Spearman–Brown Test Length Planner
Project reliability under parallel-item assumptions and round a target length upward.
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Project reliability under parallel-item assumptions and round a target length upward. All default values are fictional.
Calculation results
Projected reliability: 0.823529; proposed item count: 20.
Required multiplier: 2.42857. Rounded target length: 25 items; projected reliability at that length: 0.853659.
Assumptions and limits
Projected reliability is kr/(1+(k−1)r). Required multiplier is target(1−r)/(r(1−target)); item count rounds upward with a minimum of two. Added or removed items must behave like parallel items of the same quality. Treat the supplied reliability as fixed; this rejects target item counts beyond the safe integer range and does not propagate estimation uncertainty, establish validity or guarantee achieved reliability. Fractional proposed item counts describe a hypothetical multiplier; use whole items in practice.
CTT Spearman–Brown documentation · Read the practical guide · Download blank worksheet (TXT)