Two Types of Antecedent Variables
Both establishing operations (EOs) and discriminative stimuli (SDs) are antecedent variables that increase the likelihood of behavior, but they accomplish this through entirely different mechanisms. Confusing these two concepts is one of the most common errors BCBA candidates make on the certification exam. Understanding the distinction is also crucial for clinical practice, as the intervention strategies for each are different.
At its core, the distinction is this: an SD signals when a behavior will be reinforced, while an EO determines how much the individual wants the reinforcer right now. Both increase the probability of behavior, but they do so for different reasons.
Discriminative Stimuli (SDs)
Definition and Function
A discriminative stimulus is a stimulus in the presence of which a particular response has been reinforced in the past, and in the absence of which the response has not been reinforced (or has been reinforced less frequently). The SD signals the availability of reinforcement. It tells the organism: "If you respond now, reinforcement is available." The SD does not change how much the organism wants the reinforcer; it only signals when the reinforcer can be obtained.
How SDs Are Established
SDs are established through a history of differential reinforcement. The organism learns through experience that responding in the presence of one stimulus (the SD) produces reinforcement, while responding in the presence of another stimulus (the S-delta) does not. This learning process is called stimulus discrimination training and results in the organism responding primarily in the presence of the SD.
Clinical Example
A child has learned that when the teacher is sitting at the desk (SD), asking for help produces assistance (reinforcement). When the teacher is across the room talking to another student (S-delta), asking for help does not produce assistance. Over time, the child asks for help primarily when the teacher is at the desk. The teacher's location is the SD that controls the timing of the request. Importantly, the SD does not change how much the child needs help; it only signals when help is available.
Establishing Operations (EOs)
Definition and Function
An establishing operation is an environmental variable that has two simultaneous effects: it increases the reinforcing effectiveness of a stimulus (value-altering effect) and it increases the current frequency of behavior that has been reinforced by that stimulus (behavior-altering effect). The EO makes the individual want the reinforcer more than they did before. It changes the value of the consequence, not the signal for its availability.
How EOs Differ from SDs Mechanistically
The critical mechanistic difference is this:
- SD: Does not change the value of the reinforcer. A person who just ate a full meal will still approach a restaurant when they see the "Open" sign (SD), but they will not eat much because the EO for food (deprivation) is not present.
- EO: Changes the value of the reinforcer. A person who has not eaten for 12 hours (food deprivation as EO) will find food much more reinforcing than the same person who just ate.
Clinical Example
A child who has been working for 45 minutes without a break (deprivation of rest/escape as EO) finds break time more reinforcing than a child who just returned from recess. The prolonged work period is an EO that increases the reinforcing value of a break and increases the frequency of any behavior that has previously produced breaks (including both appropriate requesting and problem behavior).
Side-by-Side Comparison
Key Differences
- Effect on reinforcer value: SDs do not change it; EOs do change it
- Effect on behavior: SDs signal when to respond; EOs create motivation to respond
- How established: SDs through differential reinforcement history; UMOs are unlearned, CMOs through conditioning
- Temporal relationship: SDs are present at the moment of the response; EOs can operate over longer time periods
- Specificity: SDs control specific responses in specific stimulus situations; EOs affect entire response classes related to a particular reinforcer
How They Work Together
In most natural situations, both SDs and EOs operate simultaneously. Consider a thirsty person (water deprivation = EO) who sees a water fountain (SD). The EO makes water valuable right now, and the SD signals that water is available right here. Both are needed for the behavior (approaching the fountain and drinking) to occur at full strength. Without the EO (the person is not thirsty), seeing the fountain may produce little behavior. Without the SD (no water source visible), being thirsty may produce searching behavior but not fountain-approaching behavior.
The Overlap That Causes Confusion
The reason candidates confuse EOs and SDs is that both are antecedent variables that increase behavior. Additionally, some stimuli can function as both an SD and an EO in the same situation. For example, the sight of a favorite food item can function as an SD (signaling that requesting will be reinforced) and simultaneously as a CMO-T or CMO-S (increasing the value of the food through sensory stimulation). When a stimulus appears to serve both functions, exam questions typically ask you to identify the primary variable at work based on context clues.
Exam Strategies
When encountering a scenario on the BCBA exam, use these questions to distinguish EOs from SDs:
- Is the scenario about the value of the consequence changing? If yes, the variable is likely an EO. Look for clues about deprivation, satiation, painful stimulation, or other value-altering conditions.
- Is the scenario about signaling when reinforcement is available? If yes, the variable is likely an SD. Look for clues about the presence or absence of specific stimulus conditions that correlate with reinforcement availability.
- Has anything changed about how much the person wants the reinforcer? If the person's motivation has changed, an EO is involved. If their motivation is the same but their opportunity has changed, an SD is involved.
- Would the behavior occur without this variable? If the person is highly motivated but lacking the opportunity, the missing variable is an SD. If the person has the opportunity but lacks motivation, the missing variable is an EO.
This distinction is among the most heavily tested on the BCBA exam. Practice analyzing clinical scenarios by identifying which antecedent variables are operating and which mechanism (value-altering vs. occasion-setting) is at work. With enough practice, the distinction becomes intuitive.