Fixed, response-guided, and randomized phase lengths use different timing mechanisms. A fixed design names the phase endpoint in advance. A response-guided design changes phase after a prospectively described data and readiness review. A randomized design selects from permissible endpoints by chance. Each can include welfare exceptions. The report should name the mechanism actually used, preserve the decision evidence, and match any analysis to it.

Use fixed timing when the endpoint is predeclared

Record the exact observation or date, minimum data, schedule assumptions, and what happens when an observation is missed.

Use response-guided timing with a visible rule

Name the review window, evidence, owner, readiness gates, and decision record before outcomes can influence timing.

Use randomized timing with a complete assignment set

List permissible endpoints and probabilities, perform the draw prospectively, and preserve the selected and delivered starts.

Give every mechanism a humane exception path

Safety, medical need, access, consent, assent, and withdrawal can require action outside the planned timing.

Match the analysis and report to delivery

State which mechanism governed, what changed, and which confirmatory, sensitivity, or descriptive interpretation remains valid.

Build Sam's phase-length mechanism register

For the fixed versus response-guided versus randomized phase lengths question, create a versioned phase-length mechanism register. Record the target, measurement unit, eligible opportunities or observation time, scale, desired direction, raw series, missing and invalid states, context, graph version, timing mechanism, decision rule, reviewers, clinical gates, participation response, decision time, actual change, deviations, and follow-up. The record should let another qualified reviewer reconstruct why Sam's phase decision occurred without relying on memory.

Work through Sam's baseline example

Sam's team compares three protocols. The fixed version changes after observation 8. The response-guided version allows change from observations 5 through 10 after a named review. The randomized version draws one start from observations 6, 7, 8, and 9 and selects 7. These protocols can produce the same date by coincidence while supporting different claims. Show the raw series and all denominators before adding summaries. This fictional community mobility study example illustrates one decision pattern and does not create a universal stability percentage, point count, phase duration, treatment rule, or causal conclusion.

Audit the evidence available for Sam

Sam's register identifies the mechanism, allowable timing, minimum data, review owner, randomization evidence when used, assigned endpoint, clinical exception path, actual endpoint, and analysis consequence. A schedule described as randomized must include the actual prospective draw, not merely varied phase lengths. The audit also checks data-entry history, definition version, observer training, agreement when needed, graph axes, session spacing, exclusions, phase labels, source timestamps, and access to original records. Unresolved discrepancies remain visible and pause any claim that depends on them.

Address the main interpretation risk for Sam

Changing the label after seeing the result hides how timing was chosen. A response-guided start cannot inherit a randomization-test claim, and a delayed fixed phase is not automatically response-guided. Sam's record preserves the protocol and the delivered schedule separately. The review describes the observed pattern in original units, names plausible alternatives, and separates a methods judgment from a clinical recommendation. Software may calculate, graph, and surface missing evidence; qualified people interpret the series and make decisions within their authority.

Use the later phase responsibly for Sam

The team chooses the mechanism that fits the question, burden, expected response dynamics, design requirements, and welfare needs. It reports any deviation and keeps clinical care decisions distinct from statistical convenience. Visual analysis examines level, trend, variability, immediacy, overlap, and consistency across comparable phases or tiers. A supplemental statistic can summarize a defined feature. It cannot establish clinical importance, functional relation, consent, authorization, or treatment fit on its own.

Protect Sam's access and participation

Keep Sam's augmentative and alternative communication, interpreters, mobility, food, water, bathroom use, prescribed care, health support, rest, relationships, and emergency help available throughout the community mobility study. Use accessible consent and assent processes when applicable and respond to withdrawal, dissent, or distress. Necessary safety or clinical action proceeds through qualified authority even when it changes the planned phase timing.

Apply current sources to Sam's decision

Sam's source review connects current visual-analysis guidance with research showing that response-guided waiting and randomized timing answer different methodological questions. The BACB ethics hub and CASP public summary provide professional context, while the BCBA Test Content Outline identifies examination content on measurement and single-case design. The WWC Version 5.0 handbook is a research-review standard. A single-case design review describes within-phase and between-phase visual analysis. Research on waiting for baseline stability examines response-guided timing, and work on systematic visual protocols, current analytic reflections, and visual-analysis software clarifies methods and limits. ASHA supports continuous AAC access.

Rehearse Sam's review before live use

Run the phase-length mechanism register with a fictional series before it governs Sam's data. Test minimum and maximum observations, missing values, equal timestamps, trend in both directions, extreme points, floor and ceiling values, a failed access gate, assent withdrawal, delayed implementation, and an amended rule. Store expected decisions, reviewer rationale, screenshots or graph versions, software version, and correction history. The rehearsal tests mechanics while leaving the live judgment to qualified reviewers.

Close Sam's phase-decision review

Review the phase-length mechanism register with Sam, the responsible clinician, and a methodologist familiar with the design. Preserve raw data, graph, rule, decision snapshot, reviewers, client input, access and safety evidence, phase implementation, deviations, sensitivity checks, later outcomes, and inference limits. Keep the page draft and noindex until the manifest-named reviews are complete.

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