Internal/External Validity and Threats
Key Takeaways
- Internal validity concerns whether the intervention, not another variable, caused the behavior change.
- External validity concerns whether findings generalize across people, settings, behaviors, materials, or times.
- Common threats include history, maturation, instrumentation, testing effects, procedural drift, and multiple treatment interference.
- Single-case designs reduce threats through repeated measurement, staggered replications, reversals, and clear condition changes.
Validity in Domain D
Internal validity asks whether the independent variable is the most plausible reason behavior changed. External validity asks whether the effect would hold under other relevant conditions. The BCBA exam often tests whether you can separate a strong functional relation from a weak generality claim.
| Threat | What it means | Design response |
|---|---|---|
| History | Outside event occurs with treatment | Stagger starts or replicate effects |
| Maturation | Behavior changes with time or development | Use baseline trends and replications |
| Instrumentation | Measurement system changes | Train observers and monitor IOA |
| Testing | Repeated exposure changes performance | Use stable routines and probes carefully |
| Procedural drift | Staff implement differently over time | Measure procedural integrity |
| Multiple treatment interference | One condition affects another | Counterbalance or choose another design |
Single-case designs support internal validity when behavior changes in a pattern that matches the planned manipulation. A sudden improvement after treatment may be promising, but it is weak if a school vacation, medication change, or staff change occurred at the same time.
External validity is built through replication. Direct replication repeats the same procedures with similar conditions. Systematic replication tests whether the effect holds with planned variations, such as a new setting, implementer, or learner.
Exam reasoning
When a question asks for the biggest threat, look for events that coincide with the condition change. When it asks for the best design improvement, choose the option that adds prediction, verification, or replication while respecting ethics and feasibility.
A design may be internally valid but externally narrow. For example, a reversal with one client in one clinic may convincingly show that a treatment reduced disruption for that client. It does not prove the same treatment will work for all clients, settings, or functions of behavior.
A student's disruption decreases immediately after an intervention begins. The same week, the student also starts a new medication. What is the primary validity concern?
A BCBA demonstrates a functional relation for one client in one classroom. The team wants to know whether it works during lunch with another teacher. What validity issue is most relevant?
Which practice most directly addresses the threat of procedural drift during an intervention phase?