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Spearman–Brown Test Length Projection

Project reliability after changing the length of a parallel-item test.

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01 Your input
02 Your result
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FormulaProjected Reliability = factor*r/(1+(factor-1)*r); Gain = factor*r/(1+(factor-1)*r)-r
Sources and model references

Made for real tasks

  • Project reliability after changing the length of a parallel-item test.
Explore step-by-step workflows →

Worked example

Inputs

Current reliability
0.7 correlation
New-to-old length ratio
2 number

Expected results

Projected Reliability
0.8235294118 number
Gain
0.1235294118 number

Load the example in the workspace to reproduce these results. Display rounding does not change the underlying calculation.

How to use this tool

  1. Enter current reliability, new-to-old length ratio.
  2. Use the displayed units and confirm the model assumptions.
  3. Calculate and compare the outputs against your input scenario.

Try this example

{
  "r": 0.7,
  "factor": 2
}

Accuracy and scope

Assumes added or removed items behave like parallel items; not valid when content or measurement properties change.

Is my data uploaded?

This tool runs locally in your browser. Input and output are not sent to a processing server. Copy or download your result before leaving the page.