Instances of the class Observation are used to specify observations about an entity (which may or may not be an instance of a StatisticalPopulation), at a particular time. The principal properties of an Observation are observedNode, measuredProperty, measuredValue (or median, etc.) and observationDate (measuredProperty properties can, but need not always, be W3C RDF Data Cube "measure properties", as in the lifeExpectancy example). See also StatisticalPopulation, and the data and datasets overview for more details.