RBT · Behavior Assessment · 7 min read · Open lesson
Stimulus Preference Assessments
- Identify the purpose of stimulus preference assessments and describe the Registered Behavior Technician's role in implementing protocols, collecting data, and collaborating with the supervisor.
- Compare and contrast trial-based and free-operant preference assessment formats, including single-stimulus, paired-stimulus, multiple stimulus with replacement, multiple stimulus without replacement, and free-operant methods.
- Explain how preference hierarchies are established and distinguish between a stimulus preference assessment and a reinforcer assessment, accounting for variables like satiation and deprivation.
Introduction to Stimulus Preference Assessments and the RBT Role
A stimulus preference assessment is a systematic method used to identify items or activities a client prefers. In applied behavior analysis, finding preferred stimuli is an essential part of service delivery. Preference assessments help the supervising clinician identify items that may serve as reinforcers for teaching new skills or supporting behavior change. Within this process, the Registered Behavior Technician (RBT) conducts the assessment and reports the resulting data directly to the supervising Board Certified Behavior Analyst (BCBA) or Board Certified Assistant Behavior Analyst (BCaBA).
An RBT conducts preference assessments strictly following the protocol designed by the supervising BCBA or BCaBA. The technician prepares and presents items, provides access to items as directed, records the client's responses, and reports the data and relevant observations to the supervisor. It is important to remember that a client's preferences are not permanent. Preferences can change over time, across different settings, and based on daily states of deprivation or satiation. Because of these fluctuations, assessments may need to be repeated or conducted regularly according to the written behavior plan.
To ensure fair and accurate results, the RBT follows specific procedural guidelines during assessment trials. Prior to starting, the protocol may include pre-exposure. Pre-exposure involves allowing the client to briefly sample or consume each item before the assessment trials begin. This gives the client an opportunity to become familiar with items before making choices, although brief exposure does not guarantee familiarity. When the written protocol calls for a choice trial, the RBT gets the client's attention and gives the specified neutral instruction, such as 'pick one.' The RBT avoids praising specific item selections during an assessment to prevent biasing the client's choices.
The RBT must also handle session dynamics strictly according to the plan. When directed by the supervisor, the RBT can offer brief choices before teaching and record or report the client's selections as specified in the plan. If a client engages in challenging behavior when an item is removed after a trial, the RBT follows the written behavior plan and promptly informs the supervising BCBA or BCaBA. At all times, the technician implements the assessment systematically without altering procedures independently.
Single-Stimulus and Paired-Stimulus Preference Assessments
In a single-stimulus preference assessment, the RBT presents one item at a time to the client. This method is particularly useful when a client has difficulty scanning an array of stimuli or choosing between two or more options. During a single-stimulus assessment, the RBT presents the stimulus and records the client's approach to or interaction with each item as defined in the written protocol. While straightforward, presenting items one by one does not directly compare items against each other.
In a paired stimulus preference assessment, the technician presents exactly two items simultaneously. During each paired stimulus trial, the client is allowed to select only one of the two presented items. To ensure valid results, items in a paired stimulus assessment must be placed equidistant from the client to prevent proximity bias, where a client chooses an item simply because it is physically closer. Additionally, the RBT follows the written protocol for varying items' left-and-right positions across trials to reduce position bias, which occurs when a client tends to reach toward one specific side.
In a paired stimulus assessment, every item in the set is systematically paired with every other item at least once. If the client reaches for both items during a paired-stimulus trial, the RBT follows the written protocol for recording or repeating the trial rather than assuming which item was chosen. Once the client makes a clear selection of one item, the RBT provides brief access to the chosen item for the time specified in the written protocol.
A paired-stimulus assessment can provide a clear preference ranking of items because every item is evaluated directly against every other item. However, because of the comprehensive pairing process required, a paired-stimulus assessment generally takes longer than a multiple-stimulus assessment. The RBT records client selections immediately after each trial according to the written protocol so that accurate data reach the supervisor.
Multiple-Stimulus and Free-Operant Formats
A multiple stimulus preference assessment presents three or more items in an array simultaneously. Because the client must look at several choices before selecting, multiple stimulus assessments require the client to possess the scanning skills needed to view several items at once. There are two primary variations of multiple stimulus assessments: with replacement and without replacement.
In Multiple Stimulus with Replacement (MSW), the chosen item is returned to the array for subsequent trials. After the client selects an item and receives brief access, the chosen item is returned to the array, and the RBT rearranges item positions as the written protocol directs. Because the selected item is replaced, the same items remain available on each trial. A major limitation of MSW is that a client may repeatedly pick the single most preferred item on every trial, providing little data on remaining items in the array.
