Modelling cognitive trajectory. (a) Comparing time to dementia and age to describe cognitive trajectories. Mini-Mental State Examination (MMSE) trajectories of a random sample of Origins of Variance in the Oldest-Old Twin Study participants plotted as a function of age and dementia diagnosis. As an illustration of how heterogeneity of trajectories is reduced when scores are modelled using a process-based approach, MMSE scores of a random sample of participants plotted as a function of age and time to dementia diagnosis are depicted . (b) Graphical illustration of a change point model. A schematic representation of the typical change point model trajectory as assumed in the broken stick model .