Objectives

To improve the construct validity of self‐reported fatigue by establishing a formal hierarchy of scale items and to determine whether such a hierarchy could be maintained across time (aged 75–80), sex, and nationality. Design

Cohort study. Setting

Two Nordic urban locations: Jyväskylä, Finland, and Glostrup, Denmark. Participants

Baseline (1989/90) consisted of a random sample of citizens of Finland or Denmark born in 1914 (n = 837). At 5‐year follow‐up, excluding those lost to follow‐up and with baseline disability resulted in a sample of n = 690. Measurements

The Mobility‐Tiredness (Mob‐T) Scale is a six‐item scale that requires subjects to self‐report on whether they become tired performing mobility‐related tasks. Employing item response theory, an attempt was made to enhance construct validity by confirming a hierarchy of mobility‐related fatigue. Results

A formal hierarchy of fatigue tasks, maintained across time, was established using the revised Mob‐T Scale. At age 75, the scalability statistics were a homogeneity coefficient (H) of 0.80, urn:x-wiley:00028614:media:jgs12122:jgs12122-math-0001 of 3.9% and an HT value of 0.66. The corresponding figures at age 80 were 0.75, 6.9% and 0.59. The property of invariant item ordering was maintained across subgroups based on sex and nationality. Conclusion

Establishing a formal hierarchy at age 75 allowed which tasks were most debilitating to be identified more clearly and the individual's “distance” from these tasks to be gauged. Because it was possible to confirm that the item hierarchy was maintained across time (aged 75–80), researchers or clinicians can be more confident that performance over time is the result of real change and has less to do with measurement error.