A verbal operant experimental (VOX) analysis is a functional analysis of verbal behavior formalized for statistical analysis to answers the question: To what extent does the speaker respond across different verbal operants? 

Research on verbal behavior has identified four primary sources of control over language: 

• Extraverbal:

  • Tact (i.e., labeling) - the formal control of spatial contiguity. For example, the speaker sees a jar of jelly and says, "Jelly!"

  • Mand (i.e., requesting) - the thematic control of spatial discontigruity. For example, the speaker sees a jar of peanut butter and says, "Jelly!"

• Intraverbal:

  • Duplic (i.e., echoing) - the formal control of temporal contigruity. For example, the speaker hears the word, "Jelly," and says, "Jelly!"

  • Sequelic (i.e., conversing) - the thematic control of temporal discontigruity. For example, the speaker hears the words, "Peanut butter," and says, "Jelly!"

In contrast to fluent speakers, who show proportionality across these four sources of control, speakers with stimulus overselectivity show different levels of response strength. A VOX analysis identifies the extent to which the speaking repertoire is balanced.

Power Analysis

A sufficient sample size of between 24 and 48 total responses must be collected. The speaker record below can be used for:

1. Collecting data across each condition, 

2. Determining when to stop collecting data, and 

3. Entering your data into the report generator. 

Assessing Tact Control (Extraverbal, Formal)

Tact control is assessed as part of a free-operant preference assessment. Throughout this condition, the speaker has the opportunity to choose from many different items with which to engage. Each round of the assessment consists of three unique items. Once an item has been selected, the speaker is asked to label the item (e.g., “What do you have there?”). A discriminated response is under the formal control of the extraverbal stimulus. This process is repeated until three referents have been assessed under tact cotnrol.

Apart from its use in assessing the speaker’s ability to label, the free-operant preference assessment ensures random sampling (i.e., the selection is determined by the speaker) and affords cultural sensitivity (e.g., the speaker, and/or their guardian, selects the assessment materials and provides the target response). The three targets identified in the labeling condition are then also assessed under each of the remaining conditions. 

Table 1 shows tact control assessed for three items assessed in Round 1 (items 1, 2, and 3), and three different items assessed in Round 2 (items 4, 5, and 6). Note that the three items in Round 1 would also be assessed for mand, echoic, and sequelic control prior to beginning the second round of the assessment. Between 6 and 12 total labeling responses are assessed in this condition. 

Assessing Mand Control (Extraverbal, Thematic)

Mand control is assessed by presenting the three items from the tact condition simultaneously and asking the speaker to look at each one. One item is then removed from the array and the speaker is asked to name the absent item (e.g., "What's missing?"). A discriminated response is under the thematic control of the remaining extraverbal stimuli. This process is repeated until all three referents have been assessed under mand control.

Table 2 shows mand control assessed for the same three items that were identified in each round of the tact condition. Note that each round of the assessment begins with assessing tact control. The order of the remaining conditions varies for each round of the assessment. Between 6 and 12 total requesting responses are assessed in this condition. 

Assessing Duplic Control (Intraverbal, Formal)

Duplic control is assessed through discrete-trial training, again using each of the three response targets from the previous conditions. The assessor requests for the speaker to echo what they say (e.g., "Say, 'Apple.'”). A discriminated response is under the formal control of the intraverbal stimulus. A discriminated response is under the formal control of the intraverbal stimulus. This process is repeated until all three items have been assessed under duplic control.

Table 3 shows echoic control assessed for the same three items that were identified in each round of the tact condition. Note that each round of the assessment begins with assessing tact control. The order of the remaining conditions varies for each round of the assessment. Between 6 and 12 total echoing responses are assessed in this condition. 

Assessing Sequelic Control (Intraverbal, Thematic)

Sequelic control is assessed through discrete-trial training with each of the same three target responses. The listener presents the names for all three items (e.g., "Apple and Train, and Dog") and then represents the names of two items in a different order, calling for the speaker to name the third (e.g., "Dog and Train and ..."). A discriminated response is under the thematic control of intraverbal stimuli. This process is repeated until all three referents have been assessed under sequelic control.

Table 4 shows sequelic control assessed for the same three items that were identified in each round of the tact condition. Note that each round of the assessment begins with assessing tact control. The order of the remaining conditions varies for each round of the assessment. Between 6 and 12 total conversing responses are assessed in this condition. 

Multidimensional Analysis

The VOX allows for a multidimensional analysis of control over the speaker's verbal repertoire: