Measuring changes in mobility and balance offers a means of responding to the clinically important challenge of tracking progression and recovery in frail elderly patients who become ill. This article uses Rasch analysis to construct an interval scale from ordinal observations of such changes, using the Hierarchical Assessment of Balance and Mobility (HABAM) on 204 frail elderly inpatients. Principal components analysis confirmed the unidimensionality of the scale. The Rasch analysis confirmed the hierarchy of the HABAM, and pointed to gaps and redundant items. On this basis, an improved version of the HABAM was developed, suitable in its graphical form for everyday clinical use and for research, using the interval numbers that were generated.