individually. Instead, they group stimuli into classes and respond similarly to all members of a class. How stimuli become grouped together is a fundamental question in behavior analysis, and the answer has direct implications for understanding language, concept formation, and how to teach new skills efficiently. The BCBA exam tests your knowledge of three types of stimulus classes, each defined by a different basis for grouping.
Feature Stimulus Classes
Definition
A feature stimulus class (also called a perceptual or physical stimulus class) groups stimuli based on shared physical features. All members of the class share one or more common physical properties, such as color, shape, size, texture, or other sensory characteristics. The organism responds similarly to stimuli that look, sound, feel, or otherwise physically resemble each other.
Examples
A child who has been taught to identify "red" may correctly label a red ball, a red car, and a red block as "red" because they all share the physical feature of redness. The child has formed a feature stimulus class based on the common physical attribute. Similarly, a learner who can identify "circles" regardless of size or color has formed a feature stimulus class based on the shared geometric property of roundness.
Clinical Application
Feature stimulus classes are the basis for stimulus generalization. When a learner responds to a novel stimulus that shares physical features with a trained stimulus, generalization has occurred along the dimension of physical similarity. Clinicians rely on this when they teach with multiple exemplars that share the target feature but differ on irrelevant dimensions, helping the learner abstract the relevant feature.
Arbitrary Stimulus Classes
Definition
An arbitrary stimulus class groups stimuli that do not share physical features but are grouped together because they share a common functional relationship with a consequence. The stimuli become members of the same class through a common contingency, not through physical resemblance. The grouping is "arbitrary" in the sense that the physical properties of the stimuli do not predict their class membership.
Examples
Consider a set of stimuli that all function as discriminative stimuli for the same response. The sight of a vending machine, the sound of a lunch bell, and the smell of food cooking are physically very different, but if they all evoke approaching the cafeteria (because reinforcement for that behavior is available in their presence), they form an arbitrary stimulus class. The common function, not the common appearance, defines the class.
In clinical settings, different instructions from different adults may form an arbitrary stimulus class if they all evoke the same compliant response. "Sit down," "have a seat," and "take your chair" are physically different stimuli (different words, potentially different speakers), but they belong to the same functional class because they all set the occasion for the same response.
Stimulus Equivalence Classes
Definition
Stimulus equivalence, first rigorously described by Murray Sidman in the 1970s and 1980s, occurs when stimuli that are not physically similar become interchangeable through matching-to-sample training or other conditional discrimination procedures. Critically, stimulus equivalence is defined by three mathematical properties that must all be demonstrated:
The Three Defining Properties
- Reflexivity: Each stimulus matches itself (A = A). Given a sample stimulus, the individual can select the identical comparison. This is essentially generalized identity matching.
- Symmetry: If the individual is trained that A goes with B, they can demonstrate without additional training that B goes with A. The relation is bidirectional. For example, if trained to select the written word "dog" when hearing the spoken word "dog," the individual can now select the spoken word when shown the written word, without being explicitly trained on that direction.
- Transitivity: If A is related to B and B is related to C, then A is related to C without direct training. If hearing "dog" (A) is matched to the written word "dog" (B), and the written word "dog" (B) is matched to a picture of a dog (C), then hearing "dog" (A) should be matched to the picture of a dog (C) without direct training.
Why Equivalence Matters Clinically
Stimulus equivalence explains how humans can learn relationships that were never directly taught. This has enormous implications for efficiency in teaching. If a clinician trains two relations (A-B and B-C), the learner may derive four additional relations (B-A, C-B, A-C, C-A) without any additional training. This means that training two conditional discriminations can produce a class of three interchangeable stimuli with six possible relations.
In practical terms, this explains aspects of reading comprehension, vocabulary acquisition, and concept learning. A child who learns that the spoken word "cat" goes with a picture of a cat, and that the written word "cat" goes with the spoken word "cat," can now match the written word to the picture without ever being taught that specific relation. This emergent relation is a hallmark of equivalence.
Equivalence-Based Instruction (EBI)
Clinicians can leverage stimulus equivalence to teach more efficiently. By strategically selecting which relations to train directly, they can maximize the number of derived (untrained) relations that emerge. This approach, sometimes called equivalence-based instruction, has been applied to teaching reading, math facts, geographic knowledge, and many other academic skills.
Distinguishing the Three Classes on the Exam
- Feature class: Stimuli look/sound/feel alike (physical similarity)
- Arbitrary class: Stimuli share a functional relationship with a common consequence (no physical similarity required)
- Equivalence class: Stimuli become interchangeable through conditional discrimination training, demonstrated by reflexivity, symmetry, and transitivity
A common exam trap is confusing arbitrary stimulus classes with equivalence classes. Both involve stimuli without physical similarity, but equivalence classes specifically require the demonstration of all three defining properties through conditional discrimination procedures. Arbitrary classes are defined by shared functional relationships with consequences, not by matching-to-sample training.
Key Researchers to Know
Murray Sidman is the foundational researcher for stimulus equivalence. His 1994 book Equivalence Relations and Behavior is the definitive reference. For the exam, know that Sidman's work established the three defining properties and demonstrated equivalence in both typical and atypical populations. Understanding his contributions will help you answer both conceptual and application questions about stimulus classes.