In Multiple Stimulus without Replacement (MSWO), the chosen item is removed from the array and is not available on subsequent trials. In an MSWO assessment, the total number of items in the array decreases by one after each selection trial. Between trials, the RBT rearranges the order of the remaining items to control for position bias. An MSWO assessment proceeds according to the written protocol, which specifies when to stop, such as after the remaining items have been selected or when the client does not select an item within the specified response period. An MSWO assessment is generally faster than a paired-stimulus assessment and can provide a ranking of the items the client selects.
In contrast to trial-based formats, a free-operant preference assessment allows the client to move among and engage with several available items during an observation period without items being removed after each choice. During a free-operant assessment, the RBT records engagement with each item, often its duration, as specified in the written protocol. Free operant assessments are particularly useful when taking items away from a client evokes challenging behavior, as the stimuli remain freely available throughout the observation.
| Assessment Type | Item Handling After Selection | Array Size Across Trials | Key Feature or Consideration |
|---|---|---|---|
| Multiple Stimulus with Replacement (MSW) | Chosen item is returned to the array | Remains the same on each trial | Client may repeatedly select the single most preferred item, giving little data on other items |
| Multiple Stimulus without Replacement (MSWO) | Chosen item is removed and unavailable for later trials | Decreases by one item after each selection | Generally faster than paired stimulus; follows the written protocol's stopping rule, which may include a no-selection criterion |
| Free-Operant | Items are not removed after choices occur | All items remain accessible throughout observation | Particularly useful when taking items away evokes challenging behavior; records engagement duration |
Data Calculations, Preference Hierarchies, and Reinforcer Assessments
The RBT accurately records raw selection data in the client's record immediately after each trial and shares the data with the supervisor. In paired stimulus and MSW assessments, preference percentage is calculated by dividing the number of times an item was chosen by the number of times it was presented, multiplied by 100. In an MSWO assessment, items chosen in the earliest trials are ranked as higher preference than items chosen in later trials. Together, these scores allow the supervisor to establish a preference hierarchy.
A preference hierarchy ranks assessed stimuli from high-preference to moderate-preference and low-preference. If an item is not selected during an assessment, it may be low preference in that specific assessment. Rather than labeling the item non-preferred permanently, the RBT records what happened and reports it to the supervisor. This precision is important because preferences fluctuate and must be interpreted carefully by the supervising clinician.
A critical concept in behavior analysis is that a stimulus preference does not guarantee that the stimulus will function as a reinforcer. A reinforcer is a consequence whose contingent presentation or removal increases the future likelihood of a behavior under similar conditions. Preference assessments measure choices or engagement with stimuli; they do not, by themselves, show whether those stimuli increase a behavior when delivered contingently. A reinforcer assessment evaluates whether making a stimulus change contingent on a response increases that response; the supervisor determines the assessment procedures and comparison conditions. The supervisor directs this assessment.
Even a verified high-preference item may not support a particular teaching task when demands are too difficult or reinforcement is too infrequent. Environmental states also alter reinforcing value: satiation can temporarily decrease how effective a preferred item is as a reinforcer, whereas deprivation can temporarily increase a stimulus's reinforcing effectiveness. The RBT observes and reports relevant changes in client responding or motivation but does not independently restrict access to items. If an item is not supporting the target behavior as expected during sessions, the RBT records the data and informs the supervising BCBA or BCaBA. The RBT must not modify the reinforcement schedule or intervention protocol without direction from the supervisor.
Common mistakes
- Praising a client for choosing a specific toy or snack during a preference assessment, which biases the client's subsequent choices instead of remaining neutral.
- Assuming an item chosen in a preference assessment is automatically a reinforcer without understanding that reinforcers must be shown to increase behavior contingently.
- Guessing which item a client wanted when they reach for both items simultaneously in a paired-stimulus assessment, rather than following the written protocol for recording or repeating the trial.
- Independently restricting a client's access to an item outside of session to create deprivation, rather than observing and reporting changes to the supervisor.
Summary
- A stimulus preference assessment is a systematic method to identify preferred stimuli, helping the supervising BCBA or BCaBA identify potential reinforcers.
- The RBT conducts assessments strictly following written protocols, presenting items, remaining neutral, recording raw data immediately, and communicating observations to the supervisor.
- Trial-based assessments include single-stimulus (one item at a time), paired-stimulus (two items compared equidistant), MSW (chosen item replaced), and MSWO (chosen item removed).
- Free-operant assessments record engagement duration with freely accessible items and are particularly useful when item removal evokes challenging behavior.
- In paired stimulus and MSW, preference percentage is calculated as (number of times chosen / number of times presented) * 100, while MSWO ranks earliest selected items highest.
- Preference does not guarantee reinforcer effectiveness; contingent presentation or removal of a reinforcer increases the future likelihood of a behavior under similar conditions.
- RBTs must never modify protocols, change reinforcement schedules, or restrict item access independently, but must record data and inform the supervisor if an item fails to support target behaviors.
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