Definition
ABC data captures three columns for each instance of a target behavior: the Antecedent (what was happening just before), the Behavior (operationally defined), and the Consequence (what followed).
It is the most common descriptive method in a functional behavior assessment because it is fast, requires no equipment, and can be collected in real environments. It is also biased — observers see what they expect — so descriptive ABC is usually paired with experimental confirmation when stakes are high.
Example
Antecedent: instructor presents writing task. Behavior: client pushes paper away and says no. Consequence: instructor removes the paper for 10 seconds.