Free · Standardized differences

Standard Deviation Index Against a Consensus Mean

Express the difference between a laboratory mean and a consensus mean in units of the consensus-group standard deviation.

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

A laboratory mean of 100, consensus mean of 90 and consensus-group SD of 5 give an SDI of 2. The laboratory mean is two group standard deviations higher.

SDI = (laboratory mean − consensus mean) ÷ consensus-group standard deviation

Worked example

Enter these known values and leave the other values blank.

Laboratory mean
100
Consensus-group mean
90
Consensus-group standard deviation
5
Signed standard deviation index
2

Assumptions and limitations

  • Both means and the group SD must describe comparable materials, methods and units. The group SD must be positive.
  • This is a standardized difference between means, not automatically a z-test or t-test: it does not include sample-size-based uncertainty.
  • Displayed bands preserve the numerical cutoffs in this model. They do not establish regulatory acceptance, method validity or the absence of measurement bias.

Common questions

What does a negative SDI mean?

The entered laboratory mean is lower than the consensus mean.

Does an SDI inside a displayed band prove a test is acceptable?

No. Apply the acceptance criteria and uncertainty assessment required by the relevant program and method.

References

Compare signed standardized differences and band boundaries without approximate-equality overlap.

Explore standardized differences or browse all statistics calculators.