Measuring change over time: assessing the usefulness of evaluative instruments

Area of Science:

• Psychometrics
• Measurement Science

Background:

• Reliability is crucial for psychometric measures, assessing true score variance.
• Instruments can be reliable yet unresponsive to change, particularly tailored ones.
• Existing metrics may not adequately capture an instrument's ability to detect change.

Purpose of the Study:

• To introduce a novel index for assessing the responsiveness of psychometric instruments.
• To provide a metric that complements reliability and validity for change-sensitive measures.
• To highlight the importance of responsiveness in evaluating instruments for measuring change over time.

Main Methods:

• Proposed a new responsiveness index.
• Defined the index in relation to the minimal clinically important difference and variability in stable subjects.
• Discussed the implications of this statistic for sample size calculations.

Main Results:

• A new index of responsiveness was developed.
• The proposed statistic directly impacts sample size considerations.
• Responsiveness is identified as a critical attribute alongside reliability and validity.

Conclusions:

• Responsiveness is essential for instruments measuring change over time.
• The new index offers a valuable tool for evaluating instrument utility.
• Instruments designed to measure change should be assessed for reliability, validity, and responsiveness.