Reliability of estimates of changes in mental status test performance in senile dementia of the Alzheimer type.

The concept of the reliability of a measure can also be applied to its change over time. In this study we consider the growth curve approach to estimating the reliability of change, in the context of cognitive status as measured by the Mini-Mental State Examination (MMSE) and the Blessed and Tomlinson Dementia Rating Scale (DRS) in patients with senile dementia of the Alzheimer type (SDAT). The reliability of the estimates of change is shown to depend primarily upon the length of time of observation, not the number of observations made. The estimated reliability coefficient for the change in MMSE (or DRS) at 6 months is 0.16 (or 0.08); at 2 years is 0.75 (or 0.57). The concept of signal-to-noise ratio is introduced to compare reliabilities in change scores.