How to Conduct Preference Assessments: MSWO, Paired, Free Operant

Preference Versus Reinforcer

A preference assessment identifies items or activities that a learner is likely to choose. A reinforcer assessment tests whether those items actually increase future behavior when delivered contingent on a response. Preferred items often function as reinforcers, but not always. A good clinical workflow is to conduct a preference assessment to find candidates, then verify their effects with a reinforcer assessment or by monitoring data during teaching. On the BCBA exam, you should be able to distinguish between formats, choose the most suitable one, and interpret results.

Why Preference Assessments Matter

• They increase the effectiveness of reinforcement-based programs.
• They reduce reliance on caregiver report, which may be inaccurate.
• They identify items that can compete with problem behavior.
• They allow teams to update reinforcers as preferences change.
• They respect the learner by giving choice and voice.

Preparation for Any Format

1. Interview caregivers and teachers to generate a list of potential items, including edibles, toys, sensory items, social activities, and leisure materials. A structured tool such as the Reinforcer Assessment for Individuals with Severe Disabilities (RAISD) can help.
2. Select 5 to 8 items for a single assessment, covering a range of categories.
3. Control motivation: conduct assessments when the learner has not had free access to the items recently.
4. Define the selection response, such as reaching for, touching, or picking up an item, and decide how long the learner has to respond (usually 5 seconds).
5. Prepare data sheets and ensure safety by removing items that pose a choking risk or that trigger problem behavior.

Single Stimulus (Successive Choice)

Items are presented one at a time, and the clinician records whether the learner approaches or engages with each item.

Procedure

1. Present an item and say "Here is the (item)" or provide a brief sampling opportunity.
2. Record approach, engagement, or avoidance within a set time.
3. Remove the item and present the next one.
4. Present each item multiple times, in random order.

Advantages: Simple and useful for learners with limited skills. Limitations: It does not rank items against one another, and learners may approach many items, so it can overestimate preference.

Paired Stimulus (Forced Choice)

The paired stimulus preference assessment, introduced by Fisher and colleagues in 1992, presents two items at a time. The learner can choose only one, and each item is paired with every other item.

Procedure

1. Place two items in front of the learner, equidistant from the learner.
2. Say, "Pick one."
3. When the learner selects an item, give brief access (about 30 seconds for toys or a small piece for edibles).
4. If the learner selects both items simultaneously, block and re-present. If the learner makes no selection after 5 seconds, prompt a sample of each and re-present.
5. Continue until each item has been paired with every other item.
6. Calculate the percentage of trials in which each item was selected, and rank order the items.

Advantages: Produces a clear hierarchy and is reliable. Limitations: Takes more time, involves many trials, and may evoke problem behavior when an item is removed.

Multiple Stimulus With Replacement (MSW)

An array of items is presented, and the learner selects one. After brief access, the selected item is returned to the array and the array is re-presented. This continues for a set number of trials. Items are ranked by the percentage of selections. MSW can show which items are chosen consistently, but one item may dominate and obscure the relative value of others.

Multiple Stimulus Without Replacement (MSWO)

The MSWO, developed by DeLeon and Iwata in 1996, presents an array of items (typically 5 to 8), and after the learner chooses one and gets brief access, that item is not replaced. The remaining items are rearranged, and the learner chooses again. The process continues until all items have been selected or the learner stops responding.

Procedure

1. Arrange items in a line or semicircle about one foot apart in front of the learner.
2. Say, "Pick one."
3. Provide brief access to the chosen item. Do not return it to the array.
4. Rotate the positions of the remaining items to control for position bias.
5. Repeat until all items have been chosen, then conduct several sessions, usually 3 to 5, with the full set.
6. Calculate selection percentages: the number of times an item was chosen first divided by the number of sessions, or use the mean rank.

Advantages: Efficient and produces a preference hierarchy in less time than the paired stimulus format. It is among the most commonly used formats in practice. Limitations: Requires the learner to scan an array, which may be difficult for learners with limited visual attention, and can evoke problem behavior if items are removed.

Free Operant Observation

In a free operant assessment, the learner has free access to a variety of items in a room, and the clinician records how much time the learner interacts with each item. This is also called a contingent or noncontingent free operant observation.

Procedure

1. Place several items in the room and allow the learner to roam.
2. Use momentary time sampling or duration recording to measure engagement with each item.
3. Do not prompt or deliver consequences, unless safety requires.
4. Compute the percentage of time or intervals of engagement with each item.

Advantages: There is no item removal, so problem behavior is less likely. It is useful for learners who engage in problem behavior during removal of items and for learners with limited ability to make selections. Limitations: It may favor items that are easy to engage with, and it does not provide a forced-choice ranking.

Response Restriction Variation

The response restriction method, introduced by Hanley and colleagues, combines free operant observation with restriction of the most preferred items to see how engagement with other items changes. It can reveal items that are preferred when the top items are unavailable and is useful for identifying a larger range of reinforcers.

Choosing a Format

• Limited skills or problem behavior around item removal: Free operant or single stimulus.
• Need for a ranked list quickly: MSWO.
• Need for a more precise hierarchy: Paired stimulus.
• Learner who has trouble scanning arrays: Paired stimulus or free operant.
• Edible items or small leisure items: MSWO or paired stimulus.
• Preference for social or activity-based reinforcers: Use pictures, videos, or real activities in a format adapted to the learner.

Interpreting Results

Rank items by selection percentage or engagement. Items chosen first most often are high preference. Use the top 3 to 5 items as reinforcers, and rotate them to avoid satiation. Items in the middle may be useful for lower-effort tasks. Items at the bottom should not be used as reinforcers. Combine results with caregiver information and observation, and verify that high-preference items serve as reinforcers by monitoring behavior during teaching.

Reinforcer Assessment

To confirm that an item is a reinforcer, deliver it contingent on a simple response, such as touching a card or putting an object in a bin, and compare response rates with a baseline condition without the item. If responding increases, the item functions as a reinforcer. A progressive-ratio schedule can also evaluate the strength of reinforcers by increasing the response requirement until responding stops.

How Often to Repeat

Preferences change with time, satiation, and context. Repeat assessments regularly, such as weekly or before each teaching session for learners whose preferences shift quickly. Brief checks, such as a mini-MSWO with 3 to 5 items before sessions, are efficient. Observe signs that items are losing value, including decreasing responding, lack of engagement, or requests for other items.

Practical Tips

• Use small portions of edibles to prevent satiation.
• Include a range of item types, such as sensory, social, and activity-based.
• Be prepared to handle problem behavior when items are removed by using a brief access period and clear signals.
• Do not use items that are contraindicated for health or allergy reasons.
• Teach the learner to indicate preferences using picture cards or other communication systems.

How This Appears on the BCBA Exam

Questions often describe an assessment and ask you to name it, such as MSWO when items are not returned to the array. You might be asked which format is best for a learner who engages in problem behavior when items are removed (free operant) or which produces a ranked hierarchy efficiently (MSWO). You may also be asked the difference between a preference assessment and a reinforcer assessment.

Key Takeaways

• Preference assessments identify candidate reinforcers; reinforcer assessments verify them.
• MSWO is efficient; paired stimulus is precise; free operant is least likely to evoke problem behavior.
• Repeat assessments regularly because preferences change.
• Choose the format based on the learner's skills and behavior.