In many situations either the individuals, the variables, or both, possess some well defined relationships that are known a priori. Examples include linear chains or sequences (where the measurements are ordered—for example, protein sequences), time series (where the measurements are ordered in time, perhaps on a uniform time scale), and spatial or image data (where the measurements are defined on a spatial grid). Even more complex structure is possible. For example, in medicine one can have
imaging data of the brain measured on a three-dimensional grid, with repeated measurements over time.