Spearman–Brown test-length planning worksheet
StatMedAnalysis | Version 2026-11-05

Instructions: save a working copy, fill blank fields, retain assumptions and
record an independent arithmetic check. Use fictional or aggregate values;
do not enter patient identifiers. This is a planning aid, not clinical advice.

1. Define the score before calculating
Construct and intended population: ______________________________
Current item count and scoring rule: ____________________________
Reliability coefficient r, estimate and source sample: _____________
Uncertainty interval / sample limitations: _______________________
Why additional items could be parallel and of similar quality: ___
Content domains that must remain represented: ____________________

Use a current coefficient strictly between 0 and 1 for this projection.

2. Specify scenarios
Target reliability (strictly between 0 and 1): ____________________
Candidate whole item counts and respondent burden: _______________
For each k = candidate items/current items, compute kr/[1+(k-1)r].
Candidate items | k | projected reliability | time/burden
________________|___|_______________________|_____________________
________________|___|_______________________|_____________________
Required multiplier = target(1-r)/[r(1-target)]. Round item count up.
Check the projection again using the rounded whole item count.

3. Challenge the assumptions
Could added items be redundant, harder, less reliable or change the construct?
What evidence will the pilot collect on dimensionality and scoring?
What result would make you revise the projection? ________________
Do not call reliability validity or claim a guaranteed achieved coefficient.
Example (hypothetical): r=.5, 10 items, target=.8 needs k=4 and 40 items.

4. Record the decision
Chosen scenario and rationale: __________________________________
Pilot / re-estimation plan: _____________________________________
Author, date, version and unresolved limitations: ________________

Guide and primary-source links (verified 2026-11-05):
https://www.statmedanalysis.com/blog/spearman-brown-test-length
https://search.r-project.org/CRAN/refmans/CTT/html/spearman.brown.html
https://www.statmedanalysis.com/tools/reliability-spearman-brown