Standard Deviation Index Calculator

Calculate SDI to measure bias between laboratory and consensus group means

Calculate Standard Deviation Index

The mean of your test/laboratory measurements

The mean of the reference/consensus group

The standard deviation of the reference/consensus group (must be > 0)

⚠️ Consensus group standard deviation cannot be zero. Please enter a positive value.

SDI Interpretation Guide

SDI = 0

Perfect agreement

0 < |SDI| ≤ 1

Acceptable performance

1 < |SDI| ≤ 1.25

Within acceptable limits

1.25 < |SDI| ≤ 1.49

May need examination

1.5 ≤ |SDI| ≤ 1.99

Marginal - examine test

|SDI| ≥ 2

Unacceptable - action required

Key Concepts

Laboratory Mean

Mean of your test measurements (sample mean)

Consensus Group Mean

Mean of the reference group (population mean)

SDI Sign

+ indicates lab > consensus, - indicates lab < consensus

Quality Control Tips

SDI measures bias in standard deviations

Target SDI is 0.0 (no bias)

Monitor trends over time

Take corrective action when |SDI| ≥ 2

Understanding Standard Deviation Index (SDI)

What is SDI?

The Standard Deviation Index (SDI) is a measurement of bias that quantifies how far your laboratory mean is from the consensus group mean, expressed in terms of the consensus group's standard deviation units.

Why Use SDI?

  • Identifies systematic bias in testing procedures
  • Provides standardized measure for comparison
  • Helps maintain quality control standards
  • Simple calculation with clear interpretation

Formula and Calculation

SDI = (Lab Mean - Consensus Mean) / Consensus Std Dev

Where each component represents the respective statistical measure

Interpretation Guidelines

Positive SDI: Laboratory mean > Consensus mean

Negative SDI: Laboratory mean < Consensus mean

Zero SDI: Perfect agreement (ideal scenario)

Magnitude: Distance in standard deviation units

SDI Categories and Actions

Excellent (SDI = 0)

Perfect agreement. Continue current procedures.

Good (|SDI| ≤ 1)

Acceptable performance with minimal bias. Monitor regularly.

Acceptable (1 < |SDI| ≤ 1.25)

Within limits but watch for trends.

Warning (1.25 < |SDI| ≤ 1.49)

Review procedures and calibration.

Alert (1.5 ≤ |SDI| < 2)

Examine test case immediately.

Critical (|SDI| ≥ 2)

Unacceptable - urgent action required.

Applications

Laboratory Quality Control

Monitor test accuracy and identify systematic errors

Proficiency Testing

Compare laboratory performance to peer groups

Method Validation

Assess accuracy of analytical methods

Process Monitoring

Track performance trends over time