Sequence effect

Condition order becomes part of the history

Every later phase occurs after earlier exposure. A participant may learn the task, become familiar with materials, experience fatigue, develop a preference, or contact consequences that change future responding.

If all participants receive A before B, a later advantage for B can reflect the procedure, the accumulated practice, or an interaction between them. If B follows a difficult condition, contrast may make B appear especially effective. If B follows successful teaching, B may inherit a stronger skill base.

Sequence is therefore a design variable. The order, timing, and transition rules should be recorded and considered during interpretation.

Carryover is one source of sequence effects

A carryover effect is a lingering influence from an earlier condition. Sequence effects also include broader order-related processes. Fatigue may depend on cumulative session exposure. Expectancy can develop after repeated alternation. A task may become easier through practice even when no intervention feature lingers.

Use “carryover” when the earlier procedure’s influence persists into the next condition. Use “sequence effect” when the evidence supports an order-related difference but the mechanism remains broader or uncertain.

Common signals deserve investigation

Possible signals include:

• the same condition performs differently early and late in the study
• a condition looks effective only after another condition
• results differ across AB and BA sequences
• performance drifts with cumulative exposure
• phase changes become smaller across repeated comparisons
• rapid alternation produces unstable or blended responding

Each signal has alternatives. Maturation, staffing, measurement drift, procedural-integrity changes, health events, and unequal opportunities can produce similar patterns.

Design order deliberately

Several controls may help when clinically appropriate:

• counterbalance order across participants, targets, or matched sets
• randomize condition order within stated constraints
• repeat comparisons rather than relying on one sequence
• use distinct condition cues and materials
• space conditions when a plausible effect fades over time
• equalize exposure and opportunities
• predefine transition, stopping, and safety rules

Counterbalancing distributes order across cases; it does not erase history within a case. Randomization can reduce systematic bias while still leaving carryover. The design and conclusion should acknowledge remaining limits.

The BACB BCBA Test Content Outline covers internal-validity threats, single-case design features, interpretation, and comparative analyses. It is examination content rather than a step-by-step design manual.

A fictional AB and BA comparison

Ben is learning two matched self-management routines. For Target 1, the team uses Package A for six sessions, followed by Package B for six. For Target 2, it reverses the order: B for six sessions, followed by A for six.

The second condition is higher in both sequences. Calling B superior from Target 1 alone would be misleading because A also performs better when it comes second. Practice, familiarity, or another sequence influence may explain the pattern.

The matched targets are still imperfect comparisons. The team reports target definitions, session order, prompts, fidelity, opportunities, and Ben’s experience. It considers a design with additional replications before choosing between packages.

Interpret within the actual sequence

Graph the order in which data were collected. Report calendar time, session spacing, cumulative exposure, and any washout or transition interval. A table sorted by condition can hide the very pattern under review.

The WWC Single-Case Design Technical Documentation describes repeated measurement, internal validity, replication, and visual analysis. The framework reinforces the value of multiple demonstrations while leaving design choice to the research question and context.

Avoid averaging across sequences before examining each one. A pooled mean can hide opposite effects. Review level, trend, variability, immediacy, overlap, and consistency within comparable phases.

Protect learning and safety

The need for order control never justifies withdrawing effective communication, medical, mobility, or safety support. Learned skills may persist, and that persistence can be a desirable outcome.

When an irreversible or durable change is likely, select a design that works with that reality. Document the limit openly. Strong methods make the history visible rather than forcing behavior to fit a preferred sequence.

Keep an order ledger

For each session, record target, condition, planned and actual position, prior exposure, elapsed time, implementer, setting, opportunities, and deviations. Review the ledger beside the graph before attributing a difference to the current condition.

If a sequence changes mid-study, preserve both the planned and actual order. Explain the reason, date, and decision owner. This record helps distinguish a deliberate safety or clinical change from unnoticed procedural drift.