Attrition in Longitudinal Data is Primarily Selective with Respect to Level Rather than Rate of Change

An important question in longitudinal research is whether the individuals who discontinue participation differ in their level of, or their change in, cognitive functioning relative to individuals who return for subsequent occasions.

Performance in five cognitive domains was examined in nearly 5000 participants between 18 and 85 years of age who completed between one and five longitudinal occasions.

Little or no differences in cognitive performance were apparent between young adults who did or did not return for subsequent longitudinal occasions. However, among adults above about 45 years of age, returning participants had higher levels of cognitive performance, but approximately similar magnitude of longitudinal change, as participants completing fewer occasions.

These results suggest that generalizability of longitudinal comparisons may be restricted to individuals with relatively high levels of cognitive functioning, but that rates of cognitive change are nearly comparable for individuals completing different numbers of longitudinal occasions.